Adhesive Bonding: Science, Technology and Applications

Adhesive bonding

Adhesive bonding Science, technology and applications Edited by R. D. Adams

Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB1 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 2000 Corporate Blvd, NW Boca Raton FL 33431, USA First published 2005, Woodhead Publishing Limited and CRC Press LLC ß 2005, Woodhead Publishing Limited The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from the publishers. The consent of Woodhead Publishing Limited and CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited or CRC Press LLC for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress Woodhead Publishing Limited ISBN 1 85573 741 8 CRC Press ISBN 0-8493-2584-6 CRC Press order number: WP2584 The publishers' policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Project managed by Macfarlane Production Services, Markyate, Hertfordshire ([email protected]) Typeset by Godiva Publishing Services Ltd, Coventry, West Midlands Printed by TJ International Limited, Padstow, Cornwall, England

Contents

Contributor contact details

xiii

Part I Fundamentals of adhesive bonding 1

History of adhesive bonding

1.1 1.2 1.3 1.4

Early days The industrialisation of glue making The advent of synthetic polymers References

2

What are adhesives and sealants and how do they work? 23

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16

Introduction Adhesives which harden by loss of solvent Adhesives which harden by loss of water Adhesives which harden by cooling Adhesives which harden by chemical reaction Adhesives which do not harden ± pressure-sensitive adhesives Adhesion by physical adsorption Adhesion by chemical bonding The electrostatic theory of adhesion Mechanical interlocking Adhesion by interdiffusion Weak boundary layers Pressure-sensitive adhesion Future trends Sources of information References

P A F A Y , UK

3 3 10 15 19

J C O M Y N , Loughborough University, UK

23 24 24 26 27 34 35 41 45 45 45 47 47 49 49 50

vi

Contents

3

Surfaces: how to assess

52

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Introduction Surface topography Surface thermodynamics Surface chemical analysis Concluding remarks Acknowledgements References

52 53 64 67 73 73 73

4

Surfaces: how to treat

75

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

Introduction Pretreatments for metals Pretreatments for inorganic materials Pretreatments for plastics Pretreatments for elastomers Summary and future trends Literature References

75 76 78 80 85 86 87 87

J F W A T T S , University of Surrey, UK

D B R E W I S , Loughborough University, UK

Part II Mechanical properties 5

Stress analysis

5.1 5.2 5.3 5.4 5.5 5.6

Introduction A qualitative description of adhesive joint stresses Closed form, global stress analysis of adhesive joints Finite element analyses of adhesive joints Future developments References

91 91 97 107 118 119

6

Environmental (durability) effects

123

6.1 6.2 6.3 6.4 6.5 6.6

Introduction Additives to reduce photo-oxidative degradation Behaviour of structural joints to metals in wet surroundings Water and adhesives Water and adhesive interfaces Other fluids

123 123 125 133 137 140

A C R O C O M B E , University of Surrey, UK

J C O M Y N , Loughborough University, UK

91

Contents

vii

6.7 6.8 6.9 6.10

Timber joints Future trends Further information References

140 140 141 141

7

Non-destructive testing

143

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

Introduction Conventional ultrasonics Bond testers Rapid scanning methods Cohesive property measurement The interface problem and monitoring environmental degradation Conclusions References

143 145 152 154 159 160 161 161

8

Impact behaviour of adhesively bonded joints

164

8.1 8.2

164

8.4 8.5 8.6 8.7

Introduction Experimental method for impact test of adhesives and adhesively bonded joints, and characteristics of adhesives under high rate loading Stress distribution and variation in adhesively bonded joints subject to impact load Actual joint design considering impact load Future trends and further information Conclusion References

9

Fracture mechanics of adhesive bonds

189

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10

Introduction An energy criterion for failure The stress intensity factor approach The energy release rate approach Thermodynamic, intrinsic, and practical adhesion energy The effect of mode mixity Experimental evaluation of fracture energy Durability Designing with fracture mechanics Recent developments and current research areas

189 190 191 194 196 197 199 201 202 203

8.3

P C A W L E Y , Imperial College, UK

C S A T O , Tokyo Institute of Technology, Japan

D A D I L L A R D , Center for Adhesive and Sealant Science, USA

165 181 185 187 187 187

viii

Contents

9.11 9.12

Conclusions References

205 205

Fatigue

209

10.1 10.2 10.3 10.4 10.5 10.6

Introduction The stress-life approach The fatigue crack growth (FCG) approach Summary and future trends Further information References

209 213 226 235 236 237

11

Vibration damping

240

11.1 11.2 11.3 11.4

Introduction Damping in joints Prediction methods of vibration damping Experimental data on vibration damping of adhesively bonded joints Future trends References

240 241 242

10

11.5 11.6

I A A S H C R O F T , Loughborough University, UK

M H I L D E B R A N D , FY-Composites Oy, Finland

244 251 252

Part III Applications 12

Joining similar and dissimilar materials

257

12.1 12.2 12.3 12.4 12.5 12.6 12.7

Introduction Joint design Adhesive selection Surface pre-treatments Assembly issues and hybrid joining Future trends Bibliography

257 258 265 268 270 275 277

13

Bonding of composites

279

13.1 13.2 13.3 13.4 13.5

Introduction The specific nature of composite materials Design of bonded composite assemblies Surface preparation Testing

279 279 280 285 287

E J C K E L L A R , The Welding Institute, UK

P D A V I E S , Materials and Structures Group, France

Contents

ix

13.6 13.7 13.8 13.9 13.10 13.11

Influence of bondline thickness Examples of bonded composite structures Durability and long-term performance Future trends Sources of information References

291 291 296 296 300 301

14

Building and construction ± steel and aluminium

305

I J J V A N S T R A A L E N , TNO Environment and Geosciences,

The Netherlands A N D M J L V A N T O O R E N , Delft University of Technology, The Netherlands

14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9

Basic needs Adhesive characteristics required Surface preparation Strength and durability Common failures Inspection, testing and quality control Repair and strengthening Other industry-specific factors References

305 306 309 311 319 320 324 325 327

15

Building and construction ± timber

328

È L L A N D E R , SP Swedish National E SERRANO AND B KA

Testing and Research Institute, Sweden

15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12

16 16.1 16.2

Introduction and overview Basic needs and applications Wood characteristics Adhesive characteristics needed Surface preparation and bond formation Strength and durability Common failures Inspection, testing and quality control Repair Examples of use Future trends and further reading References

328 328 331 333 337 339 345 346 348 348 351 354

Automobiles

357

Introduction Basic needs

357 358

K D I L G E R , Technische UniversitaÈt Braunschweig, Germany

x

Contents

16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11

Adhesive characteristics required Surface preparation Strength and durability Common failures Inspection, testing and quality control Repair and recycling Other industry-specific factors Examples of use References

17

Boats and marine

17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9 17.10 17.11

Introduction Basic needs Adhesive characteristics required Surface preparation Strength and durability Common failures Inspection, testing and quality control Repair Examples of use Future trends References

386 386 393 397 399 404 404 405 405 415 416

Shoe industry

417

Introduction Upper materials in shoes Sole materials in shoes Types of adhesive used in shoes Solvent-borne polyurethane adhesives Waterborne polyurethane adhesives Polychloroprene (neoprene) adhesives Waterborne polychloroprene adhesives Testing, quality control and durability Future trends Acknowledgements References

417 419 421 424 424 428 433 436 439 442 449 449

18 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 18.10 18.11 18.12

M H E N T I N E N , VTT Industrial Systems, Finland

J M M A R T IÂ N - M A R T IÂ N E Z , University of Alicante, Spain

371 375 377 380 381 381 381 382 383

386

Contents

19

Electrical

J - A P E T I T A N D V N A S S I E T , Ecole Nationale d'IngeÂnieurs

xi

455

de Tarbes, France

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10

20 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8 20.9 20.10 20.11 20.12

Introduction Basic needs Adhesive characteristics Surface preparation Strength and durability: reliability Common failures Inspection, testing and quality control Examples of use Conclusion References

455 456 458 466 468 473 476 478 484 485

Aerospace

489

Basic needs Adhesive characteristics required for design and analysis Surface preparation Design of adhesively bonded joints Design features ensuring durability of bonded joints Load redistribution around flaws and porosity Effects of thermal mismatch between adherends on strength of bonded joints Inspection, testing and quality control Bonded repairs Other industry-specific factors Examples of use of adhesive bonding in aircraft structures References

489 490 495 500 505 509

Index

528

L J H A R T - S M I T H , Boeing, USA

514 515 520 521 522 525

Contributor contact details

Chapter 1 Mr Paul A. Fay Ford Motor Company Room 40/490, Trafford House 8 Station Way Basildon Essex SS16 5XX UK Tel: +44(0)1268 702671 Fax: +44(0)1268 703747 E-mail: [email protected] Chapters 2 and 6 Professor John Comyn Institute of Polymer Technology and Materials Engineering Loughborough University Loughborough Leicestershire LE11 3TU UK Chapter 3 Professor John F. Watts The Surface Analysis Laboratory School of Engineering University of Surrey Guildford Surrey

GU2 7XH UK Tel: +44(0)1483 689617 Fax: +44(0)1483 686291 Email: [email protected] Chapter 4 Dr D. M. Brewis IPTME Loughborough University Loughborough Leicestershire LE11 3TU UK Email: [email protected] Chapter 5 Professor Andrew Crocombe School of Engineering (H5) University of Surrey Guildford Surrey GU2 7XH UK Tel: +44(0)1483 689194 Email: [email protected]

xiv

Contributor contact details

Chapter 7 Professor Peter Cawley Mechanical Engineering Department Imperial College Exhibition Road London SW7 2AZ UK Email: [email protected] Chapter 8 Dr Chiaki Sato Tokyo Institute of Technology Precision & Intelligence Laboratory 4259 Nagatsuta Midori-ku Yokohama 226-8503 Japan Tel: +81(0)45 924 5062 Email: [email protected] Chapter 9 Professor David A. Dillard Engineering Science and Mechanics Department Virginia Tech Blacksburg, VA 24061-0219 USA Email: [email protected] Chapter 10 Dr Ian A. Ashcroft Senior Lecturer Wolfson School of Mechanical and Manufacturing Engineering Loughborough University Loughborough Leicestershire LE11 3TU UK

Tel: +44(0)1509 227535 Fax: +44(0)1509 227648 Email: [email protected] Chapter 11 Mr Martin Hildebrand FY-Composites Oy Mannerheimintie 44A FIN-00260 Helsinki Finland Email: [email protected] Chapter 12 Dr Ewen J. C. Kellar Principal Project Leader Advanced Materials and Processes (AMP) TWI, Granta Park Great Abington Cambridge CB1 6AL UK Tel: +44(0)1223 891162 x2495 Fax: +44(0)1223 892588 E-mail: [email protected] Chapter 13 Dr Peter Davies IFREMER ± Centre de Brest Materials and Structures Group ± ERT-MS B.P. 70 F-29280 Plouzane France Email: [email protected]

Contributor contact details Chapter 14 Dr IJsbrand J. van Straalen TNO Environment and Geosciences Van Mourik Broekmanweg 6 P.O. Box 49 2600 AA Delft The Netherlands Tel: +31 15 276 34 69 Fax: +31 15 276 30 16 Email: [email protected] Professor Dr Michel J. L. van Tooren Systems Integration Aircraft Faculty of Aerospace Engineering Delft University of Technology The Netherlands Chapter 15 Dr Erik Serrano Div. of Structural Mechanics Lund University PO Box 118 SE-221 00 Lund Sweden Tel: +46 46 222 95 88 Fax: +46 46 222 44 20 Email: [email protected] BjoÈrn KaÈllander SP Swedish National Testing and Research Institute PO Box 857 SE-501 15 BoraÊs Sweden

xv

Chapter 16 Professor Dr Klaus Dilger Institut fuÈr FuÈge- und Schweiûtechnik Technische UniversitaÈt Braunschweig Langer Kamp 8 38106 Braunschweig Germany Email: [email protected] Chapter 17 Markku Hentinen VTT Industrial Systems Product Performance Tekniikantie 12 P.O. Box 1705 FIN-02044 VTT Finland Email: [email protected] Chapter 18 Jose Miguel MartõÂn-Martinez Adhesion and Adhesives Laboratory University of Alicante 03080 Alicante Spain Tel: 34-96-5903977 Fax: 34-96-5903454 Email: [email protected] Chapter 19 Professor Jacques-Alain Petit and Dr ValeÂrie Nassiet Laboratoire GeÂnie de Production Ecole Nationale d'IngeÂnieurs de Tarbes 47, avenue d'Azereix 65016 Tarbes Cedex ± France Email: [email protected]; [email protected]

xvi

Contributor contact details

Chapter 20 Dr L. J. Hart-Smith Boeing Phantom Works 5301 Bolsa Avenue H013-C326 Huntington Beach CA 92647-2099 USA Email: [email protected]

Part I

Fundamentals of adhesive bonding

1

History of adhesive bonding P A FAY

1.1

Early days

The hardest part of writing about the history of adhesive bonding is deciding where to start, since it is impossible to know for certain when or where adhesive materials were first used. Although there is significant written and archaeological evidence to suggest that humans have been using adhesive products for thousands of years, in their drive to make objects more decorous, stronger, more useful or simply cheaper, there does not appear to have been a single `Eureka' moment when their usefulness was first discovered. Instead, the introduction of adhesives is likely to have been a gradual process, beginning with the application of naturally `sticky' products before moving on to the preparation of simple adhesives, possibly by-products from cooking. Exactly when this transformation took place, we will probably never know. It is tempting to start with the suggestion made by the Roman author and scientist Pliny the Elder, that glue was invented by Daedalus (Pliny, AD50, VII: lvi). Apart from this simple claim however, few further details of his invention are provided. According to the various accounts of the legend, Daedalus is acknowledged as the inventor of a wide variety of other things including sails for ships and numerous techniques used in sculpture, art and construction, as well as many of the tools used in carpentry (Apollodorus, c.140BC; Diodorus, c.60BC; Ovid, AD1; Pausanias, c.AD160; Pliny, AD50; Virgil, c.19BC). To add glue to this list of achievements therefore seems reasonable and appropriate. Probably the best-known part of the legend is that of Daedalus and his son, Icarus, escaping from Cretan imprisonment using wings fashioned from feathers (somewhere around 1300±1000BC). In most of the accounts, the feathers are joined using wax, although Apollodorus actually uses the term `glue'. This dramatic and highly ambitious use of adhesive bonding unfortunately ended in failure. Icarus, showing the over-enthusiasm of his youth, flew higher and higher, and as he got closer to the sun, the wax melted and he fell to his death. It is reported that Daedalus successfully completed his own journey to Sicily. Icarus would have done the same if he had heeded his father's pre-flight advice,

4

Adhesive bonding Take care to wing your course along the middle air; If low, the surges wet your flagging plumes; If high, the sun the melting wax consumes. (Ovid, AD1)

This warning resonates with the challenges still faced by users of synthetic structural adhesives today, trying to avoid extreme environmental conditions of high humidity and elevated temperatures. Unfortunately though, the legend of Daedalus is simply that, a legend, and Daedalus was almost certainly a mythical character. His story also does not stand up to scientific scrutiny. The ambient temperature, which was supposed to have risen as Icarus climbed higher, would actually have fallen, reducing the likelihood of the wax melting (although increasing the risk of brittle fracture). Even if this were not true, subsequent experiments in human powered flight have not managed to repeat Daedalus' successful journey. The best attempt to date used a lightweight aeroplane (appropriately called `Daedalus') with a wingspan of 31m and a weight of 31kg which flew, using only human power, the 115km from Crete to Santorini (Dorsey, 1990). The difficulty in replicating Daedalus' original flight makes the suggestions of both Pausanias and Diodorus, that the escape was actually made by boat (using the newly invented sails), much more credible. Even without these doubts, there is now considerable archaeological evidence that adhesive materials were actually in use much earlier than Daedalus' time. As far as we can tell, the history of manufactured adhesives appears to have started long before the existence of modern humans. Neanderthal tools dating from at least 80,000 years ago were found in Koenigsaue in the Harz Mountains in Germany in 1963. Residues of an adhesive substance were detected on them which later analysis has shown was derived from processed birch pitch (Koller et al., 2001). Similarly, tools dating from around 40,000BC found at Umm el Tlel in Syria used bitumen, which had been subjected to extreme temperature, as a hafting material joining the tools to their handles (BoeÈda et al., 1996). The production of such materials would have required careful processing at exactly the right temperatures, possibly in the absence of oxygen, and clearly demonstrates the technical abilities of the Neanderthals to manufacture relatively sophisticated adhesives. The oldest discovery of the use of adhesives by modern humans to date was made in the Nahal Hemar Cave to the northwest of Mount Sedom in Israel. When this cave was excavated in 1983, many of the artefacts unearthed were found to carry residues of a collagen-based material believed to be derived from animal skins. This adhesive has been carbon dated to over 8,000 years ago (Walker, 1998; Bar-Yosef and Schick, 1989). Studies of burial sites dating from before 4000BC revealed vessels made from broken pottery which had been repaired with sticky resins from trees. Statues found in Babylonian temples from around the same time have also been discovered, with ivory eyeballs glued into their sockets with a tar-like glue which was still holding after 6,000 years (Stumbo, 1965).

History of adhesive bonding

5

È tzi', the frozen mummy of a man A number of weapons were found with `O from the Late Neolithic period (c.3300BC) discovered in a Tyrolean glacier in 1991. They consisted of flint arrowheads and a copper hatchet bonded to wooden shafts with an adhesive-like material (Spindler, 1995). Detailed analytical techniques have identified it as a `pitch' prepared by pyrolysis of bark from birch trees (Sauter et al., 2000). The use of bitumen as an adhesive occurred `almost everywhere in Antiquity' according to Forbes (1964), who described examples dating from before 2800BC. The best surviving examples are those found in the ruins of Babylon, dating from around 1500BC, which demonstrate the use of filler materials in bitumen used for bonding red clay bricks (Alsalim, 1981). By the time of the early Egyptians, adhesives from a wide range of different sources appear to have been in regular use. They have been analysed and documented in detail by Lucas and Harris (1962) and more recently by Newman and Serpico (2000). Lucas lists the `principal adhesives employed, or possibly employed' in ancient Egypt (in alphabetical order) as: albumin, beeswax, clay, glue, gum, gypsum, natron, resin, salt, solder and starch. In an earlier work (Lucas 1927), he remarked that: Glue was well known in ancient Egypt, and a specimen recently examined may be described. This was found some years ago by Dr Howard Carter in a rock chamber over the mortuary temple of Queen Hat-shep-suÃt at Deir el Bahari . . . it had been cast and was originally rectangular in shape, but now it is shrunken and distorted owing to desiccation . . . in appearance it cannot be distinguished from modern glue, and it still responds to the usual tests.

Clear evidence of the use of glue by the Egyptians can be found in a wall carving from around 2000BC, found in the tomb of Rekhmara (Bogue, 1922, frontispiece). It shows the gluing of a thin veneer of wood to a thicker plank. The glue pot, being heated over a fire, the application of the glue with a brush and the use of weights to hold the veneer in place as the glue set can be clearly seen. The Egyptians used adhesive materials for a wide range of different applications. These included: fastening wood together; inlaying and veneering of wood; preparation of plaster and similar materials; as a binder in paints and pigments; fastening gold leaf to plaster and sealing and repairing alabaster jars (Lucas and Harris, 1962). One interesting application of animal glue was a prosthetic toe produced at some time before 600BC found on a mummy in Thebes. It was manufactured from `cartonnage' (linen impregnated with animal glue), which gave it a smooth, tan-coloured coating similar to modern-day prosthetics (Falder et al., 2003). Around the same time (c.530BC), in what is probably the earliest allusion to structural bonding of metal, Theodurus of Samos is credited with developing a new technique described as `gluing metal to metal' (Feldhaus, 1931). A little later, there are several references to adhesives and sealants in the Bible. When God was instructing how to build the ark, he told Noah to, `cover it

6

Adhesive bonding

inside and out with pitch' (Genesis 6:14). We are also told that the builders of the Tower of Babel used `bitumen for mortar' (Genesis 11:3) and that the bulrush basket which carried the baby Moses down the river was sealed with bitumen and pitch (Exodus 2:3). Despite these successful examples, the Bible also contains a number of cautions about adhesive bonding. In Ecclesiasticus (written around 200BC), it is suggested that: `He who teaches a fool is like one who glues potsherds1 together' (Ecclesiasticus 22:7) and in the Book of Jeremiah, it is said that the people and the city will be broken `as one breaks a potter's vessel, so that it can never be mended' (Jeremiah 19:11). Despite these warnings however, the use of adhesives continued to flourish. Lucretius, writing in around 50BC, considered the affinity existing between different materials and stated that, `Wood is joined together with bull's glue, so that the grain of boards often gapes open with a crack before the joints of the bull's glue loosen their hold' (Lucretius, c.50BC). Of all the classical authors, Pliny, writing in around AD50, probably had the most to say on the subject of adhesives. Apart from the claim that Daedalus invented glue (mentioned above), he also reports on the use of a number of different types of adhesive material. These include a `mineral pitch' which stuck to anything solid that touched it. He described one application of this material in defending the city of Samosata since, `When people touch it, it actually follows them as they try to get away from it' (Pliny, AD50, II: cviii). Other naturally adhesive materials described by Pliny came from trees. Examples include different varieties of `mastich' (XII: xxxvi); gum from the Egyptian thorn (XIII: xx); the resin of the pitch pine, used for coating wine casks (XIV: xxv) and `bird-lime' derived from mistletoe berries used for ensnaring birds (XVI: xciv). Pliny was obviously also familiar with animal glue. He claimed that, `The finest glue is made from the ears and genitals of bulls' but also identified glue being made from `any old skins and even from shoes'. His opinion was that the most reliable glue came from Rhodes, especially when it was white rather than `dark and wood-like' (XXVIII: lxxi). Pliny also identified two significant uses of bonding. The first of these was the manufacture of papyrus (making use of the `effect of glue' provided by muddy Nile water) (XIII: xxiii) and paper (using flour and water paste) (XIII: xxvi). The second application was the bonding and veneering of wood. According to Pliny, different types of wood were more suited to being glued. Fir wood was described as `the most adapted for being glued together, so much so that it will split at a solid place before it parts at a join' (XVI: lxxxii). In contrast, it was reported that hard oak could not be `joined by glueing'. Somewhat surprisingly, Pliny suggested that materials `unlike in substance', such as stone and wood, `do not hold together' (XVI: lxxxiii).

1.

Pieces of broken pottery.

History of adhesive bonding

7

Around the same time, the Greek physician and botanist, Dioscorides, described the preparation of glue from bull hides and whale intestines. His main interest however appears to have been the use of these materials in treating skin diseases, rather than for bonding (Dioscorides, c.AD50). Another physician of this period, Celsus (a naturalist and encyclopaedist practising medicine in firstcentury Rome), described the use of glue for cleaning and healing wounds as well as for creating splints for repairing broken noses (Celsus c.AD30). Very few written records exist regarding the use of adhesives in the period immediately following the decline of Greece and Rome and it is likely that, like so many other technologies, they fell out of common use for several hundred years. Stumbo (1965) for example, reports that `. . . the study of furniture made between the fall of the Roman Empire and the sixteenth century shows that . . . the art of gluing fell into disuse'. This appears to be not strictly true and, to trace the continuing use of adhesives, it is necessary to delve into the world of the medieval artists. A huge body of finished works and associated texts demonstrate that the use of adhesives featured heavily in art, particularly religious art, during this period. As an example, the Mappae Clavicula, believed to have been written early in the 9th century AD, contains a number of simple adhesive recipes including `glue for stone' made from a mixture of fish glue with either ox or cheese glue; glues made from tree saps and glues for gilding made from parchment. Various recipes for solder are also provided (Smith and Hawthorne, 1974). Later in the period, one of the most interesting written records describing the use of adhesives is De Diversis Artibus by Theophilus, written around 1140. This work deals with the techniques required by clerics for decorating churches, making religious vessels, illuminating manuscripts and so on (de Camp, 1977). These included the manufacture and use of adhesives made from animal hides and horn, fish bladders and casein (Dodwell, 1961). In possibly the earliest written detailed glue recipe, he describes the manufacture of casein glue as follows: Soft cheese is cut up into small pieces and washed in warm water with a pestle and mortar until the water, which you have poured on several times, comes out unclouded. Then this cheese is thinned out by hand and placed in cold water until it becomes hard. After this it is broken up finely on a smooth wooden board with a piece of wood. It is then replaced in the mortar and carefully pounded with the pestle, and water mixed with quicklime is added until it becomes as thick as lees. With this glue panels are fastened together. When they have dried, they stick so firmly that they cannot be separated by damp or heat.

Theophilus also gave some thought to quality control. He suggested that, when using animal glue: You test it in this way: moisten your fingers in the water (in which the hides and horns have been boiled), and if, when they are cool, they stick together, the glue is good; but if not, heat it until they do stick together.

8

Adhesive bonding

Towards the end of the medieval period, Cennini produced his comprehensive Libro dell'arte in 1437, now regarded as the best source on the methods of late medieval artists. Cennini provides recipes for paper glues (made from flour), cements for mending stones, dishes and glass, fish glue, goat glue and glue made from cheese (Thompson, 1933). Vasari, in his Lives of the Artists, gives a charming example of the familiarity of medieval artists with making casein adhesives. He tells the tale of Paulo Uccello in the mid-15th century, who was working on the cloisters of San Miniato near Florence but, to the dismay of the Abbot, had not completed the job. The Abbot sent various friars to look for him and, when they finally tracked him down, Uccello offered the following explanation for not finishing the commission: You've brought me to such a sorry state that I not only run away from the sight of you, I can't even go where there are carpenters working. This is all the fault of your dim-witted abbot. What with his cheese pies and his cheese soups, he's stuffed me so full of cheese that I'm frightened they'll use me to make glue. (Bull, 1965)

The use of adhesives by the medieval artists has been analysed and documented by a number of modern authors, most notably Laurie (1926) and Thompson (1956). Both confirm the widespread and successful use of glue in medieval art. Thompson, in particular, states that: Among the many troubles which beset medieval paintings in our time, one of the rarest is for the glued joints of the wood to separate; and the strength is largely due to the use of the strange, homely adhesive.

By the 14th century, references to adhesives also occur in works of literature. For example, in The Squire's Tale by Chaucer, he describes the brass horse on which a royal messenger arrived in the following terms: `The horse of brass that may not be remewed, It stant as it were to the ground yglewed' (Chaucer, 1386). In 1393, an elderly Parisian merchant wrote a text known as the Goodman of Paris for his new, much younger wife. The book contains a wealth of advice on religious and moral duties, a wife's duties to her husband, household management, gardening and pastimes. The young wife is advised that one of the ways to `bewitch' her husband is to make sure his bed is free of fleas during the summer. A suggested way of achieving this is to set `one or two trenchers [of bread] slimed with glue around the room' at night and, attracted by a nearby candle, the fleas `will come and be stuck thereto'. The husband thoughtfully also includes a recipe for making glue from the bark of the Holly tree (Power, 1992). During the 16th century, Shakespeare made a number of references to glue, of which the following are but two examples. In Titus Andronicus (Shakespeare, 1588), Demitrius tells Chiron to: `Go to; have your lath glued within your sheath, till you know better how to handle it' whilst in King John, Shakespeare has King Phillip of France saying:

History of adhesive bonding

9

. . . ten thousand wiry friends Do glue themselves in sociable grief, Like true, inseparable, faithful loves, Sticking together in calamity. (Shakespeare, 1595)

By the 17th century, scientists were beginning to give consideration to the nature of adhesion itself. Francis Bacon in his Novum Organum suggested that `there is in all bodies a tendency to avoid breaking up'. He further reports that this tendency is weak in homogeneous substances but more powerful in bodies compounded of heterogeneous substances, reasoning that the `addition of heterogeneity unites bodies'. In his arguments, he introduces the concepts of `bonding' (by which bodies refuse to be torn from contact with other bodies) and `cohesion' (by which bodies, to differing degrees, abhor their own dissolution) (Bacon, 1620). Galileo (1638) discussed the manner in which materials without an obvious fibrous structure produced such high breaking loads. He suggested that the coherence of these bodies is produced by other causes ± either nature's repugnance of a vacuum or, `this horror of a vacuum not being sufficient, it is necessary to introduce . . . a gluey or viscous substance' to bind them. A little later Newton (1717) conjectured that, `There are agents in Nature able to make the particles of bodies stick together by very strong attractions. And it is the business of experimental philosophy to find them out'. A general renaissance in the use of adhesive bonding began around this time and is clearly demonstrated by the changing construction methods used for furniture. The use of adhesives for inlaying work re-started in the 16th century and veneering in the 17th (Stumbo, 1965). It was not, however, until the 18th century that adhesives had an impact on the production and design of furniture and by the 19th century furniture makers `were starting to rely solely on the strength of the glue bonds to ensure joint security' (Tout, 2000). This changing nature of furniture construction led to a prolonged dispute among the Trade Guilds of London. The Company of Carpenters and the Company of Joiners, which could both trace their origins back to the 14th century, each organised differently trained craftsmen in the art of furnituremaking and there was a long-standing rivalry between them. It was finally resolved in 1632 by a judgment from the Court of Aldermen. They decreed that from that time onwards, the joiners alone should be entitled to make particular items. The biggest distinction was the use of glue ± for instance, the joiners were entitled to make `All sorts of cabinets or boxes dufftailed pynned or glewed'. An interesting dividing line occurred in the construction of bedsteads which were in the province of the joiners, unless they were simply `boarded and nailed together', in which case they could be made by carpenters. Elsewhere in London, the central criminal court, known as `The Old Bailey', was also making judgments involving glue. In the 18th century several defendants were convicted of stealing relatively small amounts of glue, `leaden glue pots' and associated tools and were sentenced to transportation (presumably

10

Adhesive bonding

to America) as a result. In June 1796, a Mr William Bell was charged with counterfeiting coins. Part of the case against him (other than being in possession of suitable press, dies and copper blanks) was that his hands were very dirty. Under cross-examination he attributed this to the fact that he was about to repair some furniture and had been heating his glue pot. Unfortunately for Bell, a police witness reported that there was no glue pot (or indeed any broken furniture) present. He was therefore found guilty and fined and imprisoned for `Offences against the King'. (Old Bailey, 1714±1799).

1.2

The industrialisation of glue making

During the early days in the history of adhesives, it is likely that the materials were produced on a very small scale, possibly in the kitchens of the individual users. However, by around 1700, the production of adhesives started to undergo transformation into a major industry. It has been suggested (Bogue, 1922, p. 3) that the earliest record of the practical manufacture of glue dates from Holland in around 1690. Shortly afterwards, the industry was introduced to England and established as `one of her permanent industries' around 1700. The first mention of glue in patent literature comes from a British patent for `a kind of glue called fish glue' in 1754 (British Patent, 1754). Over the next hundred years, this was followed by other patents pertaining to preparation of various types of animal glues. An alternative view put forward by Alexander (1923, p. 15), is that the `real glue and gelatin industry emerged about the beginning of the nineteenth century'. He suggests that the industry started in the area of Lyons, France where the glue factories were considered the most important of their kind in Europe for many years. The pre-eminence of the French glue industry may have been a result of the great claims made of the nutritional value of gelatin (a purer form of glue, often produced in the same factories) during the Napoleonic era. In Germany, the glue industry was also fostered and a German company which reportedly started in 1895 with three plants, had expanded to the extent that, by 1912, it controlled seventeen plants and also had factories in Austria, Russia, Belgium, Switzerland and France. Records exist of glue production in the USA since 1810 (Bogue, 1922, pp. 5±12). At this time, there were only seven establishments primarily engaged in the production of glue and gelatin, between them manufacturing products with a value of $54,000. By 1879, the number of establishments had risen to 82 but then fell to 57 by 1914, by now manufacturing over $13 million worth of products. Notably, during the period 1914 to 1919, the value of products exported from America rose by about 380 per cent whilst imports fell by about 510 per cent. Alexander reports that, prior to 1860, alongside the true glue manufacturers, a `great many tanners boiled up their own stock in open kettles' (Alexander, 1923, p. 15). The early American glue factories were situated around Boston, New York, Philadelphia and Cincinnati.

History of adhesive bonding

11

Most of the glue produced at this time was of animal or vegetable origin, mainly for bonding wood or paper products, and the manufacturing methods established by these early glue factories remained largely unchanged for over a hundred years. Teesdale (1922, p. 12) described the process of making animal glues as follows: The stock is washed and treated to remove dirt and grease, then boiled to convert the glue-forming substances into a glue solution, which is concentrated by evaporation until it will form a jelly on cooling. The jelly is then dried and the resulting product is the glue.

Although this sounds relatively clean and simple, by all accounts early glue works were not pleasant places to either work in or live near to. Lambert (1925, pp. 2±3), for instance, suggests that: The arrangement and situation of a bone factory is a matter of great importance. . . . In choosing a site for the erection of such a works, a position outside the boundaries of a town should be decided upon, in order that the offensive smell which arises from a works of this character may not give cause for complaint from a populous community.

An interesting insight is given by Fernbach (1907, p. 87) when discussing the relative merits of glues produced in the USA and the so-called `foreign glues': Labor conditions are such in England and on the continent that it is possible to subject both stock and glue-liquor to operations precluded by the high cost of labor in the United States. Where the European manufacturer can avail himself, at a trifling cost, of the services of the aged and infirm of the locality in which his factory is situate, for the purposes of hand-picking glue stock, the same labor would command many times the price in the United States and hence the increased cost of production would speedily place beyond the pale of competition the manufacturer who sought to take advantage of such process.

In these early days of industrialisation, quality control was almost nonexistent in glue factories and the final products available for use were of varying quality and performance. As an example, Teesdale (1922, p. 12) commented that `Bones are sometimes boiled without first removing either dirt or grease. This naturally fails to produce a high quality glue.' But things were beginning to improve. By 1917, it was noted, in an annual report on the progress of applied chemistry, that some of the larger glue works were employing `competent chemists' (Wood, 1917) and, in 1922, Bogue (1922, p. 368) noted that, . . . the manufacturing process has vastly improved in the last 30 years. There was a time when every conceivable part of the animal that could not be utilized for more valuable products was `dumped' into the glue kettle. But . . . that method is a thing of the past, and the industry is beginning to operate on a scientific basis.

12

Adhesive bonding

The introduction of scientific discipline had a number of effects. First, a great deal of the available knowledge of glue manufacture, testing and use was written down and published, often for the first time. Notable titles of this period include Dawidowsky (1884), Standage (1897), Rideal (1901), Fernbach (1907), Boulton (1920), Teesdale (1922), Bogue (1922), Alexander (1923), Lambert (1925), and Smith (1929). These titles collectively describe the advancement of the glue industry as it matured. Second, the importance of quality control was accepted and many controls on raw materials and manufacturing processes were implemented. Finally, comprehensive testing of the adhesive products became standard practice. There is little doubt that, at the beginning of the 20th century, testing of the manufactured glue products was of extreme importance since not only did this establish the performance and quality of the glues, but the selling price of a glue also depended directly on the results. Fernbach (1907, p. 20) stated: `Glue is sold on ``test''; that is, the price of the product is governed by its strength and certain other properties, for the measurement of which many divergent tests have been formulated.' Before the introduction of methodical testing, adhesives were often assessed using the experience of the glue maker and the application of simple human senses. A keen sense of smell appears to been a pre-requisite: Glue of good quality is practically free from smell, is unaffected by the atmosphere, and has great adhesive power. (Lambert, 1925, p. 38) Keeping qualities are of some importance. . . . Glues made from partly decomposed stock do not keep well, and have a bad odor when the glue solution is heated. Hence odor has a bearing on keeping qualities. . . . Something may be learned by smelling of a moistened flake of glue warmed in the hand. (Teesdale, 1922, p. 25)

The hands and eyes of the experienced glue-makers were also put to good use: Breaking a sample of the glue with the thumb and forefinger of each hand gives an indication of glue quality. The condition of the air must be considered, as a dry day will give different indication than a humid one. If the glue fractures evenly and bends but little, low strength and brittleness are indicated. If a thin sheet bends well and in case it breaks, shows a splintery fracture, good strength is indicated. (Teesdale, 1922, p. 25) One of the most significant and important of the tests commonly made on glue is an estimation of jelly strength. . . . In spite of numerous attempts to develop apparatus for obtaining a measure of jelly strength in terms of some tangible numeric unit, the finger test is still favored by those most expert. The finger test is akin to tea or wine tasting in that it requires long experience and great skill to obtain good results. (Teesdale, 1922, p. 37) The colour of glue varies according to the nature and quality of the raw materials from which it is prepared. . . . All liquid glues should be clear and sparkling. (Lambert, 1925, pp. 38 and 103)

History of adhesive bonding

13

In time, these methods were found to be insufficient and the need for better methods of testing and assessing adhesives was recognised, for example by Alexander (1906), In glue, above all things, appearances are deceptive. Even after a manufacturer has finished his glue he is obliged to test it in order to establish the grade of his finished product.

But there was considerable debate about the best test methods to use. Rideal, writing in 1901 (p. 107), commented that: Much controversy, especially in Germany, has centred on the large number of methods that have been proposed for glue testing, and there is little doubt that difficulties arise in the interpretation of results, and in obtaining absolute measures, or figures that shall be comparable between different observers. . . . Systematic tests, however, are of use to manufacturers in controlling their processes, and to users in avoiding loss and mistakes in purchasing, owing to the fancy prices and misleading names and descriptions frequently put forward.

This latter point was supported by Teesdale (1922, p. 23) who proposed that, `It is exceedingly desirable for glue users to test their glues rather than trust wholly to the promises of the salesmen.' Although the value of testing glue was well understood, the difficulty and complexity of the task were also well appreciated, At the outset let it be emphasized that there is no single chemical or physical test which will satisfactorily gauge the value of a glue or gelatin for all purposes. Many authors have recommended individual tests and while these may have some value for special purposes, the wisest and safest way for the factory or sales manager is to run a series of connected tests which will grade the glue or gelatin against preceding lots of the same type, and thus render possible uniform deliveries to the consumer, whatever his business may be. (Alexander, 1923, p. 173) The observations seem to show that while it would be rash to form a judgment of glue from a single test, the evidence afforded by a number may be irresistible. . . . The expert's wisest system appears to be, not to rely upon single short-cut tests of general quality but to employ a number of methods, including any having especial bearing on the present or prospective uses of the glue, and then to base his conclusions on a consideration of all the results together. (Clayton, 1902)

There was also a degree of scepticism about the value of testing, The glue chemist should have a rather clear understanding of the fundamentals of testing glue joints, or he may draw erroneous conclusions from his results. He is possibly more liable to be measuring his own ability to do `stunts' with the glue rather than measure the strength of the glue. (Teesdale, 1922, p. 41)

14

Adhesive bonding

One significant development in the acceptance of glue classification based on test results was the adoption of standard products to act as benchmarks. Fernbach, noting the value of establishing standards (Fernbach, 1907, p. 21), suggested that, The constants or measurements of quality in glue-testing are arbitrary and of value only when compared with the corresponding constants of a standard glue. The consumer is frequently at a loss to select the proper standards of comparison. He is usually content, having secured a glue that fully answers his requirements, to compare each succeeding delivery with this.

In a similar vein, Teesdale (1922, p. 63) remarked that, The readers inexperienced in glue testing may have gained the impression that the methods of test are unreliable and of little value. This is by no means the case. It is true that testing methods are arbitrary, and that they cannot in general be expressed in terms of numeric values with sufficient exactness to write a specification. It was for this reason that . . . all thought of attempting to prepare a specification without the use of a standard sample was abandoned.

For animal glues, the accepted standards for many years were those produced by the Peter Cooper glue works. Peter Cooper was a remarkable man who made significant contributions in fields as diverse as textile machines, railway locomotive design, iron and steel production, structural design of buildings, transatlantic telegraphy, education and public water supply as well as in local and national politics. Detailed descriptions of his life and work have been given by Raymond (1901), Hubbard (1909), Nevins (1935) and Mack (1949). His name is probably best remembered today by the `Cooper Union for the Advancement of Science and Art', one of America's oldest institutions of higher education which he established in 1859. Most of the fortune which allowed him to make this and many other philanthropic gestures, was made from the success of his glue works and his contribution to the development of the adhesives industry was significant. As Hubbard commented (1909, p. 13), `The glue factory was the foundation of his fortune. He made better glue and more glue than any concern in America.' He purchased the glue works in 1822 and the company bearing his name (by then the `Peter Cooper Corporation') was still producing animal glues until around 1990. Part of the reason for the success of the business was the consistency and quality of his products. Nevins (1935, pp. 59±62) describes Cooper's activities in the field of improving glue manufacturing and quotes him as saying, `I determined to make the best glue that could be produced, and found out every method and ingredient to that end.' His reputation for producing products of consistent quality quickly led to them being adopted as standards. Teesdale (1922, p. 14) noted that, A system of classification, based chiefly on the jelly strength, was devised a long time ago by Peter Cooper, by which it is possible to group the great

History of adhesive bonding

15

variety of glues into a relatively few classes, or grades. The grades established by Cooper, beginning with the strongest, were designated, respectively, A Extra, 1 Extra, 1, 1X, 1 1/4, 1 3/8, 1 1/2, 1 5/8, 1 3/4, 1 7/8, 2.

There appears to have been much support for the use of the `Cooper Grades' as standards. For example, Fernbach (1907, p. 22) commented that, These grades were for many years considered the best made, and competing manufacturers sought to produce glues corresponding with them in all respects. Hence they remain the authentic standards of comparison.

Despite their slightly odd numbering system (believed to be based on a particular instrument used by Cooper for measuring their jelly strength), the universal acceptance of them as standards allowed the glue industry to mature, developing improved manufacturing processes and controls. The development of test methods for adhesives during this period laid down the foundations for many of the test methods still in use today, including test methods for assessing uncured adhesives (such as viscosity assessments) as well as the strength and durability of adhesive joints. This groundwork prepared the industry for its most significant change ± the arrival of synthetic materials.

1.3

The advent of synthetic polymers

Although enormous efforts had been made in the manufacture of animal glue in the period up to 1925, much bigger developments were taking place which were to have the most significant effect on the history of adhesives in thousands of years ± the development of synthetic polymers. Although the first man-made polymer did not appear until the introduction of celluloid by Alexander Parkes in 1862 (Kaufman, 1963), it is interesting to note that the development of synthetic polymers was actually prophesied in 1665 by Hooke: `I have often thought that probably there might be a way found out, to make an artificial glutinous composition . . .'. Until the 1920s, most, if not all the adhesives used for structural applications were still of natural origin. Judge (1921) lists the adhesives available at this time for aircraft and automobile manufacture as: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Animal glues (hide, bone or hoof). Fish glue. Liquid glues (animal glues in liquid, ready to use state). Marine glue (made from indiarubber, naphtha and shellac). Casein glues. Waterproof glues (modified `ordinary glue'). Vegetable glues. Flexible glue (modified animal glue). Albumen glues.

16

Adhesive bonding

Over the next twenty or so years, these were rapidly replaced by modern adhesives based on synthetic polymers. By 1943, it was noted that, There are about a dozen different kinds of synthetic adhesives already finding applications in modern industry. The most important are, of course, phenol-formaldehyde and urea-formaldehyde adhesives, which are now being generously used in the manufacture of plywood for aeronautical, shipbuilding and building purposes and for many types of stress assembly woodwork where an exceedingly strong bonding material possessing maximum resistance to moisture, insects and fungi is required. The urea and phenolic adhesives are very definite improvements on even the finest animal and vegetable glues and can be recommended for all types of woodwork where the strength requirement of the joint is high or where the wood has to possess maximum resistance to the weather. (`Plastes', 1943)

Phenol-formaldehydes are generally regarded as the first true, fully synthetic polymers. They were `discovered' several times before the significance of the discovery was fully appreciated and the time was appropriate for their further development and exploitation. The first record of the successful interaction of phenol and formaldehyde to produce resins was made by Baeyer in 1872. This was followed by similar experiments by a number of other chemists, before Leo Baekeland produced a synthetic resin with marketable possibilities in 1907, sold under the trade name `Bakelite' (Morrell, 1943, p. 104). The first suggestion that phenol-formaldehyde resins could be used as adhesives appears to have been made by Baekeland around 1912 (Wood, 1963, p. 75). By 1918, trials of a thin sheet of paper impregnated with phenolic resin for use in the manufacture of plywood were under way although it was not until around 1930 that such products were commercially available. The high cost of the material limited its early use to highly demanding applications for waterproof plywood such as aircraft and boat building. In later years, phenolic adhesives were developed in different forms (such as water emulsions and dry powders) which gave them more universal appeal. The phenol-formaldehydes were the first in a long series of synthetic polymers used as adhesives. The major landmark introductions were summarised by Hartshorn as shown in Table 1.1. Urea-formaldehyde resins followed quickly after the phenol-formaldehydes. The earliest materials were produced by Hans John in 1918, who suggested their use as adhesives. (Kaufman, 1963, pp. 67±69). The development of these resins into industrial products was continued by many others, most notably by Frits Pollak throughout the 1920s and 30s (Morrell, 1943, pp. 176±178). A modern review of the development of bonded aircraft structures reports that urea-formaldehyde adhesives were being used in aircraft construction from around 1937 (Bishopp, 1997). A detailed review of the development of the various types of adhesives based on phenolic resins is provided by Robins (1986).

History of adhesive bonding

17

Table 1.1 Historical development of structural adhesives (Hartshorn, 1986) Approximate date of commercial availability 1910 1930 1940 1950 1960 1970

Adhesive Phenol-formaldehyde Urea-formaldehyde Nitrile-phenolic, vinyl-phenolic, acrylic, polyurethane Epoxies, cyanoacrylates, anaerobics Polyimide, polybenzimidazole, polyquinoxaline Second-generation acrylic

Other important developments around this time were the developments of polyvinyl acetate, polyvinyl chloride and acrylic adhesives. Vinyl acetate and vinyl chloride monomers were first synthesised in 1912 and polymerised soon after. Polyvinyl acetate was unusual among the early plastics because its physical properties made it unsuitable for use in shaped articles and its use was primarily in adhesives, paints and surface coatings. Acrylates were first prepared in around 1873 and polmerised around seven years later (Kaufman, 1963, p. 86). Acrylic polymers later formed the basis for a complex family of adhesives including cyanoacrylates, anaerobics, u.v. hardening adhesives and two-part toughened acrylic adhesives (Kinloch, 1987, pp. 182±184). The development of acrylic adhesives has been described by Martin (1977) and Boeder (1986) whilst a more colourful history of anaerobic adhesives and the Loctite Corporation is given by Grant (1983). Further details of the history of cyanoacrylate adhesives are provided by Millet (1986). Polyurethane polymers were developed by Otto Bayer in Germany in around 1937. His patented isocyanate polyaddition process led to a versatile range of new materials which found applications in coatings, paint, foams, elastomers, mouldings and many other forms. Their potential as adhesives was discovered in 1940 and, since then, a wide range of applications for polyurethane adhesives have emerged including bonding of glass, composites, rubber, wood and leather (Lay and Cranley, 2003; Edwards, 1986). Probably the single most important landmark in the history of structural adhesives is the emergence of epoxy (or `epoxoid') resins in the late 1930s. The first synthesised resins were produced by Pierre Castan in Switzerland in 1936, whilst resins using epichlorhydrin and bispenol A were first produced by Greenlee in the United States in 1939. A review of these developments (and the earlier work on which they were built) is provided by Lee and Neville (1982). Castan was, at the time he made his discovery, working for a dental products manufacturer who made attempts to market his products as casting resins for dental use. These attempts were unsuccessful and the patents were licensed to Ciba AG of Basel. Ciba continued development of the materials and, at the Swiss Industries Fair in 1946, launched an epoxy resin adhesive and four

18

Adhesive bonding

electrical casting resins ± the start of commercial exploitation of epoxy technology (Potter, 1976, pp. 8±10). Epoxy adhesives gained rapid success in aerospace, automotive, construction, electronic and woodworking applications, largely because of their ease of use, versatility and mechanical properties. Typically, they possessed high shear strengths but relatively low toughness and peel strength. Attempts were therefore made to improve these properties. Various different approaches were tried, using additives and developing epoxy hybrids (such as polyamide fortified epoxies) but the most important breakthrough came in the early 1970s with the introduction of butadiene based rubber modifiers from Goodrich. These transformed the performance of both epoxy and acrylic adhesives, adding peel, impact and fatigue resistance without compromising the existing performance characteristics (Lees, 1981). This would be an appropriate point at which to end an historical review of adhesives ± the major technological advancements which form the basis of today's structural adhesives have been introduced and the structural adhesives industry can be considered mature and sophisticated. Alongside the developments in the synthetic polymers, there have also been parallel developments in the analytical tools, surface analysis methods, stress analysis, fracture mechanics and inspection techniques necessary to exploit their potential. A number of key industries dependent on adhesive technology for their success ± most notably the aerospace, automotive and electronics industries ± have emerged and have grown and matured in step with the improvements in adhesive science. However, no history of adhesives would be considered complete without some reference to a few key adhesive inventions which are widely used outside of industry and whose brands have become household names. Paint-masking tape was first developed by Dick Drew, a 3M chemist in the 1920s. According to 3M company history, Drew was visiting a car body repair shop to test a new batch of sandpaper. He heard the workers complaining about the limitations of existing methods for masking areas which did not require painting, a particular problem posed by the popularity of two-tone styles at that time. The available methods, using heavy adhesive tape and paper, often resulted in damage to the newly applied paint. Rather than see this as an opportunity to sell more sandpaper, Drew decided to develop a better solution. By selecting a suitable paper carrier and a less `aggressive' acrylic adhesive, he was able to come up with a suitable product which is still in use today, largely unchanged (Petroski, 1994, pp. 80±82). Self-adhesive Cellophane tape was an obvious development from the success of masking tape. In the late 1920s, Cellophane was becoming a widely used packaging material and Drew investigated the use of it, coated with adhesive, as a waterproof sealing tape. This presented a number of technical and cosmetic difficulties which were overcome by the use of a primer on the Cellophane, specialised manufacturing equipment and the development of virtually

History of adhesive bonding

19

colourless adhesives. The product (launched in 1930) was another success for 3M which, with later improvements to use, appearance, performance and durability is still very widely used (Petroski, 1994, pp. 82±83). The cyanoacrylate polymer used in `Superglue' was first discovered in 1942 by scientists searching for suitable products to make clear plastic gunsights. They quickly rejected their discovery on the grounds that it stuck to everything (including human skin), creating a lot of problems. It was re-discovered in 1951 by researchers at Eastman Kodak, Harry Coover and Fred Joyner, who were trying to optically join two prisms in a refractometer. Although they wrecked the instrument, they did recognise the potential of cyanoacrylates as adhesives. In 1958 the first product, Eastman 910, was brought to market. They are now widely used and favoured in many applications because of their rapid curing, high strength and simple processing (Coover, 1980). Another partly serendipitous invention from 3M was the development of Post-it NotesÕ. In the late 1960s, Dr Spence Silver had been investigating advanced acrylic adhesives with high strength and tackiness. Along the way, one of the experimental products demonstrated exactly the opposite properties, peeling away from paper surfaces with little effort, leaving no residue. At first, no use was found for this novel material but around 1974, Art Fry, a chemical engineer at 3M found the ideal application. Fry sang in a church choir and had been using scraps of paper to mark the pages in his hymnal which would be used during two services. Often the page markers would fall out between the two services. Fry hit upon the idea of sticky bits of paper which could easily be removed and remembered Silver's unusual adhesive (Petroski, 1994, pp. 84±87). The resulting product, Post-it NotesÕ, were launched in 1980 and are now found all over the world. They have proved extremely valuable during the writing of this chapter!

1.4

References

Alexander J (1906), `The grading and use of glues and gelatine', Jour. Soc. Chem. Ind., Feb 28, 1906, No. 25, pp. 158±161. Alexander J (1923), Glue and Gelatin, American Chemical Society Monograph Series, The Chemical Catalog Company, Inc, New York. Alsalim H S (1981), `Construction Adhesives used in the Buildings of Babylon', Adhesion 5, Ed K W Allen, Applied Science Publishers, London, pp. 151±156. Apollodorus (c.140BC), The Library and Epitome as translated by Sir James George Frazer, Heinemann, London, 1921. Bacon F (1620), Novum Organum, available in translation by L Jardine and M Silverthorne, Cambridge University Press, 2000, pp. 140±141 and 191±194. Bar-Yosef O and Schick T (1989), `Early Neolithic organic remains from Nahal Hemar Cave', National Geographic Research, Vol. 5, No. 2, pp. 176±190. Bishopp J (1997), `The history of Redux and the Redux bonding process', Int J Adhesion and Adhesives, Vol. 17, No. 4, pp. 287±301.

20

Adhesive bonding

BoeÈda E, Connan J, Dessort D, Muhesen S, Mercier N, Valladas H and TisneÂrat N (1996), `Bitumen as a hafting material on Middle Palaeolithic artefacts', Nature, 380, pp. 336±338, 28 March 1996. Boeder C W (1986), `Anaerobic and Structural Acrylic Adhesives', in Hartshorn (1986), pp. 217±247. Bogue R H (1922), The Chemistry and Technology of Gelatin and Glue, McGraw-Hill Book Company, Inc., New York. Boulton B C (1920), The manufacture and use of plywood and glue, Sir Isaac Pitman & Sons Ltd, London. British Patent (1754), Number 691, to Peter Zomer, May 23, 1754. Bull G (1965), Giorgio Vasari ± The lives of the artists. A selection translated by George Bull, Penguin Books, p. 98. de Camp L S (1977), Ancient Engineers, Tandem, London. Celsus A C (c.AD30), De re medicina, available in translation by W G Spencer, published by the Loeb Classical Library, 1935. Clayton E G (1902), `The examination of glue', Jour. Soc. Chem. Ind., May 31, 1902, No. 21, pp. 670±675. Chaucer G (1386), `The Squire's Tale' in The Canterbury Tales. Coover H W (1980), `Cyanoacrylate adhesives ± A day of serendipity, a decade of hard work', Research Technology Management, Vol. 13, No. 6, November 1980, pp. 37±40. Dawidowsky F (1884), A practical treatise on the raw materials and fabrication of glue, gelatin, gelatine veneers and foils, isinglass, cements, pastes, mucilages, etc based on actual experience, (translated by W T Brannt), Henry Carey Baird & Co, Philadelphia. Diodorus Sicullus (c.60BC), The Library of History as translated by C H Oldfather, Heinemann, London, 1952. Dioscorides P (c.AD50), De Materia Medica as translated by T A Osbaldeston and R P A Wood, IBIDIS Press, Johannesburg, 2000, pp. 484±487. Dodwell C R (1961), Theophilus ± The Various Arts (translated from the Latin), Nelson, London. Dorsey G (1990), Fullness of wings: The making of a new Daedalus, Penguin, USA. Edwards B H (1986), `Polyurethane Structural Adhesives' in Hartshorn (1986), pp. 181±215. Falder S, Bennett S, Alvi R and Reeves N (2003), `Following in the footsteps of the pharaohs', British Journal of Plastic Surgery, Vol. 56, Issue 2, March 2003, pp. 196± 197. Feldhaus F M (1931), Die Technik der Antike und des Mittelalters, Akad Verlagsgesellschaft, Potsdam. Fernbach R L (1907), Glues and Gelatines, Archibald Constable and Co. Ltd, London. Forbes R J (1964), Studies in Ancient Technology, Brill, London. Galileo Galilei (1638), Discourses and mathematical demonstrations concerning two new sciences, published in Leiden (Holland), available in translation by S Drake, University of Wisconsin Press, 1974. Grant E S (1983), Drop by drop ± The Loctite story, Loctite Corporation. Hartshorn S R (1986), Structural Adhesives ± Chemistry and Technology, Plenum, New York. Hooke R (1665), Micrographia: Or some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon, Royal Society, London.

History of adhesive bonding

21

Hubbard E (1909), `Little journeys to the homes of great business men', Vol. 25 Peter Cooper, New York. Judge A W (1921), Aircraft and Automobile Materials of Construction, Vol II: NonFerrous & Organic Materials, London, Pitman, pp. 391±397. Kaufman M (1963), The first century of plastics, Celluloid and its sequel, The Plastics Institute, London. Kinloch A J (1987), Adhesion and Adhesives ± Science and Technology, Chapman and Hall, London. Koller J, Baumer U and Mania D (2001), `High-tech in the middle Palaeolithic: Neanderthal-manufactured pitch identified', European Journal of Archaeology, Vol. 4, No. 3, December 2001, pp. 385±397. Lambert T (1925), Bone Products and Manures ± A Treatise on the Manufacture of Fat, Glue, Animal Charcoal, Size, Gelatin, and Manures, 3rd edn, revised by Stocks H B, Scott, Greenwood & Son, London. Laurie A P (1926), The painter's methods & materials, Seeley, Service & Co., London. Lay D G and Cranley P (2003), `Polyurethane Adhesives' in Handbook of adhesive technology, edited by A Pizzi and K L Mittal, Marcel Decker, New York, pp. 695±718. Lee H and Neville K (1982), Handbook of epoxy resins, McGraw-Hill, New York. Lees W A (1981), `Modified epoxides; Practical aspects of toughening', J. Adhesion, Vol. 12, pp. 233±240. Lucas A (1927), `The chemistry of the tomb', Appendix II in The tomb of Tut.ankh.Amen by Howard Carter, Cassell. Lucas A and Harris J R (1962), Ancient Egyptian Materials and Industries, Edward Arnold, London. Lucretius (Titus Lucretius Carus) (c.50BC), `De Rerum Natura', Book VI (as translated by W H D Rouse in Lucretius on the nature of things, Loeb Classical Library, Harvard, Cambridge, Massachusetts, 1992, p. 573). Mack E C (1949), Peter Cooper, Citizen of New York, Duell, Sloan and Pearce, New York. Martin F R (1977), `Acrylic Adhesives' in Developments in Adhesives ± 1 edited by W C Wake, Applied Science, London, pp. 157±179. Millet G H (1986), `Cyanoacrylate Adhesives' in Hartshorn (1986), pp. 249±307. Morrell R S (1943), Synthetic resins and allied plastics, Oxford University Press, London. Nevins A (1935), Abram S. Hewitt with some account of Peter Cooper, Harper & Brothers, New York. Newman R and Serpico M (2000), `Adhesives and binders' in Ancient Egyptian Materials and Technology, edited by Nicholson P T and Shaw I, Cambridge University Press, Cambridge, 2000, Chapter 19, pp. 475±494. Newton I (1717), Opticks: or, a treatise of the reflections, refractions, inflections and colours of light. The second edition, with additions, printed by W. Bowyer for W. Innys at the Prince's Arms in St. Paul's Churchyard, London, p. 369. Old Bailey (1714±1799), The Proceedings of the Old Bailey, available on the Internet at www.oldbaileyonline.org, accessed March 2004. Ovid (AD1), Metamorphoses, Book Eight, as translated by John Dryden et al., Wordsworth, 1988. Pausanias (c.AD160), Description of Greece, as translated by W H S Jones, Heinemann, London, 1961.

22

Adhesive bonding

Petroski H (1994), The evolution of useful things, Vintage Books, New York. `Plastes' (1943), Plastics in Industry, Chapman & Hall, London, p. 162. Pliny (AD50), Natural History, as translated by H Rackham, Loeb Classical Library, Harvard. Potter W G (1976), Uses of epoxy resins, Chemical Publishing Company, New York. Power E (1992), The Goodman of Paris (Le MeÂnagier de Paris). A Treatise on Moral and Domestic Economy by a citizen of Paris, translated by Eileen Power, The Folio Society, pp. 115 and 198. Raymond R W (1901), Peter Cooper, Riverside Biographical Series, Houghton, Mifflin and Co, Boston and New York. Rideal S (1901), Glue and Glue Testing, Scott, Greenwood & Son, London. Robins J (1986), `Phenolic Resins' in Hartshorn (1986), pp. 69±112. Sauter F (2000), Jordis J, Graf A, Werther W and Varmuza K, `Studies in organic archaeometry I: Identification of the prehistoric adhesive used by the `Tyrolean Iceman' to fix his weapons', ARKIVOC, 2000, Vol. 1, Part 5, pp. 735±747. Shakespeare W, Titus Andronicus (1588), Act II, Scene 1; King John (1595), Act III, Scene 4. Smith P I (1929), Glue and Gelatine, Sir Isaac Pitman & Sons Ltd, London. Smith C S and Hawthorne J G (1974), `Mappae Clavicula ± A little key to the world of medieval techniques', Transactions of the American Philosophical Society, New Series, Vol. 64, Part 4, July 1974. Spindler K (1995), The man in the ice, Phoenix, London. Standage H C (1897), Cements, pastes, glues, and gums, Crosby Lockwood and Son, London. Stumbo, D A (1965), `Historical Table' in Adhesion and Adhesives, 2nd edn, Vol. 1 (Adhesives), edited by R Houwink and G Salomon, Elsevier, Amsterdam, 1965, pp. 534±536. Teesdale C H (1922), Modern Glues and Glue Handling, The Periodical Publishing Co., Grand Rapids, Michigan. Thompson D V (1933), Il Libro dell'Arte ± Cennino D'Andrea Cennini. The Craftman's Handbook, translated by Daniel V Thompson, Dover Publications, New York. Thompson D V (1956), The materials and techniques of medieval painting, Dover Publications, New York. Tout R (2000), `A review of adhesives for furniture', Int J Adhesion and Adhesives, Vol. 20, No. 4, pp. 269±272. Virgil (c.19BC), The Aeneid, Book 6, as translated by John Dryden, Wordsworth, 1997. Walker A A (1998), `Oldest glue discovered', Archaeology Online News, May 21, 1998, (http://www.archaeology.org). Wood J T (1917), `Leather and Glue', in Annual Reports of the Society of Chemical Industry on the Progress of Applied Chemistry, Vol. II, 1917, p. 374. Wood A D (1963), Plywoods of the world ± Their development, manufacture and application, Johnston and Bacon, Edinburgh and London.

2

What are adhesives and sealants and how do they work? J COMYN

2.1

Introduction

`An adhesive may be defined as a material which when applied to surfaces of materials can join them together and resist separation.' This definition was proposed by Kinloch (1987) and it would include some materials not normally considered as adhesives such as mortar and solder. There are other substances which are outside this definition but which show the phenomenon of adhesion; these include paints and printing inks. In terms of the substances involved, the principle component of an adhesive or sealant is an organic polymer, or one or more (usually two) compounds which can chemically react to produce a polymer. At the time of application the adhesive or sealant must be a liquid, as this enables it to make intimate molecular contact with the adherends; that is it must wet the surfaces. It must then harden (cure) to a cohesive solid. Pressure-sensitive adhesives are an exception in that they do not harden, but remain permanently sticky. Adhesives and sealants can be classified by the manner in which they harden. This can be by loss of solvent, loss of water, cooling or chemical reaction. Once hardened, the polymer in an adhesive can be linear or crosslinked. The act of crosslinking renders polymers insoluble and infusible, and greatly reduces creep. All structural adhesives are crosslinked. All polymers have a glass-transition temperature (Tg). Below this temperature they are relatively hard and inflexible, and above it they are soft and flexible; the words glassy, and rubbery or leathery are used to describe the two conditions. The glass transition is a manifestation of the motion of segments of the polymer chain. These are immobile in the glass, but free to move in the rubbery state. Rubbery and glassy adhesives both have their uses, pressuresensitives are examples of the former and all structural adhesives are glasses. However, it is unacceptable for adhesives to pass from one state to another during service. Curing (hardening) takes place within the bulk of the adhesive or sealant, and adhesion occurs at the interface. Van der Waals forces will always contribute to

24

Adhesive bonding

adhesion as these are the normal attractions between atoms and molecules; this provides the physical adsorption theory of adhesion. The other theories are chemical bonding, mechanical interlocking, diffusion, electrostatic and weak boundary layer.

2.2

Adhesives which harden by loss of solvent

Contact adhesives are probably the best known solvent-based adhesives. The adhesive is basically a solution of polymer in organic solvents, which is applied to both surfaces to be bonded. Some time is allowed for the solvent to evaporate and the surfaces are then pressed together. Neoprene (polychloroprene) adhesives are prominent examples. They have good tack, rapid development of bond strength and are resistant to oils and chemicals. The formulation of a typical neoprene contact adhesive is shown in Table 2.1. Table 2.1 Composition of a polychloroprene contact adhesive Component Polychloroprene Magnesium oxide Zinc oxide Antioxidant (butylated hydroxytoluene BHT) Resins (p-tert-butyl phenolics) Solvents (mixture of acetone, hexane and toluene)

Parts per hundred resin (phr) 100 4±8 5 2 30±50 600

Polychloroprene is unstable and degrades with the liberation of hydrogen chloride; the metal oxides and antioxidant are there to reduce degradation. The oxides act as acid acceptors and the antioxidant as a free radical scavenger. Resins improve adhesion and cohesive strength. Just before application 1±2% of a diisocyanate (e.g. diphenylmethane diisocyanate, DDM) can be added as a crosslinking agent. These normally react with active hydrogen atoms, but it is not clear how they react here. Uses include DIY contact adhesives, shoe soling, rubber dinghies and rubber to metal bonding. Clear adhesives, which are available to the public, can be solutions of a copolymer of butadiene and acrylonitrile in organic solvents.

2.3

Adhesives which harden by loss of water

There is much pressure from environmental, and health and safety regulators, to reduce or eliminate the use of solvents in adhesives, and the adhesives and sealants industry is responding by developing water-based systems to replace

What are adhesives and sealants and how do they work?

25

Table 2.2 Enthalpy of vaporisation of some common solvents Solvent Water Acetone Ethyl acetate n-Hexane Toluene


Hv/J gÿ1 2440 534 404 508 413

them. There are, however, two fundamental problems, one being the low rate at which water evaporates because of its high enthalpy of vaporisation, which is compared with values for some common solvents in Table 2.2. The second problem is with latex adhesives which are a most important water-based class. The water-soluble materials which are essential to manufacture and stabilise latices remain in the adhesive after drying, so increasing water absorption and the sensitivity of joints to water.

2.3.1 Water solutions and pastes Starch is cheap and available in plenty, maize and corn being the main sources for adhesive use. The main uses are for bonding paper, board and textiles. Applications include corrugated board, paper bags, tube winding, wallpaper paste and remoistenable adhesives. Water-moistenable adhesives include polyvinyl alcohol which is used on postage stamps.

2.3.2 Latex adhesives The product of emulsion polymerisation is a latex of polymer particles with adsorbed stabiliser, which is normally an anionic surfactant (soap). The particle diameters are of the order of 1 m and the amount of water is normally 50±55%. Polymer latices are best known as emulsion paints which are based on polyvinyl acetate (PVA). Whether used as surface coatings or adhesives, they are spread on surfaces and a continuous film is formed as the water evaporates. The lowest temperature at which a continuous film can be formed is the minimum filmforming temperature MFT, which is close to the glass transition temperature. Perhaps the best known example is DIY wood adhesive which is a PVA latex. Here the adhesive hardens by water migrating into pores in the wood. Phthalate plasticiser can be added to reduce brittleness. Because of water-sensitivity, it is only suitable for indoor applications. Another example is `Copydex' which is natural rubber latex with ammonia added as a stabiliser. In contact with water, adhesive bonds with latex adhesives may release surfactants, which will have the effect of lowering surface tension and changing

26

Adhesive bonding

the thermodynamic work of adhesion (Comyn, Blackley et al., 1993a). Some latices based on copolymers of vinyl acetate were dried to give films which were then immersed in small quantities of water. The surface tensions fell from 72.8 mNmÿ1 to values in the range 39 to 53 mNmÿ1 in the first hour and then remained fairly static (Comyn, Blackley et al., 1993b). All the interfaces were stable in pure water and remained stable so long as the work of adhesion remained positive. This was confirmed by experiments in which such bonds rapidly disintegrated in water. Adhesive bonds to latex adhesives can thus be self-destructive in water.

2.4

Adhesives which harden by cooling

Hot melt adhesives are one part materials which are applied to substrates as a hot liquid, and rapidly form an adhesive bond as they cool. Their application is readily automated. They can be used to bond paper and board, many plastics and wood but a problem with bonding metals is that the substrate conducts heat too rapidly, restricting the extent of wetting.

2.4.1 Ethylene vinyl acetate (EVA) hot melts EVA random copolymers containing up to 30% vinyl acetate are used, and the effect of adding VA to polyethylene is to reduce crystallinity and increase polarity. Melt viscosity is very dependent on molecular weight. Tackifiers are added to reduce viscosity and improve wetting. Waxes can be added to lower cost and reduce viscosity. Fillers such as calcium carbonate lower cost and increase viscosity. Antioxidants are needed to protect the adhesive during application and service life. Butylated hydroxytoluene (BHT) is a popular antioxidant but it is so volatile that it can evaporate from hot melt adhesives, and can co-evaporate with the solvents from contact adhesives. Less volatile antoxidants have higher molecular weights and cost more. Antioxidants feature in section 6.2 of Chapter 6 (Environmental (durability) effects). Uses include cardboard boxes, bookbinding, iron-on patches and edge-tapes on chipboard.

2.4.2 Polyamide hot melts Polyamide hot melt adhesives have lower melting points than polyamide (nylon) plastics, and this is achieved by employing a mixture of monomers, which has the effect of reducing interchain N-H---O=C hydrogen bonding. They have better heat resistance than EVAs but cost more, but give good tack without needing additives. The polyamide terpolymers 6,6-6,6-10, 6,6-6,12 6,6-6,6-12 and 6,6-9,6-12 are used for bonding textile fabrics, where they are softened by steam, but this

What are adhesives and sealants and how do they work?

27

facility also lowers their wash resistance. They have good dry cleaning resistance.

2.5

Adhesives which harden by chemical reaction

2.5.1 Epoxides Epoxides are the best known and most widely used structural adhesives. There are only a few commercial epoxide resins, but they can be mixed with a wide range of hardeners, which include amines and acid anhydrides. Advantages are that no volatiles are formed on hardening and shrinkage is very low. A disadvantage is that they can cause skin diseases. The most commonly used epoxide resin is based on the diglycidylether of bisphenol-A (DGEBA). It has structure 1, where n is about 0.2. The pure compound is a solid but the commercial product is more conveniently a liquid.

Commercial epoxy resin based on DGEBA

The structure of another commercial resin is shown in structural formula 2.

Tetraglycidyl diaminodiphenylmethane

Both aromatic and aliphatic amines are used as hardeners, and the stoichiometry is that one epoxide ring will react with one amine-hydrogen atom in a condensation polymerisation. The reaction of a primary amine group with epoxide rings is shown below.

Reaction of primary amine with 2 epoxide groups

28

Adhesive bonding

Some typical aliphatic amine curing agents are triethylene tetramine (TETA) which is 6-functional and bis(aminopropyl)tetraoxaspiroundecane which is 4functional. NH2CH2CH2NHCH2CH2NHCH2CH2NH2 triethylene tetramine

3,9-Bis(aminopropyl)-2,4,8,10-tetraoxaspiro(5,5)undecane

Epoxide adhesives with aliphatic amines can be cured at room temperature or the process can be accelerated by heating. Typical cure times are 14 hours at room temperature or 3 hours at 80 ëC. Cure with aromatic amines requires elevated temperatures, typically 2 hours at 150 ëC, and the cured adhesives have higher glass transition temperatures and the joints tend to be more durable. Some aromatic amine hardeners are shown below.

1,3-Diaminobenzene

4,40 Diaminodiphenyl sulfone

One-part adhesives can be made with hardeners which require elevated temperatures. Such a hardener is dicyandiamide (H2N-C(=NH)-NH-CN) which has the added advantage of being insoluble in DGEBA at room temperature, dissolving when the adhesive is heated. Such adhesives are often supplied in the form of a film which is stored in a refrigerator, and often contains a textile fabric or carrier to assist in handling the adhesive and in controlling glue-line thickness.

2.5.2 Phenolic adhesives for metals When phenol is reacted with an excess of formaldehyde under basic conditions in aqueous solution, the product, which is known as a resole, is an oligomer containing phenols bridged by ether and methylene groups, and with methylol groups substituted on the benzene rings. This is shown in Fig. 2.1. If used as adhesives they would be heated to 130±160 ëC in the joint, where further condensation of methylol groups takes place to give a crosslinked polymer thus: 2-CH2OH = -CH2OCH2- + H2O

What are adhesives and sealants and how do they work?

29

Figure 2.1 Reaction of phenol with formaldehyde to form a resole.

To avoid the formation of voids filled with steam, joints with phenolic adhesives have to be cured under pressure, usually between heated steel platens of a hydraulic press. Because they are brittle, other polymers are added to phenolics to toughen them. These include polyvinylformal, polyvinylbutyral, epoxides and nitrile rubber.

2.5.3 Structural acrylic adhesives Structural adhesives containing acrylic monomers are cured by free radical addition polymerisation at ambient temperatures. The principal monomer is methylmethacrylate, but others may be present such as methacrylic acid to improve adhesion to metals by forming carboxylate salts and heat-resistance, and ethylene glycol dimethacrylate for crosslinking. Often polymethylmethacrylate is also present; this has the effect of increasing viscosity and reducing odour. The formulation of a typical structural acrylic adhesive is given in Table 2.3. Chlorosulfonated polyethylene is a rubbery toughening agent. Cumene hydroperoxide and N,N-dimethylaniline are the components of a redox initiator. The adhesive would be supplied in two parts (resin and catalyst). The catalyst contains one of the initiator components, and all the other components are in the resin. Most conveniently the resin can be spread on one surface and the catalyst on the other. After being joined for about one minute the adhesive will have cured sufficiently to hold the joint together, and maximum strength will develop in about ten minutes. It is also possible to premix the components. Table 2.3 Formulation of a structural acrylic adhesive Component Methylmethacrylate Methacrylic acid Ethylene glycol dimethacrylate Chlorosulfonated polyethylene Cumene hydroperoxide N,N-dimethylaniline

Parts by weight 85 15 2 100 6 2

30

Adhesive bonding

The most widely used initiator system is a hydroperoxide and a condensation product of aniline and butyraldehyde, which can also generate free radicals by reacting with sulfonyl chloride groups in the toughening rubber, leading to some grafting of acrylic polymer to the rubber particles. Cements for fixing artificial joints to human bones, and porcelain caps to teeth are also based on MMA. In the case of the latter the dentist uses phosphoric acid to prepare the surface and dries it with cold air, and uses UV to cure the adhesive. There is a large volume decrease of 20.7% when MMA is polymerised. Such a large change could introduce significant stresses into joints, but can be reduced by adding particulate fillers. Shrinkage is also the reason why adhesives tend to have poor gap-filling properties.

2.5.4 Rubber toughening of structural adhesives Many structural adhesives have rubbery polymers dissolved in them. When the adhesives cure the rubber precipitates as droplets about 1 m diameter, the driving force for this being the incompatibility which generally occurs between polymers. Adhesive joints break by the growth of a crack, and rubber particles act as crack stoppers. Fracture energies and impact strengths are increased. Rubbers which are used in this way include polyvinylformal, polyvinylbutyral, chlorosulfonated polyethylene ATBN and CTBN. The latter are acronyms for copolymers of butadiene and acrylonitrile with either amine or carboxylic end groups. In an epoxide adhesive the end groups will react with the resin to give chemical bonding at the particle-matrix interface.

2.5.5 High-temperature adhesives The maximum temperatures at which structural adhesives can be used is limited by the glass transition temperature and chemical degradation. The upper limit for acrylic adhesives is set by the glass transition temperature of polymethylmethacrylate (105 ëC) and the limit for epoxides of about 200 ëC is due to chemical degradation. There are a number of adhesives which can operate at higher temperatures than epoxides and phenolics. These tend to be expensive and require high cure temperatures. The best known are perhaps the polyimides, which were developed by NASA in the USA. They are made by a condensation polymerisation between a dianhydride and a diamine. In the example shown in Fig. 2.2, pyromellitic dianhydride is reacted with 1,4-diaminobenzene. The first step in the reaction gives a polyamic acid which is soluble and fusible, and it would be applied to the substrates at this stage. Cure is then at high temperature and under pressure; the resulting polyimide is insoluble and infusible.

What are adhesives and sealants and how do they work?

31

Figure 2.2 Synthesis of a polyimide.

2.5.6 Formaldehyde condensate adhesives for wood Some adhesives for wood are condensates of formaldehyde with phenol and resorcinol (1,3-dihydroxybenzene). Others are condensates with either urea or melamine, where reaction with formaldehyde results in the replacement of amine hydrogen atoms by methylol groups. Tetramethylolurea has not been isolated. All these compounds undergo condensation polymerisation via methylol groups, to give crosslinked products. The reactions take place at ambient temperatures after the addition of a catalyst. The adhesives are water-based and water is produced on cure; it is removed by migration into the wood, making these adhesives suitable only for porous adherends.

2.5.7 Anaerobic adhesives Anaerobic adhesives cure in the absence of oxygen, which inhibits polymerisation. They are usually based on dimethacrylates of polyethylene glycol, but end-capped polyurethanes are also used. They contain a redox free radical initiator, and are usually supplied in air-permeable polyethylene containers only partially filled, to maintain an adequate supply of oxygen. Uses include nutlocking, strengthening cylindrical fits and gasketing.

2.5.8 Cyanoacrylates The molecule shown below is ethyl cyanoacrylate, and because it contains two strongly electron-withdrawing groups (-CN and -COO-) it is very susceptible to anionic polymerisation. This is initiated by water which is adsorbed on all

32

Adhesive bonding

surfaces in the atmosphere, and is complete within seconds. The actual initiating groups in water are the basic hydroxide ions (OH-). Because the surface of glass is alkaline, cyanoacrylates are packed in polyethylene rather than glass containers. Sulfur dioxide is added as a stabiliser. Methyl, n-butyl and allyl cyanoacrylates are also used. CH2=C-CN | COOCH2CH3 ethyl cyanoacrylate

2.5.9 Polyurethanes Polyurethane adhesives are made by reacting a low molecular weight polymer with at least two -OH end groups with a diisocyanate. The polymers can be polyethers, aliphatic polyesters or polybutadiene. The basic chemical reaction is -NCO + -OH = -NHCOOisocyanate ol urethane In two-component polyurethane adhesive the polymer and isocyanate are mixed and then applied to the adherends. Any hydroxyl groups on the surfaces (e.g. on paper, wood or glass) will possibly react with isocyanate to form covalent bonds between adhesive and substrate. One-part adhesives consist of low molecular weight, linear polymer molecules, which have isocyanate (-NCO) end groups. Water vapour from the atmosphere diffuses into the adhesive and causes the following chemical reactions which join the molecules together to form larger linear molecules. -NCO + H2O = -NH2 + CO2 -NCO + -NH2 = -NH-CO-NHurea unit However, a further reaction is that of isocyanate with urea units, and a consequence of this is that the adhesive which was firstly linear, now becomes crosslinked. -NCO + -NH-CO-NH- = -NH-CO-N| CO-NHbiuret unit

What are adhesives and sealants and how do they work?

33

2.5.10 Silicones One-part silicone adhesives are often termed (RTV) room temperature vulcanising, and consist of polydimethylsiloxane (PDMS) with molar masses in the range 300±1600, with acetate, ketoxime or ether end groups. These are hydrolysed by moisture from the atmosphere to form hydroxyl groups, which subsequently condense with the elimination of water. The reactions for acetate end groups are shown below. They are best known as sealants for use in the bathroom. -SiOCOCH3 + H2O = -SiOH + CH3COOH" acetate -SiOH + -SiOH = -Si-O-Si- + H2O The rate of cure is controlled by water diffusion, which is slow in comparison with the chemical reactions. There is a sharp advancing front of cured sealant, and the cured material acts as a barrier for water permeation. Any water which passes through this barrier quickly reacts with uncured sealant, and thus the barrier is thickened (Comyn, Day et al., 1998). Two-part silicones, which are essential for thick sections, normally contain water and are catalysed with stannous octoate for fast cure, or dibutyltindilaurate for slower cure. Silicone adhesives are soft and compliant, and have good chemical and environmental resistance. Joints with silicones can operate over a wide temperature ranging from about ÿ60 ëC to 200 ëC. The glass transition temperature is ÿ120 ëC.

2.5.11 Polysulfides Polysulfides are primarily used as sealants and a major use is to seal the edges of double glazing units, both to hold the units together and prevent the ingress of moisture. They are made by reacting bis(2-chloroethyl formal) with sodium polysulfide as shown below, and where x is about 2 and n about 20. The addition of a small quantity of trichloropropane leads to branch points, which in turn lead to crosslinking on cure. ClCH2CH2OCH2OCH2CH2Cl + NaSx = -(CH2CH2OCH2OCH2CH2Sx)n- + NaCl Polysulfide sealants are formulated with mineral fillers to reduce cost and modify flow properties, phthalate plasticisers and silane coupling agents. They are two-part systems and curing agents include manganese dioxide and chromates. Cure involves oxidative coupling of -SH end groups to form -S-S-, and has a complex free radical mechanism.

34

Adhesive bonding

2.6

Adhesives which do not harden ± pressure-sensitive adhesives

These are the adhesives which are used on sticky tapes and labels. They do not harden but remain permanently sticky. These are viscous polymeric fluids with a glass transition temperature below the temperature of use. Common additives are antioxidants and tackifiers. They can be applied to a label or tape from solution, emulsion or hot melt. The major types of pressure-sensitive adhesives are based on natural rubber, styrene butadiene rubber, block copolymers, amorphous poly--olefins and acrylics.

2.6.1 Polymers Used in 1845 for surgical plasters, natural rubber is still a major material in PSAs. They essentially comprise natural rubber, antioxidant and tackifier, and are normally coated from solution (e.g. heptane, toluene). The rubber is masticated to break down gel and reduce molar mass. Latex styrene butadiene rubber (SBR) is a random copolymer. The glass transition temperature of such materials is linear with composition, and increases with the amount of styrene. They are applied from solvents. Block copolymers are of type ABA where A are styrene blocks and B is initially of isoprene or butadiene. However, the latter blocks can be hydrogenated to improve oxidative stability. The two phases are incompatible, each phase having a separate glass transition temperature. The rubbery phase is

T/K

300

250

200 0

4

8 12 16 Number of carbon atoms

20

Figure 2.3 Effect of the number of carbon atoms in the alkyl group on the brittle point of poly-n-alkylacrylates.

What are adhesives and sealants and how do they work?

35

continuous and the styrene phase consists of small particles 20±30 m diameter. The styrene content is generally above 25%. They are widely used as thermoplastic elastomers and can be melt-coated. Tackifiers are necessary, petroleum resins being the most common. Atactic polypropylene is an amorphous and a sticky solid. This and its copolymers with ethylene, butene and hexane are used as PSAs. They bond well to low energy surfaces such as polyethylene. Acrylic adhesives using monomers from methyl to ethylhexyl acrylate give a wide range of physical properties. Figure 2.3 plots the brittle points of poly-nalkyl acrylates as a function of the number of carbon atoms in the alkyl group. Brittle point intially falls as the alkyl groups increase in size, but then rises as the long side chains begin to crystallise. These can be used without tackifier, so avoiding possible migration problems. They are also resistant to attack by oxygen and UV and do not discolour. They can be applied from solution, latex or hot melt. For PSAs only copolymers are used, the acrylic monomers being butyl acrylate, 2-ethylhexyl acrylate and isooctyl acrylate, with vinyl acetate as a common comonomer.

2.6.2 Tackifiers Tackifier resins must be compatible with polymers used in PSAs, be of low molar mass, and have a higher glass transition temperature than the base polymer. Most are brittle glassy solids with Tg in the range 30±60 ëC. They are also used in hot melt adhesives. Rosin acid is obtained from pine trees and it can be esterified with glycerol or pentaerythritol to produce a tackifier. It can be hydrogenated to remove C=C bonds and reduce oxidation. Hydrocarbons tackifiers are made by cationic polymerisation using Lewis acid initiators with C5 and C9 monomers. They are low molar mass polymers with very irregular structures. Aromatic resins are made from indene with small amounts of styrene, methyl styrene and methylindenes. The main monomers for aliphatic resins are cis- and trans-1,3-pentadiene. With terpene resins the monomers (-pinene, -pinene and dipentene) are dimers of isoprene.

2.7

Adhesion by physical adsorption

2.7.1 Introduction Physical adsorption contributes to all adhesive bonds and so is the most widely applicable theory of adhesion. The basis is that van der Waals forces, which occur between all atoms and molecules when they are close together, exist across interfaces. These are the weakest of all intermolecular forces, but their strengths are more than adequate to account for the strengths of adhesive joints. Van der Waals forces are of three types, namely the forces of attraction between

36

Adhesive bonding

molecules with permanent dipoles, those between a permanent dipole and a nonpolar molecule, and those between nonpolar molecules. The potential energies for these attractions are all proportional to r-6, where r is the distance of separation. Such forces of adhesion are very short range and are experienced by only one or two layers of molecules in the interfacial layers.

2.7.2 Contact mechanics Contact between rubber spheres can demonstrate the presence of attractive forces across the interface. Equation 2.1 is due to Hertz and it gives the diameter of the zone of contact d when two elastic spheres of diameter D are pressed together with a force F. Here E is Young's modulus of the material of the spheres and  is Poisson's ratio. d 3 ˆ 3…1 ÿ  2 †FD=E

2:1

Johnson, Kendall and Roberts (1971) measured d for some natural rubber spheres and found deviations from the Hertz equation at low loads, but conformity at high loads. Data are shown in Fig. 2.4. At low loads the zones of contact were greater than predicted by Hertz. This was due to the forces of attraction between the surfaces of the two spheres, and it was shown that the diameter of the zone of contact was now given by eqn 2.2, where W is the work of adhesion. d 3 ˆ 3…1 ÿ  2 †DfF ‡ 3WD=4 ‡ ‰3WDF=2 ‡ …3WD=4†2 Š1=2 g=E 2:2

Contact diameter (mm)

5.0

0.50

0.05

1

10 Load (mN)

100

Figure 2.4 The diameter of the contact spot between two rubber spheres of 22 mm radius, l as measured in air, m in water and n in a solution of sodium dodecyl sulfate. After Johnson, Kendall, et al. (1971).

What are adhesives and sealants and how do they work?

37

The use of eqn 2.2 gave a value of W ˆ 71  4 mJmÿ2 for dry rubber (that is the surface free energy of the rubber is 35 mJmÿ2) and 6.8  0.4 mJmÿ2 in the presence of water. When immersed in a 0.01M solution of the surfactant sodium dodecyl sulfate the Hertz equation was obeyed, because the work of adhesion was now very low < 1 mJmÿ2. Equation 2.2 has received much attention in the literature and is generally called the JKR equation.

2.7.3 Contact angles The physical adsorption theory of adhesion can be explored by the observation of liquid contact angles. In Fig. 2.5 some molecules of one liquid are lying upon some molecules of another; both are non-polar so only dispersion forces will be acting across the interface. The force by which molecule A is attracted to its own kind is the surface tension of liquid 1 ( 1 ), but what is the force which attracts it to the other liquid? Fowkes (1964) considers that it is the geometric mean of the two surface tensions, and Wu (1973) considers it to be the harmonic mean, i.e., Fowkes, Interfacial attraction = ( 1 2 †1=2

2.3

1 ˆ 1= 1 ‡ 1= 2 Interfacial attraction

2.4

Wu,

For liquids surface tension and surface free energy are numerically the same, but the dimensions differ. These are usually mNmÿ1 and mJmÿ2 respectively. For a liquid or a solid the surface free energies are the sum of dispersion (d) and polar (p) components, i.e.

L ˆ L d ‡ L p

2:5

S ˆ S d ‡ S p

2:6

The outcome is that the contact angle  for a liquid on a solid is given by eqn 2.7.

L …1 ‡ cos †=2… L d †1=2 ˆ … S d †1=2 ‡ … S p L p = L d †1=2 d 1=2

2:7 p

d 1=2

This means that if L …1 ‡ cos †=2… L † is plotted against … L = L † , the graph should be linear with intercept … S d †1=2 and slope … S p †1=2 , so permitting the determination of the polar and dispersive components of the surface free energy of the solid. Such plots have been referred to as Owens-Wendt plots, examples are given in Figs 2.6 and 2.7 (Comyn, Blackley et al., 1993b). Figure 2.6 is for dried films of an emulsion adhesive based on a copolymer of vinyl acetate and butyl acrylate. Here S d ˆ 6:4  2:1 mJmÿ2 and S p ˆ 38:5 6:3 mJmÿ2. Figure 2.7 is for the release agent zinc stearate which was pressed into discs. Here S d ˆ 22:4  0:1 mJm ÿ2 and S p ˆ 0:06  0:05 mJm ÿ2 ,

38

Adhesive bonding

Figure 2.5 Forces acting on a molecule at a liquid-liquid interface.

Figure 2.6 Plot based on eqn 2.6 for liquids on a dried film from a latex adhesive. From left to right the liquids are dimethylformamide, dimethylsulfoxide, ethane diol and water. After Comyn, Blackley et al. (1993b).

What are adhesives and sealants and how do they work?

39

Figure 2.7 Plot based on eqn 2.7 for liquids on zinc stearate. From left to right the liquids are n-hexadecane, dimethylformamide, dimethylsulfoxide and water. After Comyn, Blackley et al. (1993b).

showing that it is the non-polar alkyl groups which dominate the surface, rather than the polar zinc carboxylate units.

2.7.4 Thermodynamic work of adhesion The thermodynamic work of adhesion, that is the work required to separate unit area of two phases in contact, is related to surface free energies by the Dupre equation. It is the minimum work needed to separate the phases, and energies needed to break adhesive bonds often exceed this by a significant amount, because of work done in deforming the adhesive layer or the adherends. An example where much work is done on stretching the adhesive is a pressuresensitive adhesive which forms filaments before the adhesive detaches. If the phases are separated in dry air, work of adhesion WA is given by eqn 2.8. WA ˆ A ‡ S ÿ AS

2:8

But if separation is in the presence of water it is given by eqn 2.9. WAW ˆ AW ‡ SW ÿ AS

2:9

Here the subscripts A, S and W denote adhesive, substrate and water. The separation processes are illustrated in Fig. 2.8. Fowkes (1964) gives eqn 2.10 for the interfacial free energy ( 12 ) between phases 1 and 2. It can be used to obtain the interfacial free energies in eqns 2.8 and 2.9.

40

Adhesive bonding

Figure 2.8 Separation of an adhesive from a substrate in dry air (top), and in water (bottom).

12 ˆ 1 ‡ 2 ÿ 2… 1 d 2 d †1=2 ÿ 2… 1 p 2 p †1=2

2:10

If the thermodynamic work of adhesion is positive then the bond is stable, and conversely a negative value indicated instability. The parameter which has created most interest in the literature is the work of adhesion in the presence of water, as this can be used to predict joint durability. The data in Table 2.4 for the vinylidene chloride-methyl acrylate copolymer bonded to polypropylene is quoted from a paper by Owens (1970). Owens coated a polypropylene sheet with an aqueous dispersion containing 80 parts

What are adhesives and sealants and how do they work?

41

Table 2.4 Work of adhesion for interfaces in air and in liquids Work of adhesion/mJmÿ2

Interface

Air

Liquid

Interfacial debonding in liquid?

Epoxy/steel

291

22 ethanol -166 formamide -255 water

No Yes Yes

Epoxy/aluminium

232

-137 water

Yes

Epoxy/silica

178

-57 water

Yes

22±40 water

No

Epoxy/carbon fibre composite

88±90

Vinylidene chloridemethyl acrylate copolymer

88

37 1.4 -0.9 -0.8

water Na n-octylsulfate soln Na n-dodecylsulfate soln Na n-hexadecylsulfate soln

No No Yes Yes

vinylidene chloride, 20 parts methyl acrylate and 4 of acrylic acid. The dispersion was surfactant free and the polypropylene surface had been flame treated. The resulting laminates were placed in some surfactant solutions, and to quote Owens, `In every case where WA,L upon immersion in the liquid is negative, the coating spontaneously separated from the substrate, becoming completely detached. Where WA,L was positive, spontaneous separation did not occur. Where separation occurred between coating and substrate, it did so within 15 minutes. The films that did not show separation were left immersed for six months. At the end of this time, they still were not separated, and some effort was required to remove the coatings from the films.'

2.8

Adhesion by chemical bonding

The chemical bonding theory of adhesion invokes the formation of covalent, ionic or hydrogen bonds or Lewis acid-base interactions across the interface. Typical strengths of these are shown in Table 2.5, where they are compared with van der Waals forces which are the source of physical adsorption. The interactions are listed roughly in order of size, and it can be seen that the strongest are considerably stronger than the weakest. The ionic interactions have been calculated for an isolated pair of ions in a vacuum and those involving aluminium and titanium might occur when epoxide adhesives are used with these metals. Strengths of covalent bonds are typical for bonds of these particular types. It is a possibility that C-O bonds are formed when isocyanate adhesives are used on substrates with hydroxyl groups such as wood and skin. The Si-O bond is formed when silane coupling agents are used on glass.

42

Adhesive bonding Table 2.5 Typical strengths of chemical bonds and van der Waals interactions Type of interaction

Energy kJmolÿ1

Ionic Na+ClAl3+O2TI4+O2-

503 4290 5340

Covalent C-C C-O Si-O C-N

368 377 368 291

Hydrogen bond -OH-----O=C- (acetic acid) -OH-----OH (methanol) -OH-----N (phenol-trimethylamine) F------HF F------HOH

30  2 32  6 35  2 163  4 96  4

Lewis acid-base BF3 + C2H5OC2H5 C6H5OH + NH3 SO2 + N(C2H5)3 SO2 + C6H6

64 33 43 4.2

van der Waals forces dipole-dipole dipole-induced dipole dispersion

2 0.05 2

Hydrogen bonds involving fluorine are stronger than other types, and this is because fluorine is the most electronegative element; here the values are taken from Jeffrey (1977). The data for Lewis acids and bases are actually enthalpies of mixing and are taken from Drago, Vogel et al. (1971).

2.8.1 Covalent bonds There is much evidence that covalent bonds are formed with silane coupling agents. They are generally considered to chemically react with both substrate and adhesive, so forming a system of covalent bonds across the interface which is both strong and durable. If wood is treated with an adhesive containing an isocyanate, it is possible that these would react with hydroxyl groups on cellulose or lignin to produce urethane linkages as shown in section 2.5.9. Using solid state 15N nuclear magnetic resonance (NMR) spectroscopy, Bao, Daunch et al. (1999) found little evidence for this when two types of wood (Aspen and Southern Pine) were treated with polymerised

What are adhesives and sealants and how do they work?

43

diphenylmethane diisocyanate. However, evidence for such linkages was obtained by Zhou and Frazier (2001) but now using NMR spectroscopy with both 15N and 13C nuclei. Another approach to improve adhesive bonding to wood is to graft acid anhydrides to its surface, and then react the resulting carboxylic acid group with the adhesive. Mallon and Hill (2002) used 13C NMR and FTIR to show that succinic anhydride reacts with hydroxyl groups on wood, and the acid groups can subsequently be reacted with hexamethylene diamine.

2.8.2 Ionic bonds The potential energy E‡ÿ of two ions separated by distance r is given by eqn 2.11. Here z1 and z2 are the valencies of the ions, e is the electronic charge, 0 is the permittivity of a vacuum and r is the relative permittivity of the medium. E‡ÿ ˆ

z1 z2 e2 40 r r

2:11

It has been demonstrated using IETS (Mallik, Pritchard et al., 1985) that when the ester-containing polymers polymethyl methacrylate and polyvinyl acetate are placed in contact with aluminium oxide, peaks arise which are assigned to the carboxylate ion. Specifically these are due to the symmetric and asymmetric vibrational modes of ±COOÿ which are located at about 1450 cmÿ1 and 1610 cmÿ1. More recently Devdas and Mallik (2000) showed using IETS that a number of carboxylic acids adsorbed on alumina show such peaks; an example is that pyruvic acid CH3CH2COCOOH shows the peaks at 1450 and 1605 cmÿ1. Perhaps the strongest evidence for interfacial ion-pairs is the fact that carboxylic acids enhance adhesion to metals, and commercial adhesives, such as structural acrylics, often incorporate this feature. The ion-pair mechanism allows partial weakening of joints in the presence of water, with recovery when the joints are dried out. This is in contrast to the physical adsorption theory which predicts the reduction in strength to zero as water displaces adhesive from the metal oxide, and no recovery as a glassy adhesive would have insufficient molecular mobility for it to re-establish intimate contact with the substrate.

2.8.3 The unique properties of water Water is a liquid with extreme properties. If ion-pairs are significant interfacial forces then it is the high relative permittivity which causes weakening. If physical adsorption is the mechanism of adhesion, then it is the high surface tension of water which enables it to displace adhesives from metallic surfaces.

44

Adhesive bonding

2.8.4 Hydrogen bonds Hydrogen bonds probably contribute to the attachment of postage stamps to envelopes where the adhesive (polyvinyl alcohol) and paper (cellulose fibres) both contain -OH groups. Wood is also rich in cellulose and the reactive adhesives based on formaldehyde contain hydroxyl or amine groups capable of participating in hydrogen bonds. Agrawal and Drzal (1996) consider that hydrogen bonding is very important in the adhesion of a polyurethane formed from toluene diisocyanate and 1,4butane diol bonded to float glass, but dipole-dipole forces also contribute. Nagae and Nakamae (2002) investigated the nylon 6±glass fibre interface using laser Raman spectroscopy. Shifts in peaks due to >C=O and >NH groups indicated the formation of interfacial hydrogen bonds, but these were weaker than those in the bulk nylon 6.

2.8.5 Lewis acid-base interactions Conventional or Brùnsted acids are donors of protons (hydrogen ions H+) and the bases are proton acceptors. The concept dates from 1923. In 1938 G.N. Lewis proposed a broader definition in that an acid is an electron acceptor and a base is an electron donor. Boron trifluoride is an example of a Lewis acid and ammonia is a Lewis base. Because of the low position of boron in the periodic table, BF3 is electron deficient which means that it has an sp3 orbital containing no electrons. In ammonia there is a non-bonded sp3 orbital but this now contains two electrons. The two molecules join together by the two electrons being shared as shown in Fig. 2.9; heat is liberated. It is Lewis acids and bases which have attracted much attention in adhesion science in recent years. The application to adhesion has been reviewed by Chehimi (1999).

Figure 2.9 The reaction of boron trifluoride (a Lewis acid) with ammonia (a Lewis base) by the sharing of a pair of electrons.

What are adhesives and sealants and how do they work?

2.9

45

The electrostatic theory of adhesion

The electrostatic theory originated in the proposal that if two metals are placed in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives. However, Randow, Williams et al. (1997) investigated the adhesion of some commercial `cling films', as used in food packing, to glass, steel and polyolefin substrates. The cling films were made of plasticised PVC, low density polyethylene or plasticised polyvinylidene chloride. Surface smoothness was the factor which most increased adhesion by increasing the area of contact, but otherwise adhesion depended on physical adsorption and static electrification. Measurements which supported this were of contact angles, and of residual electric charge on both films and substrates after separation. All films showed sparking when repeatedly applied to glass and noises were produced on an AM radio during peeling.

2.10 Mechanical interlocking If a substrate has an irregular surface, then the adhesive may enter the irregularities prior to hardening. This simple idea gives the mechanical interlocking theory, which contributes to adhesive bonds with porous materials such as wood and textiles. An example is the use of iron-on patches for clothing. The patches contain a hot melt adhesive which, when molten, invades the textile material. Mechanical interlocking to wood (oak) of a thermoplastic adhesive based on polypropylene has been demonstrated by scanning electron microscopy (Smith, Dai et al., 2002). The adhesive conformed to features on the wood as small as 1 m, and penetrated pores' openings 15 m across to depths greater than 150 m. Larger pores were penetrated to depths of hundreds of m. The extent of interlocking depended on the porosity of wood, the viscosity of the molten adhesive, and the pressure and duration of bonding. A related matter is whether roughening a surface increases the strength of an adhesive joint. Harris and Beevers (1999) found no differences in adhesion to mild steel and aluminium alloy blasted with alumina grits of different particle sizes. Shahid and Hashim (2002) used a structural epoxide adhesive with mild steel adherends in cleavage joints. The surfaces had been grit-blasted or diamond-polished, and surface profiled. Results are shown in Table 2.6, where all differences in strength seem to be the same, within experimental scatter.

2.11 Adhesion by interdiffusion The diffusion theory takes the view that polymers in contact may interdiffuse, so that the initial boundary is eventually removed. Such interdiffusion will occur only if the polymer chains are mobile (i.e. the temperature must be above the

46

Adhesive bonding

Table 2.6 Effect of surface roughness on joint strength Average roughness (Ra)/m

Cleavage strength/Nmÿ2

Coefficient of variation

0.04  0.02 0.98  0.05 2.97  0.18 4.23  0.25 6.31  0.28

15.8 18.3 17.5 17.0 16.4

2.9 1.3 1.9 3.2 4.0

glass transition temperature) and compatible. As most polymers, including those with very similar chemical structures such as polyethylene and polypropylene are incompatible for thermodynamic reasons, the theory is generally only applicable in bonding like linear rubbery polymers (autohesion), and in the solvent-welding of thermoplastics. Voyutskii was an originator of the diffusion theory (1963). Nevertheless, there are a small number of polymer pairs made compatible by specific interactions. One pair is polymethylmethacrylate and polyvinylchloride where hydrogen bonding leads to a negative heat of mixing. Diffusion at the interface between PVC and poly--caprolactone has been demonstrated (Price Gilmore et al. 1978, Gilmore Falabella et al. 1980) using energy dispersive x-ray analysis. Voyutskii (1971) has shown some electron micrographs of the interfaces of polymethylacrylate-PVC and polybutylmethacrylate-PVC prepared at 210±220 ëC. Mixing at the interface was much greater with the first pair. Schreiber and Ouhlal (2003) annealed a number of polymer pairs in contact for up to 72 hours at 60±160 ëC and found substantial increases in adhesive strength for polypropylene/linear low density polyethylene and polystyrene/ PVC, but not with polystyrene/PMMA and PVC/polyvinylidene chloride. With the two polyolefins in contact only dispersion forces are available, and only in the case of polystyrene/PVC are there favourable acid-base attractions. The data `point to significant contributions to bond strength arising from diffusion when dispersion forces and favourable acid-base interactions act at the interface'. After an induction period, plots of strength against the square root of time are linear which suggests a diffusion process. The explanation offered for the induction periods was that low molar mass polymer was removed first. In the case of polypropylene/linear low density polyethylene the data give an activation energy of 23 kJ molÿ1 which is stated to be consistent with diffusion processes in polyolefins. Two polymers which can be joined by swelling with solvent and then pressing together are polystyrene and polycarbonate. Here the solvent has the effect of depressing the glass transition temperature below the working temperature, so that when adherends are pressed together there is an adequate level of molecular motion for interdiffusion. After bonding solvent diffuses from the joint and evaporates. Titow et al. (1973) have examined the strength and

What are adhesives and sealants and how do they work?

47

structure of joints in polycarbonate welded with either 1,2-dichloroethane or dichloromethane. The original interface is completely removed and there is no evidence of a residual parting plane. More recently the solvent welding of polycarbonate has been studied by Change and Lee (1996).

2.12 Weak boundary layers The weak boundary layer theory proposes that clean surfaces can give strong bonds to adhesives, but some contaminants such as rust and oils or greases give a layer which is cohesively weak. Not all contaminants will form weak boundary layers, as in some circumstances they will be dissolved by the adhesive. However, in some cases, contaminants such as oils and greases can actually be removed by the adhesive dissolving them (Brewis, 1993). This is an area where acrylic structural adhesives are superior to epoxides because of their ability to dissolve oils and greases.

2.13 Pressure-sensitive adhesion Pressure-sensitive adhesives are viscous liquids, and remain so when incorporated in an adhesive joint. Nevertheless it is essential that they adhere to substrates, and they will do so by one or more of the mechanisms which have been already described. Physical adsorption will contribute in every case, and in most cases it may be the only mechanism, but chemical bonding via ion-pairs may contribute if the adhesive contains carboxylic acid groups and the substrate is a metal. Static electrification is another possible contributor. Zosel (1998) considers that the work of separation in tack w is given by eqn 2.12 where WA is the thermodynamic work of adhesion and  is a viscoelastic factor which is a function of temperature and rate. WA is a property of the interface and  of the adhesive. w ˆ WA … ‡ 1†

2:12

Figure 2.10 shows a number of stress-strain plots for the debonding of polybutylacrylate from some cylindrical steel probes tips with average roughnesses of 0.02 m and 2 m; it illustrates two points. The area under these curves is w. The first is that the peaks at low strain are due to clean detachment of the adhesive from the probe, and the elongated shoulders at higher strain are due to the formation and stretching of fibrils in the adhesive. The second point is that after a contact time of 1 s, stronger joints are formed with the smooth probes. This is because the viscous adhesive has not had sufficient time to make contact with the rougher surface, so WA has not been maximised. Figure 2.11 shows the effect of increasing contact time. WA is the driving force for pressure-sensitive adhesion, but this is opposed by the viscosity of the adhesive.

48

Adhesive bonding

Figure 2.10 Effect of surface roughness on tack (after Zosel, 1998).

Toyama, Ito et al. (1970) measured tack and peel forces needed to remove plastic surfaces from three pressure-sensitive adhesives. The plastic surfaces were, in order of increasing critical surface tension ( c), PTFE, high-density polyethylene, polystyrene, PMMA and nylon 6. Figure 2.12 shows a plot of peel force against c for the three adhesives, and here there is a value of c which

Figure 2.11 Effect of contact time on tack (after Zosel, 1998).

What are adhesives and sealants and how do they work?

49

Peel force/kNm-1

2

1

0 30

gc mNm -1

40

Figure 2.12 Relationship between peel force and critical surface tension of the substrate for l acrylic, q polyvinyl ether and m natural rubber adhesives. Contact time was 168 h (after Toyama, Ito et al. 1970).

gives maximum adhesion. Similar behaviour was shown by the tack data. The authors noted that it is possible that maximum work of adhesion coincides with the substrate having a critical surface tension near to those of the adhesives.

2.14 Future trends Predicting the future depends on the author's uncertain clairvoyance, but his view is that new adhesive systems will not appear in the foreseeable future, instead improvements will be made to existing ones. The performance of structural adhesives will improve, and organic solvents will be virtually removed from industrial adhesives. There will be a growth in latex adhesives. The use of adhesives in all types of manufacture will increase. It is adhesion by Lewis acids and bases which is currently the mechanism of adhesion receiving most attention. Our understanding of adhesion will develop steadily, and this will be assisted by experimental data from modern surface analysis techniques.

2.15 Sources of information Further information on mechanisms of adhesion can be found in the cited references. Recent reviews are by Comyn (in press) and Kendall (2001). In

50

Adhesive bonding

describing the basic chemistry of adhesives and sealants, fewer citations to the primary literature are given. The books listed below fill this deficiency. Expansion of the basic chemistry of adhesive materials is given by Comyn (1997) and with much technological detail by Skeist (1990). Epoxides are dealt with by Ellis (1993) and in considerable detail by May (1988). Structural adhesives are the topic of a book by Hartshorn (1986). Blackley (1997) and Warson (1993) consider latex adhesives, and Pizzi (1983, 1989) deals with adhesives for wood. A review of the literature on polysulfides has been presented by Lee (1999). Pressure-sensitive adhesives are the subject of a book by Satas (1999).

2.16 References Agrawal R K and Drzal L T (1996), J Adhesion, 55, 221. Bao S, Daunch W A, Sun Y, Rinaldi P L, Marcinko J J and Phanopoulos C (1999), J Adhesion, 71, 377. Blackley D C (1997), Polymer Latices; Science and Technology 2nd edn. Vol 3 Applications of Latices Chapman & Hall. Brewis D M (1993), Int J Adhes Adhes, 13, 251. Change K C and Lee S (1996), J Adhesion, 56, 135. Chehimi M M (1999), Ch 2 in Adhesion Promotion Techniques: technological applications, Eds. Mittal K L and Pizzi A, Marcel Dekker Inc. Comyn J (1997) Adhesion Science, Royal Society of Chemistry. Comyn J (in press), Ch. in Handbook of Adhesives and Sealants, Ed P Cognard. Comyn J, Day J and Shaw S J (1998), J Adhesion, 66, 289. Comyn J, Blackley D C and Harding L M (1993a), J Adhesion, 40, 163. Comyn J, Blackley D C and Harding L M (1993b), Int J Adhes Adhes, 13, 163. Devdas S and Mallik R R (2000), Int J Adhes Adhes, 20, 341. Drago R S, Vogel G C and Needham T E (1971), J Amer Chem Soc, 93, 6014. Ellis B (1993) (Ed), Chemistry and Technology of Epoxy Resins, Blackie Academic & Professional, Glasgow. Fowkes F M (1964), Ind Eng Chem, 56, (12) 40. Gilmore P T, Falabella R and Laurence R L (1980), Macromolecules, 13, 880. Harris A F and Beevers A (1999), Int J Adhes Adhes, 19, 445. Hartshorn S R (Ed) (1986) Structural Adhesives, Chemistry and Technology, Plenum Press, New York. Jeffrey G A (1997), An Introduction to Hydrogen Bonding, Oxford University Press. Johnson K L, Kendall K and Roberts A D (1971), Proc Roy Soc London, A324, 301. Kendall K (2001), Molecular Adhesion and its Applications: The Sticky Universe, Kluwer Academic/Plenum Publishers, New York. Kinloch A J (1987) Adhesion and Adhesives; Science and Technology, Chapman & Hall, London, p. 1. Lee T C P (1999), Properties and Applications of Elastomeric Polysulfides, Rapra Review Report 106, Rapra Technology Ltd, Shawbury. Mallik R R, Pritchard R G, Horley C C and Comyn J (1985), Polymer, 26, 551. Mallon S and Hill C A S (2002), Int J Adhes Adhes, 22, 465. May C A (1988) (Ed), Epoxy Resins, Chemistry and Technology, 2nd edn., Marcel

What are adhesives and sealants and how do they work?

51

Dekker Inc., New York. Nagae S and Nakamae K (2002), Int J Adhes Adhes, 22, 139. Owens D K (1970), J Appl Polymer Sci, 14, 1725. Pizzi A (1983) (Ed), Wood Adhesives: Chemistry and Technology, vol. 1, Marcel Dekker. Pizzi A (1989) (Ed), Wood Adhesives: Chemistry and Technology, vol. 2, Marcel Dekker. Price F P, Gilmore P T, Thomas E L and Laurence R L (1978), J Polymer Sci, Polymer Symposium, 63, 33. Randow C L, Williams C A, Ward T C, Dillard D A, Dillard J G and Wightman J P (1997), J Adhesion, 63, 285. Satas D (1999) (Ed), Handbook of Pressure Sensitive Adhesive Technology, 3rd edn, Satas and Associates. Schreiber H P and Ouhlal A (2003), J Adhes, 79, 135. Shahid M and Hashim S A (2002), Int J Adhes Adhes, 22, 235. Skeist I (1990) (Ed), Handbook of Adhesives, 3rd edn, Van Nostrand Reinhold. Smith M J, Dai H and Ramani K (2002), Int J Adhes Adhes, 22, 197. Titow V W, Loneragan R J, Johns J H T and Currell B R (1973), Plast Polym, 41, 149. Toyama M, Ito T and Moriguchi H (1970), J Appl Polymer Sci, 14, 2039. Voyutskii S S (1963), Autohesion and Adhesion of High Polymers, Interscience. Voyutskii S S (1971), J Adhesion, 3, 69. Warson H (1993), Polymer Emulsion Adhesives, Solihull Chemical Services. Wu S (1973), J Adhesion, 5, 39. Zhou Z and Frazier C E (2001), Int J Adhes Adhes, 21, 259. Zosel A (1998), Int J Adhes Adhes, 18, 265.

3

Surfaces: how to assess J F WATTS

3.1

Introduction

Adhesive bonding relies on the establishment of intermolecular forces between a substrate and the polymeric adhesive itself. To this end it is invariably necessary to pretreat the substrate in some manner so as to confer the required surface properties; this may be a simple abrasion treatment, or a more sophisticated method such as acid anodising. In a similar vein, chemical methods such as a corona discharge treatment used on polyolefins, or the application of a primer solution based on an organosilane adhesion promoter, may be used to ensure the required durability of an adhesive joint. In all cases, the performance of the adhesive joint is directly related to the successful application of such a pretreatment, and an important part of the development of a new pretreatment procedure or the quality assurance of an established process is the assessment of the surface characteristics, both in terms of topography and chemistry. This chapter considers the methods that are commonly used by the adhesive bonding technologist for the assessment of the surface characteristics of solid substrates prior to bonding. The methods that will be covered are the investigation of surface topography by stylus profilometry, electron microscopy and scanning probe microscopy, the assessment of the wetting and spreading of liquids on solid surfaces and the surface chemical analysis of surfaces by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Depending of the size of the organisation, some or all of these methodologies will be available in-house but, particularly in the case of surface chemical analysis, it may be necessary to use expertise from a commercial analysis company or an established university research group. The need to assess the surface properties of an adherend is most likely related to one of two rather fundamental questions: on the one hand, there is a need to know the condition of the surface as delivered (from internal or external sources): this encompasses the need to be aware of the presence of temporary protective coatings that may give rise to weak boundary layers in the eventual adhesive joint. But also, there is a requirement to determine the chemical and physical changes that have been brought about by a specific pre-treatment applied as part of the

Surfaces: how to assess

53

adhesive bonding system. In the latter category, the need may be related to a quality assurance requirement, but it is more likely to be encountered during the development of a new or improved pre-treatment process.

3.2

Surface topography

3.2.1 Scanning electron microscopy (SEM)* Surface topography is one of the most important surface characteristics of metallic substrates and the usual manner of investigation is the use of a scanning electron microscope to provide a high magnification image of the material under investigation. Optical (or light) microscopy is not really sufficient, not because it lacks the range of magnification of a SEM, although this is an important feature, but because of its poor depth of field and depth of focus. In optical microscopy, features not in the image plane appear either under or over focused (i.e. blurred), whereas a SEM is able to accommodate very large depths of field as exemplified by SEM images of small insects that often appear in the popular press. As the general operating principles of an SEM are generally well appreciated, they will not be repeated here but the reader who needs a brief overview of operating principles is referred to standard texts such as Goodhew et al. (2000). The importance of surface topography is illustrated by the images of Fig. 3.1. These are from nominally the same sheet steel stock and the difference in surface structure is readily apparent. The same steel treated by emery abrasion or by gritblasting is shown in Fig. 3.2 and the enhanced rugosity that is provided by such simple mechanical treatments is clearly seen. This effectively increases the degree of interfacial contact area between the adhesive and the substrate and, at a very simplistic level, may enhance the level of adhesion and durability so obtained. It is sometimes convenient to use a SEM to examine failure surfaces of joints. The micrograph of Fig. 3.3 is the failure surface of an aluminium substrate bonded with a structural adhesive from Sautrot et al. (2005). Although on the basis of the microscopy one would tend to classify this as an interfacial failure, there are a few small islands of adhesive left on the metal side of the failure surface which vary in size from a few micrometres up to around 100 m. As the polymer is an insulating material it will generally charge during electron microscopy and this is evident in the micrograph of Fig. 3.3 as darker contrast around the adhesive residue. As this chapter will show later, the definition of the locus of failure is a rather complex task and depends on the level of sophistication of the assessment methods available, but for the time being the example of Fig. 3.3 will be considered an interfacial failure. Some pretreatments lead to characteristic morphologies on a very fine length scale which can be clearly defined only by high resolution SEM. The most * SEM is taken to mean scanning electron microscope or scanning electron microscopy depending on context.

54

Adhesive bonding

Figure 3.1 Appearance of cold rolled steel surfaces, nominally the same but from different batches.

widely cited example of this type of surface is the classic work of Venables (1984) on the morphology of acid anodised aluminium. The approach taken by Clearfield et al. (1991) was to record stereo pair SEM micrographs and to present these along with an isometric drawing of the supposed morphology. These micrographs are, quite rightly, regarded as classics in the adhesion bonding literature and Fig. 3.4 shows a set of images (the stereo pair micrographs and the associated isometric view) of phosphoric acid anodised aluminium, the standard pretreatment for the adhesive bonding of aluminium for aerospace applications in the USA.

Surfaces: how to assess

55

Figure 3.2 Characteristic morphology of (a) abraded steel and (b) grit-blasted surfaces.

In the case of polymeric substrates, SEM is not quite so useful. The modifications brought about by an adhesion enhancing treatment, such a corona discharge, are quite subtle and the need to coat the insulating surface with carbon, gold, gold-palladium, or a similar material to prevent electrostatic charging, can add another level of complexity to sample preparation. Although SEM is useful for identifying debris at a surface or delamination in the case of composite materials atomic force microscopy (AFM) is generally preferred to

56

Adhesive bonding

Figure 3.3 Interfacial metal failure surface from grit-blasted, adhesively bonded aluminium (courtesy of Marie-Laure Abel and Marie Sautrot).

SEM for the examination of polymer surfaces as images are not blurred by subsurface information as may occur with the SEM.

3.2.2 Stylus profilometry In any assessment of surface roughness it is desirable to move away from the qualitative images of the SEM to an approach which can provide a quantitative assessment of surface roughness. This can be achieved by a variety of techniques including the scanning probe microscopies of scanning tunnelling microscopy and atomic force microscopy, but the most straightforward method is the use of stylus profilometry. This is a standard metrology tool which is widely used in engineering to assess the surface profile (or roughness) of a machined component. The concept is simple in that a diamond stylus is dragged across the surface and records the short range undulations (roughness) and long range undulations (waviness) of the surface in a graphical manner, either from direct deflection of the stylus or by using an interferometric approach. The interpretation of roughness data is considered at length in the relevant national standards (e.g. BSI (1972), DIN (1990)) and international standards (e.g. ISO (1997)), but the most important terms are roughness average (sometimes known as centre line average), Ra, and RMS roughness, Rq. The term Rz to define maximum excursion of the profile from the hypothetical centre line is sometimes used but is not very helpful. Figure 3.5 shows some profiles and

Surfaces: how to assess

57

Figure 3.4 High resolution SEM stereo pair images of PAA aluminium and sketch of the proposed morphology.

indicates some of the potential problems associated with the interpretation of stylus profilometry. One of the main issues with the data is that the vertical scale is often greatly magnified in relation to the horizontal scale, as indicated in Fig. 3.5(a±c). In Fig. 3.5(b) the 1 cm marker in the horizontal direction is equivalent to 670 m in the vertical direction while in Fig. 3.5(c) the dimension reduces to

58

Adhesive bonding

Figure 3.5 Stylus profilometry data, (a) real profile of a surface (aspect ratio 1:1), (b) the same surface with as aspect ratio of 15:1, (c) aspect ratio of 300:1, (d) definition of surface roughness.

34 m. The terms Ra and Rq introduced above can be defined by reference to Fig. 3.5(d) as follows: Z 1 l Ra ˆ jy…x†jdx l 0 1 Rq ˆ l

Z

l 0

‰y…x†Š2 dx

Additional parameters are available to describe the bearing area, the autocorrelation coefficient as well as many others which are described in the standards cited above. The main disadvantage of such an approach is that although it gives quantitative information regarding the deviation from the centre line of the profile, it tells us nothing about the distribution of heights, the length scale of the surface profile, or the variations as a function of distance along the length of the scan. For this reason, profilometry, when used for reasons other than to check the profile of a machined component for metrology purposes, should always be combined with a microscopic technique to visualise the surface (e.g. SEM or AFM). The stylus profilometry results for the steel surfaces of Figs 3.1 and 3.2 are presented in Fig. 3.6, while the data of Table 3.1 gives the Ra values of the steel surfaces recorded from the profiles of Watts and Castle (1984).

Surfaces: how to assess

59

Figure 3.6 Stylus profilometry traces for the steel surfaces (a) grit blasted (Fig. 3.2(a)), (b) as received (Fig. 3.1(a)), (c) emery abraded (Fig. 3.2(b)).

Table 3.1 Ra value for various steel pretreatments Steel pretreatment Diamond polish Emery abrasion As received I As received II Grit-blast

SEM

Ra/m

ö Fig. 3.2(a) Fig. 3.1(a) Fig. 3.1(b) Fig. 3.2(b)

0.05 0.85 1.70 1.70 3.80

3.2.3 Scanning probe microscopy (SPM) Scanning probe microscopy consists of two related, but rather different, techniques based around scanning tunnelling microscopy (STM) and atomic (or scanning) force microscopy (AFM or SFM). STM can produce very high spatial

60

Adhesive bonding

resolution and readily achieves atomic visualisation. The principle is based on the ability of electrons to tunnel between the surface of a conducting sample and a fine probe held close (about 1 nm) to the surface. The sample is located on a piezoelectric scanner which moves the sample under the tip, to adjust tip-sample distance, and also scans the sample relative to the tip to enable an image of surface topography to be built up. There are two basic modes of operation in STM, the constant height mode and the constant current mode. The tunnelling current decays exponentially with distance from the surface. Thus, if the distance changes by 10% (i.e. about 0.1 nm) the tunnelling current will change by an order of magnitude. In the constant height mode the tip travels in the horizontal plane relative to the sample surface and the tunnelling current varies with topography and local electronic properties of the surface. The tunnelling current, at each pixel point, is essentially able to provide the topographic image of the surface. In the constant current mode the motion of the scanner provides the data set, as a feed back loop ensures that the tunnelling current is kept constant within very stringent constraints. This method is slower than the constant height mode but is better suited for analysis of very irregular surfaces. Figure 3.7 shows a STM image from Zhdan (2002) which illustrates atomic resolution of copper in the constant current mode. Whilst STM is an important technique for the surface scientist, and is often operated in an ultra-high vacuum enclosure it is perhaps a little esoteric for the

Figure 3.7 A STM image at atomic resolution of an annealed copper surface.

Surfaces: how to assess

61

Figure 3.8 Schematic of an atomic force microscope, showing tip, cantilever, piezoelectric scanner and detector assembly.

adhesion scientist and the SPM methodology that is most widely used in the assessment of surfaces for adhesive bonding is the AFM. A schematic of an AFM is shown in Fig. 3.8 and, although closely related to the STM, the basic principle relies on the attraction between a sharp tip (often of silicon nitride) and the surface under examination. The tip is located at the free end of a cantilever of low spring constant (about 1 Nmÿ1). The force between the tip and the sample causes the cantilever to deflect. A detector arrangement, based on a laser reflecting from the cantilever and a quadrant array photodetector, records the deflection of the cantilever as the tip is scanned relative to the sample. The extent of deflection of the cantilever can then be used to produce an image of surface topography. The forces involved are repulsive at very close proximity (about 0.1 nm) but, as the cantilever is withdrawn, they become attractive as shown in Fig. 3.9. This leads to the definition of two forms of AFM operation, contact (about 0.2 nm distance on Fig. 3.9) and non-contact AFM (0.4±0.6 nm). In contact AFM, the tip makes gentle contact with the sample and, in the case of very soft or delicate samples (pressure sensitive adhesive tape for example), may indent the surface giving erroneous results. To overcome this problem, the tip may be vibrated near the surface (at a mean distance of 1±10 nm). However, the force between tip and sample may be very small (10ÿ12 N), making it more difficult to measure than in the contact mode, which is several orders of

62

Adhesive bonding

Figure 3.9 Force curve showing interatomic force as a function of distance.

magnitude higher. There is also the possibility of the tip being pulled into the sample, so stiffer cantilevers are sometimes required. A compromise between these two modes of operation is tapping mode AFM (TM-AFM) in which a sinusoidal vibration is applied to the tip and it gently taps against the surface. Since shear and lateral forces are reduced, sample damage is, in most cases, negligible. AFM has a wide range of magnifications from close to atomic resolution to approximately 500, so in many ways it competes with the SEM as a method of assessment of surface topography. The complementarity of AFM and SEM have been discussed at some length by Castle and Zhdan (1997). Instruments are now available in portable format, low cost versions, geometries to handle very large specimens, and a plethora of versions handle difficult samples such as cells and wet surfaces, making it a very versatile technique indeed. As far as those involved in adhesive bonding are concerned, the advantages of an AFM over other forms of surface characterisation are threefold. First, all samples, metal, polymer, ceramic, viscoelastic adhesives and so on can be handled successfully without the need for further sample processing. Second, by processing of the image data as a line scan, it is possible to carry out a `sectional analysis', which is essentially stylus profilometry at the nanometric scale. The third use of the AFM is as a localised surface forces apparatus. In this mode of operation one can measure the forces between tip and specimen surface directly, and by using tips functionalised with particular chemical species it is possible to probe chemically heterogeneous region at the surface. This can be particularly useful for probing the adhesion properties of a polymer blend, for instance. In a similar

Surfaces: how to assess

63

Figure 3.10 Polyolefin film following corona discharge treatment for (a) 0 s, (b) and (c) 20 s and (d) following peel test.

manner force modulation can be used to probe localised elastic properties in the cross-section of a polymer composite, or adhesive joint: and in this manner, an interphase zone can be identified mechanically, as illustrated in the elegant work of Gao and Mader (2002). The utility of AFM in adhesion research is shown in the following example related to the corona treatment of polyolefin film, from the work of Zhdan (2002). In its untreated form the polymer film appears rather flat and featureless as in Fig. 3.10(a). But, as corona treatment proceeds, so the gradual development of `polypoids' on the surface becomes apparent, as in Fig. 3.10(b). A higher magnification view, presented in `plan' view is shown in Fig. 3.10(c), whilst Fig. 3.10(d) shows the appearance of the bonded surface following a peel test. This phenomenon is a general occurrence on the discharge treatment of thermoplastic polymer films and is thought to relate to the heterogeneous nature of the discharge, regions of high intensity leading to localised melting of the polymer giving this characteristic morphology. The change in morphology is not the whole story, and at the same time as such morphological changes occur, so the

64

Adhesive bonding

surface oxygen concentration increases, leading to an increase in surface free energy (see following sections).

3.3

Surface thermodynamics

3.3.1 Wetting and spreading of liquids on solid surfaces Following the well known Young Equation:

sv ˆ sl ‡ lv cos  representing the equilibria established by a sessile drop on a solid surface, a contact angle () can be defined, as in Fig. 3.11. This represents the angle of the tangent of the drop at the triple point between solid, liquid, vapour and the free energy of the solid substrate sv, and the interfacial free energy of the liquid and solid sl. The surface free energy, or surface tension, of the liquid lv will be known and  provides a readily observed manifestation of the interaction of a liquid with a solid. Thus, if we consider water as the wetting liquid, a high surface energy substrate such as an oxide will wet fairly readily, while a low surface energy solid such as a polymer will not wet so readily and the liquid will form a very high contact angle (perhaps even discrete spheres) on the surface. Thus, the simple expedient of observing the characteristics of a small drop of water on a solid substrate tells the observer much about the free energy and wettability of the solid surface. This can be important in two quite different areas, the degreasing of metals and the surface treatment of polymers. Although the Young Equation forms the underpinning basis for understanding the behaviour of the solid/liquid interface and the spreading of liquids on a surface, there are several routine tests that find widespread use that merely offer a go/no-go situation and can be used quite satisfactorily for quality assurance purposes by untrained personnel. The two most popular approaches in this category are the water break test and the use of dyne pens.

Figure 3.11 Thermodynamic equilibria of a sessile liquid drop on a solid substrate.

Surfaces: how to assess

65

3.3.2 The water break test There are various forms of the water break test, which is essentially a method for assessing the surface cleanliness of metal substrates, for assessing the effectiveness of a cleaning process in the removal of any residual organic contamination resulting from protective greases or mechanical working lubricants. Such carbonaceous films will be hydrophobic (non-wetting) in nature, and the test involves withdrawing the metal panel under test from a container full to the brim with distilled water. On withdrawing a clean substrate, the water will drain uniformly over the surface. In the presence of residual contamination, the draining water film will break up into a discontinuous layer around the contaminated regions. Although this is a very subjective test it is quick to carry out and lends itself to process control purposes. One form of the test is embodied in the relevant US standard (ASTM, 2002).

3.3.3 Dyne test markers The name of this method requires a little explanation. The unit of surface free energy in the SI system is mN mÿ1 (equivalent to mJ mÿ2) but within the (old) cgs system, the numerically equivalent unit is a dyne cmÿ1. The concept of liquid test markers was developed many years ago when the cgs system was current and, for this reason, they are still universally referred to, by users and manufacturers alike, as dyne pens or markers. The concept of dyne markers is extremely simple. They are usually supplied as a kit containing pens with `inks' of well defined surface tensions, usually between 30 and 60 nN mÿ1. In order to assess the surface under test, and they are used almost exclusively to establish the printability of polymer surfaces following surface treatment, the marker pen is applied to the substrate. If it marks (i.e. the liquid wets) the surface, then the substrate has been treated to a level required for wetting to take place. These markers will always have a coloured dye included in the liquid so that they can be used in the conventional sense as marker pens for product identification. In general, these markers will be used as a go/no-go test during processing, or to identify the treated side of a polymer film. By using a range of dyne pens, it is possible to estimate the surface tension of a liquid that will just wet the surface and thus gain an indication of the level of treatment of the polymer surface. The advantage of this system is that it is quick, cheap (<¨10 per pen) easy to use and, if the pen is new, can provide quite accurate results. The drawbacks are the short shelf life of pens, easy contamination of the liquid and the lack of a numerical value relating directly to substrate properties, so only a simple ranking order can be deduced. Notwithstanding such shortcomings dyne markers continue to play an important role in the surface treatment industry.

66

Adhesive bonding

3.3.4 Determination of surface free energy The determination of the surface free energy of a solid substrate is a rather complex process and, for many adhesive bonding situations, it is not strictly necessary as the determination of the contact angle can sometimes be sufficient. However, the contact angle may also act as a starting point for calculating the surface free energy. The contact angle may be recorded by a number of methods, but the most popular are by direct observation of the Wilhelmy plate method which makes use of a force balance to evaluate the force necessary to withdraw a glass slide if necessary coated with the polymer of interest from the test liquid. Many commercial instruments are available and all will be supplied with a dedicated computer which will contain the necessary software to make calculations of surface free energy and related parameters. It is possible to assess the surface free energy by extending the Young Equation and by recognising that the surface free energy can be sub-divided into components that represent the degree of bonding attributable to dispersion forces ( D), polar forces ( P), hydrogen bonding ( H), and so forth, such that:

ˆ D ‡ P ‡ H ‡ . . . where the exact number of components depends on the material type and thus the bonding types involved. It is possible to arrive at the following relationship, the derivation of which is given by Packham (1992): 1 ‡ cos  ˆ 2

D 1=2 …yD … P P †1=2 L S † ‡2 L S … L † … L †

where the subscripts S and L represent the solid substrate and wetting liquid respectively. The approach involves the use of a series of liquids, of known values of the dispersive and polar contributions to surface free energy, to measure the contact angles on the solid substrate of interest. This then yields a number of simultaneous equations of the type shown above. As they contain only two unknowns, the dispersive and polar contributions of the surface free energy of the substrate, these can be readily evaluated. An alternative approach is to rearrange the equation in the form: " # P 1=2

L …1 ‡ cos † P 1=2 … L † ˆ … S † ‡ … SD †1=2 … LD †1=2 2… LD †1=2

L …1 ‡ cos † The equation is now in the form of y ˆ mx ‡ c, and a graph of " # 2… LD †1=2 … LP †1=2 can be plotted against ; the gradient of the best-fit line will be … SP †1=2 , … LD †1=2

and the line intercept … SD †1=2 . The surface energy of the unknown solid, S, is then the sum of the two terms, S ˆ SD ‡ SP . Data of this type for poly(dimethyl siloxane) (PDMS) tested with water, diodomethane and hexadecane is shown in

Surfaces: how to assess

67

Figure 3.12 Wetting curve for water, di-iodomethane and hexadecane on PDMS; inset shows the contact angle for the three liquids on PDMS (data from Choi (2003)).

Fig. 3.12. Analysis of Fig. 3.12 yields the following values of surface free energy for PDMS: D = 0 mJ mÿ2, P = 22 mJ mÿ2, = 22 mJ mÿ2.

3.4

Surface chemical analysis

Compared with the water break test or dyne pens, the instrumentation here presents investment at the other end of the financial scale, with a capital cost of the order of ¨1.0m. being necessary. Within the category of surface chemical analysis, the techniques being considered are X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These methods find various uses in adhesive bonding investigations, but there are several main areas of investigation. These include the assessment of surface properties, the removal of contamination or the investigation of the chemistry of surface pretreatments; the forensic analysis of adhesive bonding failures, the study of interfacial failure surfaces with the aim of identifying the presence of weak boundary layers or other phenomena responsible for failure and finally the direct probing of the interfacial chemistry responsible for adhesion itself. The last named is perhaps the holy grail of the adhesion scientist and presents the most challenging scenario for surface analysis. The eventual aim is to engineer specific chemistry of an interface so as to provide specific properties in the adhesive joint.

68

Adhesive bonding

3.4.1 X-ray photoelectron spectroscopy and Auger electron spectroscopy As XPS and AES are both techniques based on low energy emitted electrons, they will often be found together on the same system as they make use of the same analyser. It is the analytical use of low energy electrons that gives these two techniques their surface specificity, with analysis depths of 6 nm being representative of both techniques; the figure depends on material type and electron kinetic energy and can be increased slightly or reduced to monolayer dimensions by judicious adjustment of experimental parameters. The interested reader is referred to standard texts for further details on these techniques such as Watts and Wolstenholme (2003) and Briggs and Grant (2003), but the basic principle will be reviewed here to provide a backdrop to their use in adhesion science. The essential difference between the two is that XPS makes use of an X-ray beam (usually AlK or MgK) whilst AES uses a finely focused electron beam. The spatial resolution attainable with the two techniques varies with system specification but, on current systems, XPS will be in the regime of 1±0.5 mm for a standard system with spot sizes as small as 10 m for a high performance small area XPS system. In the case of AES one would expect electron beam sizes of 50 m down to 20 nm. Thus, AES provides a surface analysis at high spatial resolution but, as a result of the use of a primary electron beam, it is applicable only to conducting surfaces such as metals. XPS, on the other hand, is able to provide quantitative analysis of the surfaces of all materials providing they are stable within the UHV chamber of the spectrometer. XPS is also known by the acronym ESCA, electron spectroscopy for chemical analysis, and it is the chemical specificity of XPS that has made it a popular choice for surface analysis in adhesion science as described by Watts (1987, 1988, 1998) and Watts and Abel (2001) in several reviews over the last two decades. An example of the surface specificity of XPS is readily seen in the two XPS survey spectra of Fig. 3.13. Figure 3.13(a) is the spectrum from an extremely clean aluminium surface: the surface composition calculated from this is shown within the figure and the level of adventitious carbon contamination adsorbed from the atmosphere is seen to be very low at 17.0 atomic %. When the surface is deliberately handled prior to analysis, the spectrum changes quite dramatically as shown in Fig. 3.13(b), the carbon level increases (to 34.7 at%) and the silicon 2p and 2s peaks become well defined features of the spectrum, presumably because of the presence of silicone oils present in the hand cream used by the operator. This example shows not only the extreme surface sensitivity of XPS, but also the need to be extremely careful, when touching surfaces. The chemical specificity refers to one of the major strengths of XPS in that it can readily distinguish between a particular element in different environments by way of the XPS chemical shift. To extract information, it is necessary to

Surfaces: how to assess

69

Figure 3.13 XPS survey spectra of aluminium, (a) as received and (b) following handling contamination. The boxes above the spectra indicate the surface composition, in atomic percent, of each element (courtesy of Marie-Laure Abel and Sue Doughty).

record the electron spectra at high resolution. The carbon 1s spectrum of a urea formaldehyde/epoxy coating is shown in Fig. 3.14 and the complexity of the spectrum indicates that there is more than one component in the spectrum. By adopting the appropriate peak fitting procedure, it is possible to unravel the complexities and to assign the individual components to the appropriate chemistry as indicated on the spectrum. Auger electron spectroscopy is not so widely used in adhesion and adhesives research although, when used in conjunction with inert ion sputtering, it can provide compositional depth profiles of, for instance, acid anodised conversion coatings. XPS can also be used for depth profiling in this way, and AES can also provide chemical state information of many of the elements of the periodic table. This juxtaposition of many of the advantages of the two electron spectroscopic methods has meant that in recent years, XPS, with its ready quantification and applicability to polymers and ceramics, has tended to be the method of choice, AES being used only when sub-micrometre spatial resolution is required.

70

Adhesive bonding

Figure 3.14 C1s spectrum showing the variety of chemical shifts associated with carbon 1s electrons from a complex organic coating.

3.4.2 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) Secondary ion mass spectrometry is a surface mass spectrometry technique. The solid sample under investigation being bombarded by a primary ion beam (Ga+, Ar+, Cs+, C60+ are often used) which sputters atomic ions, cluster ions and neutrals from the surface. By extracting the ions and injecting them into an appropriate design of mass spectrometer, it is possible to obtain a mass spectrum of the sample surface. The sputtering process implies that the SIMS process is essentially destructive as material is removed from the sample surface. This is the basis of a technique known as dynamic SIMS in which the intensities of particular ions are monitored as a function of sputter time to provide a compositional depth profile. This technique gives excellent depth resolution and is widely used in the microelectronics field, although the method does not provide a surface analysis. To achieve this, the ion dose must be kept below a critical value of 1013 ions cmÿ2 for the duration of the analysis procedure. This is known as the static SIMS (SSIMS) regime, as only about 1% of the surface atoms, ions or molecules are disturbed. The favoured mass spectrometer for SSIMS is the time-of-flight design as it has a very high transmission compared with other designs and provides parallel (rather than sequential) detection of all ions that will be used to generate the spectrum. Added to these features it can

Surfaces: how to assess

71

also provide very high mass resolution (ion masses are routinely quoted at 0.0001 Da*), making it the analyser of choice for SSIMS analyses today. By using a pulsed electron flood gun, insulating samples are readily analysed in both the positive SIMS and negative SIMS modes. SSIMS provides an analysis depth that is slightly less than XPS and AES but has the additional advantage that it can provide molecular specificity, rather than the chemical environment deduced from the XPS chemical shift. It can also provide analyses at high spatial resolution (< 1 m) as mass selected images and can be used for depth profiling in conjunction with a high flux ion source. ToFSIMS has been fully described by Briggs and Vickerman (2001), and it is becoming an increasingly important complementary technique to XPS in the adhesion field, but the ready ability of XPS to provide a quantitative analysis, and the apparent relative simplicity of the spectra, will ensure that it remains a valuable and widely used surface analysis method for the foreseeable future. As an example of the analytical utility of ToF-SIMS, it is convenient to consider the interaction between an organosilane adhesion promoter and a gritblasted aluminium surface as described in the work of Abel et al. (2000a). This combination has the potential to replace Cr(VI) containing solutions for the pretreatment of aluminium for structural adhesive bonding in the aerospace

Figure 3.15 Interaction between an organosilane adhesion promoter and an aluminium surface. * Dalton (Da) is the unit preferred in mass spectrometry indicating unified atomic mass/charge (m/ Z). It should be noted that unified atomic mass (u) is not the same as atomic mass unit (amu) in that the former is relative to the 12C isotope (u ˆ ‰12 12 C=12) whilst the latter is relative to the 16O isotope (amu ˆ ‰16 16OŠ=16). The difference between the two is, however, small, being 318 ppm.

72

Adhesive bonding

Figure 3.16 High resolution ToF-SIMS spectrum on the m/z = 71 Da region showing the Si-O-Al+ ion indicative of a covalent bond of the type indicated in Fig. 3.15.

industry. The silanol groups of the adhesion promoter interact with hydroxide groups on the metal surface, undergoing a condensation reaction to provide a covalent bond between the substrate and the organosilane with water as the condensation product, as shown in Fig. 3.15. A high resolution ToF-SIMS spectrum, Fig. 3.16, shows the presence of a Al-O-Si fragment at 70.9534 Da, indicative of the formation of a covalent bond. The spectrum of Fig. 3.16 is much less than 1 Da wide and other fragments at a nominal mass of 71 are

Surfaces: how to assess

73

clearly seen. Thus, without the outstanding mass resolution of the ToF analyser, it would not have been possible to resolve the individual components; others are attributable to hydrocarbon and organosilicon fragments. This work forms part of an ongoing investigation regarding the mode of action of organosilane adhesion promoters in the author's research group. Other important results have been the identification of the manner in which the organofunctional end of the molecule interacts with a structural adhesive as shown by Rattana et al. (2002), and the use of XPS and ToF-SIMS to study the interaction of epoxy analogue molecules with aluminium and silane treated aluminium as described by Abel et al. (2000b).

3.5

Concluding remarks

There is a wide variety of reasons that leads one to assess the properties of a substrate for adhesive bonding. These range from simply ensuring it is clean, through the need to assess the quality of a pre-treatment, or perhaps to relate it to performance, to the forensic examination of failure surfaces either from a laboratory test or an in-service failure. The variety of possible methods that can be used is equally as broad, ranging from a simple water break test to a sophisticated method such as surface chemical analysis. The choice of test method will depend on many factors, not least the test environment, financial constraints, and the aim of making the analysis. Quality assurance and research activities will have very different requirements. To obtain a clear, concise, and accurate picture of a surface it will be essential to use more than one method and the best combination is probably to use AFM for surface topography, contact angle measurements to deduce the surface free energy, and XPS to provide surface chemical analysis. This will enable quantitative comparison of the important characteristics of the surface and enable the changes brought about by, for example, surface treatment, to be related to adhesive bond performance.

3.6

Acknowledgements

The author thanks Dr Peter A Zhdan and Dr Marie-Laure Abel for the provision of AFM and XPS/TOF-SIMS data respectively.

3.7

References

Abel M-L, Fletcher, I W, Digby, R P, Watts, J F (2000a), Surf Interf Anal, 29, 115±125. Abel, M-L, Rattana, A, Watts, J F (2000b), Langmuir, 16, 6510±6518. ASTM (2002), Standard F 22-02: Standard Test Method for Hydrophobic Surface Films by the Water-Break Test, ASTM International, West Conshohocken, PA, USA. Briggs, D, Grant, J T (2003), Surface Analysis by X-Ray Photoelectron Spectroscopy and

74

Adhesive bonding

Auger Electron Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester and Manchester, UK. Briggs, D, Vickerman, J C (2001), ToF-SIMS: Surface Analysis by Mass Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester and Manchester, UK. BSI (1972), British Standard BS 1134: Part I. Assessment of Surface Textures: General Information and Guidance. Castle, J E, Zhdan, P A (1997), J Phys D Appl Phys, 30, 722. Choi, J W (2003), PhD Thesis, University of Surrey. Clearfield, H M, McNamara, D K, Davis G D (1991), in Adhesive Bonding, ed. L-H Lee, Plenum Press, New York, USA, pp. 203±237. DIN (1990), DIN 4768, The Determination of Roughness Parameters Ra, Rz, Rmax by Means of Stylus Instruments: Terms and Measuring Conditions. Gao, S L, Mader, E (2002), Composites: Part A, 33, 559. Goodhew, P J, Humphreys, F J, Beanland, R (2000), Electron Microscopy and Analysis, Taylor & Francis. ISO (1997), ISO 4287, Geometric Product Specifications ± Surface Texture: Profile Method ± Terms, Definitions and Surface Texture Parameters. Packham, D E (1992), Handbook of Adhesion, Longman Science and Technical, Harlow, Essex, UK. Rattana, A, Hermes, J D, Abel, M-L, Watts, J F (2002), Int J Adhes Adhes, 22, 205±218. Sautrot, M, Abel, M-L, Watts, J F (2005), J Adhes, in press. Venables, J D (1984), J Mater Sci, 19, 2431. Watts, J F (1987), in Surface Coatings 1, eds A D Wilson, H Prosser and J W Nicholson, Applied Science Publishers, London, pp. 137±187. Watts, J F (1988), Surf Interf Anal, 12, 497±503. Watts, J F (1998), in Handbook of Surface and Interface Analysis: Methods for Problem Solving, eds J C Riviere, S Myhra, Marcel Dekker Inc, pp. 781±833. Watts, J F, Abel, M-L (2001), in State-of-the-Art Application of Surface and Interface Analysis Methods to Environmental Materials Interactions: In Honour of James E Castle's 65th Year, eds D R Baer, C R Clayton, G D Davis, G P Halada, The Electrochemical Society, Pennington, NJ, USA, Volume 2001 ± 5, pp. 80±91. Watts, J F, Castle, J E (1984), J Mater Sci, 19, 2259±2272. Watts, J F, Wolstenholme, J (2003), An Introduction to Surface Analysis by XPS and AES, John Wiley & Sons Ltd, Chichester, UK. Zhdan, P A (2002), Surf Interf Anal, 33, 879.

4

Surfaces: how to treat D BREWIS

4.1

Introduction

To achieve a satisfactory adhesive bond, it is usually necessary to carry out some form of pretreatment. Many pretreatments are available ranging from a simple solvent wipe to the use of a series of complex chemical processes. The method chosen depends on the nature of the substrate, the conditions to which the adhesive joint will be subjected, safety, environmental factors and cost. A pretreatment can act by removing potential weak boundary layers, WBLs, by altering the substrate topography, by modifying the chemistry of the substrate surface or by a combination of these mechanisms. If a cohesively weak layer is present in an adhesive joint then failure will occur at a low applied load. Such a layer is often termed a weak boundary layer. Typical examples include lubricants, polymer additives and weak metal oxides. Topography affects the level of adhesion because, together with the rheological properties of the adhesive and the surface energies of the adhesive and substrate, it will determine the degree of contact between the adhesive and substrate. Some combinations of adhesive rheology and topography may result in mechanical keying. Surface chemistry is important because it affects the wetting of a surface and the degree of interaction across the substrate-adhesive interface where wetting has occurred. Treatments may be divided into physical and chemical methods. The former includes solvent degreasing and grit blasting. Physical treatments may remove cohesively weak layers from a substrate, i.e. potential WBLs, and they may also modify topography. Chemical treatments, which include the flame treatment of plastics and anodising procedures for metals, by definition cause chemical modification to the surfaces involved. It will be seen that different groups of materials, i.e. metals, inorganic glasses, plastics, elastomers, etc., tend to have their own specific pretreatments. However, some pretreatments are effective with different groups of materials, for example, silanes can greatly enhance the performance of joints involving either metals or inorganic glasses.

76

Adhesive bonding

It is relatively easy to achieve high initial joint strengths. However, a satisfactory performance in service will require careful selection of a pretreatment. Water in particular can cause a serious loss of joint strength in service and pretreatments vary markedly in their ability to provide the necessary durability of the joints involved. To understand the effect of pretreatments on joint performance, it is necessary to have information on the physical and chemical nature of surfaces. Since about 1960, it has been possible to routinely determine the topography of a surface by means of electron microscopy. Since 1960, the performance of electron microscopes, especially in terms of spatial resolution, has increased greatly. Atomic force microscopy AFM, and related techniques, have become available in recent years and these can also provide valuable information on topography. Since about 1970, techniques to provide information on the chemistry of surfaces have become commercially available. These include X-ray photoelectron spectroscopy XPS (also known as ESCA), Auger electron spectroscopy AES and Static secondary ion mass spectrometry SSIMS; the last mentioned technique became available a few years after XPS and AES. These chemical techniques are discussed in Chapter 3 (Section 3.4). Other techniques that have provided useful information include reflection infra-red analysis and the use of contact angles. It is thus possible to acquire direct information on the physical and chemical states of a surface both before and after pretreatment. The use of the above techniques will be exemplified in relation to pretreatments of different types of materials. Primers, usually in the form of thin organic coatings, are often used as an addition or alternative to pretreatments. Primers can provide several advantages. They usually have much lower viscosities than adhesives and can therefore achieve greater contact with the substrate. They can have greater interaction with the substrate and adhesive (see Section 4.3). They can contain corrosion inhibitors; this can be important for metals. Finally, they can protect a surface until the bonding process is carried out.

4.2

Pretreatments for metals

Physical and chemical methods, often in combination, are used to pretreat metals. Physical methods, which include solvent degreasing, abrasion and grit blasting, may be sufficient if the bonding requirements are modest. However, if service conditions are demanding, for example, if the joints will be exposed to water or to high stresses, then it is likely that a chemical treatment will be necessary. Pretreatments for metals have been the subject of much research. This is especially true in the case of aluminium where particular emphasis has been placed on aerospace applications. There is of course great interest in commercial and military aircraft and much research has been carried out by manufacturers of aircraft, defence establishments, adhesive manufacturers, suppliers of

Surfaces: how to treat

77

pretreatment materials and academic institutions. Methods to pretreat aluminium have been reviewed by Critchlow and Brewis.1 Etching of aluminium with chromic acid CAE was found to give greatly enhanced performance compared to physical methods. However, anodising in chromic CAA or phosphoric acids PAA was generally found to be even more effective especially in relation to the durability in wet conditions. Chromic acid is highly toxic and corrosive; further, anodising is a complex multistage process. Much effort has been made to find safer and simpler pretreatments. Alternatives examined include the use of silanes, usually following a grit blasting process, the use of conversion coating and plasma spraying. These have failed to match CAA and PAA treatments. However, anodising in boric-sulphuric acid2 or a.c. anodising in sulphuric or phosphoric acid3 have shown great promise. Further, much effort is being made to optimise the use of silane primers as a viable alternative to CAA and PAA treatments.4 There has been much debate about the reasons for the success of anodising treatments. It is now generally agreed that topography and oxide stability have a critical effect on resultant joint performance. Much research has also been carried out on the pretreatment of titanium alloys. This metal is used in aerospace and other advanced technologies. Treatments to enhance the bondability of titanium have been reviewed by Critchlow and Brewis.5 Considerable success was achieved with an alkaline peroxide etch as shown by the durability data in Table 4.1. However, the method is not suitable for large-scale continuous production. Two anodising methods, one using sodium hydroxide and the other a chromic acid-fluoride mixture have been shown to give excellent durability. This is demonstrated by the wedge test results in Fig. 4.1. Steels are usually used in more mundane applications than aluminium or titanium and cost considerations demand relatively simple pretreatments. Grit blasting, sometimes in conjunction with a silane, is often used with mild steel. Stainless steel is likely to be used in more demanding applications and more Table 4.1 Effect of exposing stressed and unstressed adhesive bonded Ti-6Al-4V double overlap joints, pretreated by the 65 ëC alkaline hydrogen peroxide process, to a hot wet climate6 Exposure period, years 0.5 1 2 3 4

Strength retention, % Unstressed

Stressed 5%

Stressed 10%

Stressed 20%

96.4 94.1 92.8 92.1 91.2

95.0 93.4 94.7 91.5 88.4

97.7 95.1 93.4 91.3 90.8

97.1 96.7 94.7 90.5 90.6

78

Adhesive bonding

Figure 4.1 Wedge test data for titanium alloy exposed to 50 ëC/96% RH (GB: grit blast; AHPE: alkaline hydrogen peroxide etch; C: catalytic AHPE; CAA: chromic acid, fluoride anodise; SHA: sodium hydroxide anodise).6

complex pretreatments may be necessary. Chromic acid etching and anodising in nitric acid provide good durability to the effect of water. Silanes have been shown to enhance the durability of bonded steel structures. Walker,7 for example, demonstrated the benefits of a range of silanes with mild and stainless steels. Gettings and Kinloch8 found that priming with glycidoxypropyltrimethoxysilane ( -GPS) considerably improved the durability of grit blasted steel joints, whereas two other silanes did not. Using secondary ion mass spectrometry (SIMS) they detected FeOSi+ ions with mild steel and FeOSi+ and CrOSi+ ions with stainless steel using -GPS but not the other silanes. This indicates the formation of primary bonds between the steels and GPS (see Fig. 4.2).

4.3

Pretreatments for inorganic materials

Relevant inorganic materials include glasses, ceramics and concrete. Although these materials are very different, the same factors affect adhesion, namely surface chemistry, topography and the cohesive strength of surface regions. For example, a weak region might be represented by organic grease on glass or a cement layer at a concrete surface. To achieve high joint strengths it would be necessary to remove both these layers. In the former case a solvent could be used, whereas the latter could be removed by grit blasting.

Surfaces: how to treat

79

Figure 4.2 Effect of silane coupling agent on the durability of mild steelepoxide joints. Adapted from reference 8.

Glass has been the subject of the most research within this group of materials and discussion will be limited to this material. Glass has a high surface energy provided it is not covered with an organic layer, and under dry conditions it is relatively easy to obtain high joint strengths. However, under wet conditions joint strengths may be greatly reduced due to the strong interaction between glass and water. The adverse effect of water on glass-polymer interfaces was noted in the early 1940s when glass fibre-polymer composites were exposed to water. The strength and stiffness of the composites were dramatically reduced by immersion in water. This led to an extensive research programme to improve bond durability. The result was a range of silane `coupling agents', some examples of which are given in Fig. 4.3. Some of the silanes greatly increased the resistance of the glass-polymer interface to water, thus reducing the loss of mechanical properties. Silanes are usually applied in water, at a concentration of about 1%. Hydrolysis of the alkoxy groups occurs with the formation of silanol groups. SiOR + H2O ÿ! SiOH + ROH

80

Adhesive bonding

Figure 4.3 Examples of commercial silane coupling agents.

The silanol groups in the silane are able to react with a glass surface to form a primary bond (Si±O±Si). The X groups in the silane may be able to react with a chemical group in the adhesive or alternatively chain entanglement between the polysiloxane and adhesive may occur; both these mechanisms lead to a relatively stable bond. The hydrolysed silane thus acts as a bridge or coupling agent between the glass and the polymer. This is true whether the polymer is an adhesive, paint or the matrix of a composite. The mechanisms by which silanes enhance durability have been discussed by Plueddemann.9

4.4

Pretreatments for plastics

Plastics and elastomers have much lower surface energies than metal (oxides) or glass. The interaction between polymers and adhesives is therefore generally much lower than between metals (oxides) and polymers, for example. Within the plastics group surface energies vary considerably. Polytetrafluoroethylene (PTFE), polypropylene (PP) and nylon 66 have values of 19, 30 and 40 mJ mÿ2, respectively. The wetting of PTFE by an adhesive will be less complete than that between nylon 66 and the same adhesive. Also the magnitude of the interaction across the interface where wetting has occurred will be considerably greater in the case of nylon 66. To achieve satisfactory bonding with PTFE, polyethylene (PE), polypropylene and similar plastics it is usually necessary to chemically modify the surfaces of these polymers, i.e., introduce chemical groups which can interact relatively strongly with the adhesives concerned. The range of chemical treatments is outlined below. The treatments have been developed mainly for polyolefins and PTFE although they are sometimes used for other plastics. For plastics containing suitable chemical functionality, for example, the amide group in the case of nylon 66, it may be possible to achieve satisfactory adhesion without a pretreatment. If a cohesively weak layer exists on a plastic such as nylon

Surfaces: how to treat

81

66 then a physical method to remove this layer may be perfectly satisfactory; hence solvent degreasing or grit blasting are frequently used with such plastics. There are many methods to chemically modify the surfaces of polyolefins such as PE and PP. Several of these methods date back to about 1950; these include treatment with a flame, corona discharge, chromic acid immersion and exposure to chlorine gas activated by UV. The first three methods became firmly established for the treatment of PE and later PP. The corona discharge method, which involves decomposing air into active species including oxygen atoms and ozone by the application of a high voltage, is still the preferred method for treating film. Flame treatment, which involves exposing the plastic for a fraction of a second, is still widely used for treating cylindrical objects such as bottles and also for less regular shapes such as car bumpers. Chromic acid has been widely used for treating three-dimensional objects, but environmental considerations make it generally unacceptable. In the 1960s, the use of low-pressure plasmas to improve the bondability of the polyolefins and other plastics was studied; see for example, references 10 and 11. In the 1980s interest was renewed in the uses of halogen gases to pretreat polyolefins. Treatment of PE or PP for a few seconds with mixtures of fluorine and inert gases gives large improvements in bondability.12 Various other pretreatments for polyolefins have been examined, although they have not found widespread industrial use. These include organic peroxides, ammonium peroxydisulphate, and sodium hypochlorite.

4.4.1 Flame treatment The object to be treated, e.g. a bottle, is passed over one or more burners, each of which possesses a large number of closely-spaced jets. The burners are fed with an air-hydrocarbon gas mixture, whose proportions are controlled within definite limits. The treatment time is usually in the region of 0.04±0.1 seconds (Fig. 4.4). In one of the first detailed studies of the flame treatment of a polyolefin Ayers and Shofner13 found that higher peel strengths were achieved with an excess of air over the stoichiometric requirements to burn all the alkane gases used. The authors concluded that the optimum treatment time was about 0.02 s and that the optimum distance between the polymer and the top of the inner cone was about 10 mm. Sutherland and co-workers found that the flame treatment of a PP homopolymer resulted in good adhesion over a broad range of conditions.14 A study of the flame treatment of low-density polyethylene showed very high levels of oxidation, although the oxidised layer was less than 10 nm thick.15

4.4.2 Corona treatment The main application of corona treatment is film for the packaging industry. Film is passed, typically at 3 m sÿ1 over an earthed metal electrode (Fig. 4.5).

82

Adhesive bonding

Figure 4.4 Schematic representation of the flame treatment.

The distance between the electrode and the film is usually 1±2 mm. A high frequency (10±20 kHz) generator and step-up transformer produce a high voltage, which causes the electrical breakdown of the air with the formation of atoms, ions, electrons and other active species. Factors which determine the effectiveness of the treatment include the power input, line speed, the magnitude of the air gap, the relative humidity, the chemical nature of the polymer, the additives used and the time between processing the polymer and the treatment. Adhesion is also affected by the time between the treatment and bonding.

Figure 4.5 Schematic representation of the corona discharge treatment.

Surfaces: how to treat

83

Table 4.2 Concentration of different chemical groups after corona discharge treatment of polyethylene17 Group

Concentration* Initial Water-washed

Peroxide Hydroxyl Carbonyl Epoxide Carboxylic acid -NO3

1.2 1.7 1.8 2.3 1.6 0.8

0.9 1.1 0.9 1.1 0.8 0.4

*Moles of functional species per initial unreacted carbon atom (102).

Surface analysis has shown that the corona discharge treatment introduces various groups, such as carbonyl, into the polymer surface. Typically 5±10 atomic percent of oxygen is introduced into a polyolefin film. There are many publications on the corona treatment of polymers. Especially useful contributions include references 16 and 17. In the latter, the various groups introduced by the corona treatment were quantified (Table 4.2). It is known that additives such as slip agents and antioxidants can adversely affect corona treatment if this is not done in-line, i.e., immediately after processing. If the additives migrate to the surface before the treatment is carried out, then inferior adhesion may result. This is because there will be a tendency to chemically modify the additives rather than the underlying polymer chains.

4.4.3 Low-pressure plasma treatment In this method, power is applied to a gas or a monomer at low pressure (typically 1 torr) and a plasma consisting of ions, electrons, atoms and free radicals is formed. There are several parameters which control the effect of a plasma on a polymer including: · · · · · ·

the the the the the the

nature of the gas gas pressure and flow rate discharge power excitation frequency nature of the polymer temperature of the polymer.

Improved adhesion may be due to a variety of mechanisms: · · · ·

removal of contaminants by ablation crosslinking grafting of monomers to a polymer surface introduction of functional groups.

84

Adhesive bonding

The last effect can take place during exposure to a plasma or upon exposure to the atmosphere when free radicals can react with oxygen or water. Improvements in adhesion for a wide variety of polymers have been demonstrated by a number of workers (see, for example, references 11 and 18).

4.4.4 Treatment of PTFE As noted, many pretreatments are effective with polyolefins, but with the exception of plasma treatments none gives substantial improvements in the bondability of PTFE. This is because PTFE is very resistant to oxidation. However, powerful reducing agents can bring about major chemical changes, paradoxically resulting in the introduction of a variety of oxygen-containing functional groups. Effective treatments for PTFE were developed in the 1950s. These were sodium in liquid ammonia19 and sodium naphthalenide in tetrahydrofuran.20 Many other methods have since been investigated including plasma treatment, direct electrochemical reduction, treatment with an alkali metal amalgam and reduction with benzoin dianion. However, none of these methods is as effective as the original two, which remain the main commercial pretreatments. The effect of a commercial etchant of the sodium naphthalenide type on various fluoropolymers is shown in Table 4.3.21 The treatment is especially effective with PTFE. It results in extensive defluorination and the introduction of oxygen-containing groups.

Table 4.3 Effect of `Tetra Etch' treatmenta on the surface composition of PTFE, PVFb and ECTFEc, and the failure loads of composite lap shear joints involving these polymers with an epoxide adhesive21 Polymer Treatment time

Surface compositions by XPS (atom %) C Cl F

O

Failure load (N)

PTFE

None 10 s 1 min.

38.4 87.6 82.2

± ± ±

61.6 0.8 0.9

± 11.6 16.9

420 4280 4260

PVF

None 10 s 1 min. 60 min.

70.4 72.4 75.4 87.3

± ± ±

28.8 26.7 23.0 11.4

0.8 0.9 1.6 1.3

360 800 2090 3020

ECTFE

None 1 min.

53.2 72.5

14.3 3.7

32.5 17.7

± 6.0

240 3300

a

`Tetra Etch' is a product of W Gore & Associates. It is a sodium complex in organic ether; it produces similar results to sodium naphthalenide inTHF. b PVF poly(vinyl fluoride). c ECTFE is a copolymer of ethylene and chlorotrifluoroethylene.

Surfaces: how to treat

4.5

85

Pretreatments for elastomers

In many cases, elastomers are joined to other materials during the process of vulcanisation. However, in other cases, elastomers are joined to other materials after vulcanisation. With this second group, it is often necessary to pretreat the elastomers before bonding. Pretreatments range from physical methods such as a solvent wipe or abrasion to chemical methods such as treatment with trichloroisocyanuric acid. Physical methods can be used to remove cohesively weak layers from the polymer. This is essential to good bonding unless these layers can be absorbed by the adhesive. However, physical methods will only be effective if the underlying elastomer possesses suitable functional groups, e.g., with polychloroprene, or if a diffusion mechanism operates in a subsequent adhesion operation, e.g., bonding. Chemical methods may also remove weak layers or chemically modify them so that they are more compatible with the adhesive; in addition, chemical methods may roughen a surface. However, an effective chemical method will also modify the chemistry of the elastomer so that the interaction with the adhesive is increased. This will usually be essential with elastomers based on hydrocarbon polymers. These elastomers are much more widely used than those containing functional groups. In general, elastomers contain a greater variety and quantity of additives than plastics; fifteen or more components in a particular formulation is quite common. These additives, or compounding ingredients as they are often called, may well create a cohesively weak layer on the elastomer surface. On the other hand, plastics usually contain a small number of additives and in relatively low concentrations. Over the last fifty years many methods have been developed to pretreat elastomers largely on an empirical basis. From a pretreatment viewpoint, elastomers can be divided into three groups, namely those containing polar groups such as nitrile rubber and polyurethanes, hydrocarbon systems having few if any C=C bonds, and hydrocarbon systems possessing substantial numbers of C=C groups; the last type are termed unsaturated elastomers. Those with polar groups can usually be successfully bonded after a physical treatment, e.g., a solvent wipe or abrasion; these elastomers will not be considered further in this section. Of the second group, ethylene-propylene elastomers (EP) and butyl rubber are the most important examples. These elastomers often do not respond to treatments which were found to be very successful with polyolefin plastics. In one study22 butyl rubber was treated with chromic acid, corona discharges, flames, bromination, UV radiation and potassium permanganate. These treatments had little effect on bondability using an epoxide adhesive. It was concluded that chain scission occurred with the result that suitable functional groups were not introduced in sufficient quantity into long polymer chains. Also, the fragments formed by chain scission could act as a weak boundary layer. Bragole23 found that UV treatment of EP coated with a thin layer of benzophenone resulted in large increases in the adhesion of acrylic, epoxide and

86

Adhesive bonding

Table 4.4 Effect of pretreatments on the peel strengths (N mmÿ1) of SBR-epoxideSBR25 Pretreatment Toluene wipe Abrasion on grinding wheel Acidified hypochlorite Treatment with conc. H2SO4 TCICA in ethyl acetate

Peel strength 0.2 1 12 12 11

Locus of failure I I R R R

I ± apparent interfacial R ± cohesive within rubber SBR ± styrene butadiene rubber

urethane paints. Hydrocarbon elastomers with sufficient unsaturation respond much more readily to pretreatments. For example, Pettit and Carter24 found the treatment of natural rubber and styrene-butadiene copolymers with chlorine gas, sodium hypochlorite or trichloroisicyanuric acid (TCICA) resulted in large increases in peel strengths using a polyurethane adhesive. Oldfield and Symes25 found acified hypochlorite, sulphuric acid and TCICA in ethyl acetate were very effective with a styrene-butadiene rubber (Table 4.4). The last method has been widely used over the last thirty years, but the use of organic solvents is being phased out. This led Dahm et al.26 to develop a water-borne treatment for unsaturated elastomers. They found that an acidified aqueous solution of chloramine T gave high bond strengths with a styrene-butadiene copolymer with failure in the elastomer.

4.6

Summary and future trends

It is now well established that three factors relating to the conditions of a surface can have a major effect on the bondability of a substrate. These are the mechanical strength of surface regions, topography and surface chemistry. Pretreatments are effective by altering one or more of these factors. Weak layers on surfaces must be removed prior to bonding unless the layer can be absorbed by the adhesive. It may be possible to remove these layers by physical means, e.g., the use of an organic solvent or grit blasting. Physical methods can also modify the topography of a substrate. Chemical methods can alter all three parameters. To make olefinic or fully fluorinated polymers bondable, it is usually necessary to chemically modify the surface. If bonded metals are to be subjected to water, it is necessary to form the most stable oxide and to provide the correct surface topography. For each group of materials, a wide range of pretreatments exists and a satisfactory joint performance can usually be achieved. However, in recent years legislation has restricted the use of certain pretreatments, e.g., the use of

Surfaces: how to treat

87

chromium compounds. Pretreatments involving the creation of large quantities of effluent are a particular problem. In general, dry methods are favoured, e.g., corona treatment of polypropylene, as pollution is minimised. There will therefore be a strong trend towards pretreatments which have a minimum impact on the environment. Surface analytical techniques have made a major impact on our understanding of joint performance. The surfaces to be bonded can now be well characterised and information on interactions across interfaces obtained. Now the surface chemistry of substrates is well understood, more effort will be directed towards achieving chemical bonding with adhesives (or primers). This will lead to enhanced performance especially in terms of durability. In assessing the durability of adhesive joints more attention will be paid to real service conditions and the nature of the stresses involved. In particular, more testing under fatigue and impact conditions will be carried out.

4.7

Literature

The main journals in English relevant to this section are the Journal of Adhesion, the International Journal of Adhesion and Adhesives and the Journal of Adhesion Science and Technology. Important books include: Garbassi F, Morra M and Occhiello E (1994), Polymer surfaces, Chichester, John Wiley & Sons. Mittal K L and Pizzi A (1999), Adhesion promotion techniques, New York, Marcel Dekker. Crowther B (2001), Handbook of rubber bonding, Shrewsbury, Rapra Technology Limited. Minford J D (1993), Handbook of aluminium bonding technology and data, New York, Marcel Dekker.

4.8

References

1. Critchlow G W and Brewis D M, `Review of surface pretreatments for aluminium alloys', Int J Adhesion and Adhesives, 1996 16 255±275. 2. Critchlow G W, Yendall K A and Andrews F R, `An environmentally benign oxidation treatment for bonding aluminium alloys', 24th Annual Meeting of the Adhesion Society, 25±28 February 2001, Williamsburg, USA. 3. Bjorgum A, Lapique F, Walmsley J and Redford K, Int J Adhesion and Adhesives, 2003 23(5) 401±412. 4. Abel M-L, Watts J F and Digby R P, `The influence of process parameters on the interfacial chemistry of -GP on aluminium: a review', J Adhesion, 2004 80 291±312. 5. Critchlow GW and Brewis DM, `Review of surface pretreatments for titanium alloys', Int J Adhesion and Adhesives, 1995 15 161±172. 6. Poole P, `Adhesion problems in the aircraft industry', in Brewis D M and Briggs D, Industrial Adhesion Problems, 1985, Oxford, Orbital Press, 258±284. 7. Walker P, `Organosilanes as adhesion promoters', J Adhesion Sci Tech, 1991 4 279± 305.

88

Adhesive bonding

8. Gettings M and Kinloch AJ, `Surface analysis of polysiloxane/metal oxide interfaces', J Mater Sci, 1977 12 2511±2518. 9. Plueddemann E P, `Reminiscing on silane coupling agents', J Adhesion Sci Tech, 1991 4 261±277. 10. Schonhorn H and Hansen R H, `Surface treatment of polymers for adhesive bonding', J App Polym Sci, 1967 11 1461±1474. 11. Hall J R, Westerdahl C A L, Bodnar M J and Levi D W, `Effect of activated gas plasma treatment time on adhesive bondability of polymers', J App Polym Sci, 1972 16 1465±1477. 12. Brass I, Brewis D M, Sutherland I and Wiktorowicz R, `The effect of fluorination on adhesion to polyethylene', Int J Adhesion and Adhesives, 1991 11 150±153. 13. Ayres R L and Shofner D L, `Preparing polyolefin surfaces for inks and adhesives', SPE Journal, 1972 28 51±55. 14. Sutherland I, Brewis D M, Heath R J and Sheng E, `Modification of polypropylene surfaces by flame treatment', Surf Int Anal, 1991 17 507±510. 15. Briggs D, Brewis D M and Konieczko M B, `X-ray photoelectron spectroscopy studies of polymer surfaces. Part 3. Flame treatment of polyethylene', J Mat Sci, 1979 14 1344±1348. 16. Strobel M, Lyons C S, Strobel J M and Kapaun R S, `Analysis of air corona treated polypropylene and poly(ethylene terephthalate) films by contact angle measurements and X-ray photoelectron spectroscopy', J Adh Sci & Tech, 1992 6 429±443. 17. Gerenser L J, Elman J F, Mason M G and Pochan J M, `ESCA studies of corona discharge treated polyethylene surfaces by use of gas phase derivitisation', Polymer, 1985 26 1162±1166. 18. Liston E M, Martinu L and Wertheimer M R, `Plasma surface modification of polymers for improved adhesion ± a critical review', J Adh Sci & Tech, 1993 7 1091±1127. 19. Benderly A A, `Treatment of Teflon to promote bondability', J App Polym Sci, 1962 6 221±225. 20. Nelson E R, Kilduff T J and Benderly A A, `Bonding of Teflon', Ind Eng Chem, 1958 50 329±330. 21. Mathieson I, Brewis D M, Sutherland I and Cayless R A, `Pretreatments of fluoropolymers', J Adhesion, 1994 46 49±56. 22. Cope B C, Brewis D M, Comyn J, Nangreave K R and Carne R J P, `Surface treatment of butyl rubber', in Allen K W, Adhesion 10, 1986, London, Elsevier Applied Science Publishers, 178±191. 23. Bragole R A, `Factors affecting the adhesion of paints to non-polar plastics and elastomers', J Elastomers & Plastics, 1974 6 213±217. 24. Pettit D and Carter A R, `Behavior of urethane adhesives on rubber surfaces', J Adhesion, 1973 5 333-349. 25. Oldfield D and Symes T E F, `Surface modification of elastomers for bonding', J Adhesion, 1983 16(2) 77±96. 26. Dahm R H, Brewis D M and Fletcher I W, `Mechanistic studies of pretreatments of elastomers', 6th European Adhesion Conference, 10±13 September 2002, Glasgow, UK, 10M Communications.

Part II

Mechanical properties

5

Stress analysis A CROCOMBE

5.1

Introduction

Adhesive bonding is an effective method of joining many materials. As such, adhesives are used in a wide variety of applications, from aircraft to electronics, and in many of these applications, to function correctly, the adhesive joint has to transmit load. These are known as structural adhesive joints. The sources of these loads are many and varied and will be outlined in section 5.2. These loads produce stress and strain within the joint. Stress is the intensity of loading at any point and strain is the deformation that is caused by this stress. Material failure can often be determined from knowledge of the stresses or strains sustained. Clearly it is important to design structural joints to be able to transmit their loads safely. The analysis of stresses and strains induced in a joint is an important part of this joint design process. The topic of stress analysis forms the basis of the current chapter. The aim of this chapter is to provide the reader with a well rounded review of stress analysis of adhesive joints. It is designed to meet the needs of a range of readers, from users through to designers and analysts. This has been achieved by having two main parts. The first, section 5.2, provides a qualitative description of the stresses that arise in adhesive joints and the sources of these stresses. The second main part, sections 5.3 and 5.4, is more quantitative in nature and outlines various methods used for stress analysis of joints and presents some salient results. However, even this more quantitative section is structured in such a way that simple guidelines and formulae are easily accessible, even to the nonspecialist. A final section presents some ideas on future developments and the role that these developments will play in further advancing the use of adhesives as a joining technology.

5.2

A qualitative description of adhesive joint stresses

This section reviews the complete field of adhesive joint stress analysis in a qualitative way. The sources of adhesive stresses, which are not always direct

92

Adhesive bonding

Figure 5.1 Two-dimensional stresses in the adhesive layer.

mechanical loading, will be discussed. Different levels of studying the stress and strain in the joints will be presented and a general description of the distributions of these stresses within the joint will be outlined.

5.2.1 Stress and strain Although there are many sources of joint loading, all stresses induced in the adhesive will either be shear stresses () or direct stresses (). The same is true for all materials and structures. These are illustrated in Fig. 5.1 for a twodimensional representation of an adhesive joint. It can be seen that there are two direct stresses and one shear stress and that the subscripts refer to the direction of the stresses. The extension to three dimensions will result in three direct stresses and three shear stresses. Direct and shear stresses are caused by forces acting perpendicular and parallel to an area respectively. Direct stresses result in an extension (or compression) of the material, known as a direct strain () whilst shear stresses cause a sliding deformation, known as a shear strain ( ). The stresses and strains are linked by the stiffness of the material. This can be expressed quite generally for a full three-dimensional problem as equation 5.1a which can be reduced to the very familiar one-dimensional situation as equation 5.1b where E is the Young's modulus of the material. For a fuller account of general stress strain relationships the interested reader is directed to any of the large number of reference texts on stress analysis (e.g., Boresi et al., 1993). ‰Š ˆ ‰DŠ‰Š;  ˆ E


5:1a,b

5.2.2 Sources of adhesive joint stresses As outlined above there are many sources of stress in adhesive joints. Some of the main ones are briefly discussed here. Mechanical loading This is the most obvious source of stress in joints. Such loads can be represented in terms of forces, moment and torques in the substrates arising from a wide range of external loads. The transient nature of these loads will vary and include

Stress analysis

93

Figure 5.2 Typical shear and peels stresses in adhesive joints.

high rate impact events (where dynamic effects are important) and loads sustained over long periods (either oscillating or steady) in addition to loads that are essentially quasi-static in nature. Most of the stress analysis that has been undertaken and hence reported in this chapter is for quasi-static loading. However, specific sections have been included for dynamic and rate dependent analyses that are more appropriate for the other types of transient loading. In order to understand how these loads can cause stress in the adhesive it is helpful to consider the simple cases in Fig. 5.2. In the first it can be seen that the load transfers from the substrate to the ground. This load transfer is achieved by shear in the adhesive layer, as seen by the forces on a section of the substrate. As load is transferred the substrate loading reduces and hence induces less shear in the adhesive. This results in a shear stress distribution that decays from the beginning of the overlap and this is the characteristic shape of shear stresses in adhesive joints. The second case illustrates the transfer of load that is perpendicular to the adhesive layer. Here it can be seen that the load transfer is achieved through direct stress in the adhesive perpendicular to the adhesive layer. This component of stress is often referred to as the peel stress. Again it can be seen that as load is transferred the substrate deforms less and hence induces less peel stress in the adhesive. In fact the substrate experiences a small clockwise rotation such that part of it, some way from the beginning of the overlap, compresses the adhesive

94

Adhesive bonding

Figure 5.3 Bending moment action on the substrate.

and this results in a decaying and oscillating distribution that is characteristic of the peel stress in joints. As seen in Fig. 5.2, when considering stresses in adhesive joints, usually the shear stress (parallel to the interface) and the transverse direct (peel) stress (perpendicular to the interface) are the two dominant components of stress. A good appreciation of the stress distribution within an adhesive joint can often be gained by considering the loads acting in the substrate at the end of the joint. In two dimensions there are three substrate loads to consider; the axial force and the shear force (which are shown in Fig. 5.2(a) and (b) respectively) and a bending moment, shown in Fig. 5.3. Whilst the axial force and the shear force mainly produce adhesive shear and peel stresses respectively, the bending moment generates significant peel and shear stresses, both of which are distributed as shown in Fig. 5.2. From this, the stress distribution in typical structural joints can be determined. For example, a single lap joint will experience a combination of axial and shear forces and a bending moment acting in the loaded substrate at the overlap end, Fig. 5.4(a). The axial force and moment both produce shear stress in the same direction in the adhesive and the shear force and the moment both produce peel stresses in the same direction in the adhesive. Thus the shear and peel stress distributions are qualitatively as shown in Fig. 5.2(a) and (b), respectively. A T-peel specimen will have equal and opposite shear forces and bending moments in the substrates, Fig. 5.4(b). The moments and shear forces in both substrates produce peel stresses that are all in the same direction and so are distributed as in Fig. 5.2(b). However, the moment and shear force in the right substrate generate shear stresses in an opposite direction to those caused by the

Figure 5.4 Substrate loading in single lap and peel joints.

Stress analysis

95

loading on the left substrate and hence they cancel, resulting in no shear stresses. Specific analyses leading to quantitative results for adhesive stresses for various joints are presented in section 5.3. Thermal stresses Materials expand and contract with changes in temperature. The degree of expansion or contraction depends on the temperature change and the coefficient of thermal expansion (CTE) of the material. When a single material of any shape experiences a uniform change of temperature there will be a corresponding change in its dimensions without any accompanying stress. This is not the case when structures consist of different materials that are integrally joined, as in adhesively bonded joints. The most significant temperature change often occurs in joint manufacture on cooling after an elevated temperature curing cycle. The adhesive generally has a higher CTE than the substrates being joined and hence wants to contract more. The stress analyses for mechanical loading can be readily modified to incorporate thermal stresses. The most severe thermal stresses occur when the substrates have different CTEs. In these situations the substrate with the higher CTE contracts more and compresses the other substrate and vice versa. This transmission of internal load from one substrate to the other is achieved by inducing a shear stress through the adhesive thickness, which decays as the load is transmitted. This is illustrated in Fig. 5.5. In some situations these induced thermal shear stresses have been sufficient to cause joint failure before any loading is applied. When the substrates have the same CTE the adhesive does not transmit internal load from one substrate to another and the main stress that is induced in the adhesive is a longitudinal tensile direct stress as the adjacent substrates prevent the free thermal contraction in the adhesive. These tensile stresses are induced by shear stresses along the interface at the overlap. These adhesive

Figure 5.5 Adhesive shear stresses caused by differential thermal straining of substrates.

96

Adhesive bonding

stresses are generally much lower than the shear stresses induced when the substrates have different CTEs. Swelling stresses A third source of stress in adhesive joints is the swelling that occurs as the adhesive absorbs moisture in a wet environment. These can be treated in the same way as thermal stresses, replacing the CTE with a coefficient of swelling (CS) and the temperature change with a moisture concentration. However, unlike the temperature in the previous paragraph, the distribution of moisture within the joint will usually not be uniform. This means that although the closed form analyses discussed in section 5.3 can be readily adapted to accommodate thermal strains (as outlined above) such an extension can only be made for swelling strains when the adhesive layer is saturated and the moisture distribution is uniform. In other situations stress analyses must be carried out using the more general techniques such as finite element (FE) methods. These are outlined in section 5.4. In these situations the swelling will be greatest near the exposed ends of the overlap. This localised swelling can generate compressive stresses across the adhesive layer which can reduce the local peel stresses that may be induced by mechanical or thermal loading (Abdel Wahab et al., 2002) and hence enhance the strength of the joint. As with thermal strains, swelling strains are likely to be greater if one substrate swells and the other does not (i.e., composite to metal joints). As well as swelling, moisture affects the stress distribution in a joint by plasticising the adhesive, resulting in a lowering of the modulus and flow stress whilst increasing the ductility. This adds yet more complexity to the analysis, which is usually solved using FE methods. However this behaviour has recently been accommodated in closed form analysis (Crocombe, 2002a) and will be discussed in more detail in section 5.3.

5.2.3 Local singular adhesive stresses The stress distributions discussed above can be termed global (or far field) as they have been derived without taking into account any local stress raising effects. In addition to fillers and micro-voids there are two other main sources of stress raisers in adhesive joints, both of which lead to singular (infinite) stresses in a linear stress analysis. One of these is a crack and this will be dealt with in more detail under the chapter on fracture mechanics. The form of the stress in the region of the crack tip depends on whether the crack lies on the interface (in which case it oscillates very close to the crack tip, see e.g., Rice, 1988) or within the adhesive layer (in which case it approaches an infinite value smoothly, see e.g., Anderson, 1995). The other sources of singularities are the bi-material junctions that occur at the overlap ends (A) or at a substrate corner (B) that is embedded in an adhesive fillet, Fig. 5.6. The stresses are infinite only if the

Stress analysis

97

Figure 5.6 Points of bi-material singularity in an adhesive joint.

junction is completely sharp. Although, in practice, this is not the case, it is generally considered to be too complex to analyse the real geometry. It can be shown (Adams and Harris, 1987) that the stresses follow the local singular field up to a distance from the singular point that is of the same order as the geometric irregularity (which is often measured in microns). At such points of singularity it is inappropriate to use stress as a criterion for failure. It is more appropriate to characterise the stress field using stress at a distance or a critical generalised stress intensity. This latter parameter will be considered in a more detailed discussion of singular stress analysis in section 5.4.

5.3

Closed form, global stress analysis of adhesive joints

These analyses are based on the solution of differential equations formulated for a specific joint configuration. Initial analyses were one dimensional and solutions were presented in the form of explicit equations for the adhesive stress components. However, later analyses became more complex, through the introduction of generalised boundary conditions, non-linearity and the introduction of a second dimension (across the adhesive thickness) and solutions to the now complex equations were increasingly undertaken using computers. These solutions, however, are quite separate from finite element (FE) analyses which are undertaken on computer-based, general stress analysis packages. These require the construction of a FE mesh to define the particular structure being analysed. The application of this technique to adhesive joints will be discussed in section 5.4. The simplifications made to derive the equations for closed form analyses means that no account can be taken of any singular (local) stress fields discussed in section 5.2.3 above. However, it is possible to use the results of a global analysis as boundary conditions for a local analysis. This aspect will also be considered in section 5.4. Further, the global results from these closed form solutions have been used with considerable success for the structural design of adhesive joints. The following sections review the closed form analyses for a number of common adhesive joint configurations. Emphasis

98

Adhesive bonding

is placed on lap joints as these are the most common configuration of joint, but other configurations are also considered.

5.3.1 Lap joints The two common forms of lap joint are the double and single lap joints, shown in Fig. 5.7. The main difference between the two systems is that, due to its eccentric loading, the single lap joint, rotates as it is loaded. This results in substrate loads (axial and shear forces and moments) that vary non-linearly with the applied joint loading. In contrast, by considering the portion of the joint above the centre substrate mid-plane it can be seen that the double lap joint can be treated as a special case of the single lap joint where the lower (centre) substrate experiences no rotation and its thickness is half that of the centre substrate. The first analysis of the double lap joint is probably that due to Volkersen (1938). In this analysis the substrates deform only in tension whilst the adhesive deforms only in shear. This is the situation that is illustrated in Fig. 5.2(a). This has become known as the shear lag analysis and the adhesive shear stress distribution peaks at the overlap ends, again as illustrated in Fig. 5.2(a). By applying the boundary conditions of full load at one end of the substrate and zero load at the other end the following equation for adhesive shear stress  normalised by the average shear stress  av, can be obtained: …X † !cosh…!X † …t1 ÿ t2 † !sinh…!X † ‡ ; ˆ av 2sinh…!=2† …t1 ‡ t2 † 2cosh…!=2†   t1 GL2 2 where ! ˆ 1 ‡ t2 Et1 t3

5:2

Here, t1, t2 refer to the outer and half the centre substrate thicknesses, t3 is the adhesive thickness, X is the distance from the overlap centre normalised by the overlap length L and E and G are the substrate and adhesive, Young's and shear modulii respectively. Solution of this shows that the maximum shear stress occurs at the end of the overlap where the loaded substrate is most flexible. Goland and Reissner (1945) presented two extremes for an analysis of single lap joints, namely a relatively inflexible adhesive and a relatively flexible adhesive. Work on adhesively bonded joints, as outlined below, concentrates on the latter although many bonded systems lie between these two extremes. The

Figure 5.7 Single and double lap joints.

Stress analysis

99

Figure 5.8 Single lap joint bending moment factor.

work of Volkersen (1938) has been extended in two key ways. Bending was included in the substrate and this enabled expressions to be developed for adhesive peel as well as shear stresses. Also an expression was developed that gave the substrate bending moment, and consequently the shear force, in terms of the applied axial load. This was achieved by splitting the joint into three parts; the overlap and the two free substrate regions. Each part was analysed for deflection, and continuity conditions were applied at the points of connection. In this way, the three substrate loads, illustrated in Figs 5.2 and 5.3, could all be included in the analysis (unlike Volkersen's analysis where only the axial force is included). The moment, M, is expressed in terms of a bending moment factor, k (illustrated in Fig. 5.8). As shown, as the applied load P increases, the line of action of the load gets closer to the mid-plane of the substrate and hence its moment arm about point O decreases. Thus k decreases from a value of 1 at low loads towards 0.26 as the load increases as shown in Fig. 5.8. The parameters E, t and  refer to the substrate modulus, thickness and Poisson's ratio respectively and c is half the overlap length. In the analysis the adhesive is allowed to transmit both shear and peel forces. By considering equilibrium and deformation in both the substrates and the adhesive, equations for adhesive peel and shear

100

Adhesive bonding

stresses were derived. The distributions are as illustrated in Fig. 5.2 with both stresses reaching maximum values at the overlap end and decaying into the overlap. Following this, Volkersen (1965) extended the analysis of double lap joints to include the adhesive peel stresses. As the centre substrate does not bend, only its extension was included. However, due to the internal stresses, the outer substrates bend, generating compressive (negative) and tensile (positive) adhesive peel stresses at ends A and B of the overlap respectively, see Fig. 5.7. This is similar to the peel stress distribution shown in Fig. 5.9 for HartSmith's work. Thus in a balanced double lap joint, failure is more likely to occur at B than A. One reason for undertaking stress analysis is to predict joint strength. Many modern adhesive systems exhibit significant non-linearity in their stress strain behaviour and hence analyses based on linear material behaviour may be of limited use for strength assessment. To overcome this Hart-Smith (1973a) extended double lap joint analyses, modelling the adhesive as an elasto-plastic material. He showed that the actual form of the adhesive stress strain curve was less important than the area under it (energy dissipated) and hence assumed an elastic-perfectly plastic response. The analyses were simplified by uncoupling the shear and peel stresses, essentially using a shear lag analysis (substrate extension only) for the shear stress and considering substrate bending and shear to determine the peel stress. Elasto-plasticity was included only in the shear stress formulation and this was achieved by deriving equations for an elastic inner region and plastic outer region and then applying boundary conditions at the overlap ends and continuity at the inner-outer region boundaries. The

Figure 5.9 Schematic representation of adhesive stress distribution from HartSmith's (1973a) non-linear double lap joint analysis.

Stress analysis

101

resulting shear stress distribution is illustrated schematically in Fig. 5.9. Thermal strains have been incorporated into the expression for substrate deformation. The peel stress distribution is similar to that of the latter Volkersen (1965) analysis and is also illustrated in Fig. 5.9. By assuming that joint failure occurs at a critical level of plastic strain, Hart-Smith develops a chart that gives predicted joint strength as a function of a joint geometry parameter and the ratio of failure to initial yield, shear strains. At this time Hart-Smith (1973b) also extended the work of Goland and Reissner (1945) for single lap joints. Plasticity was included in the adhesive shear behaviour as in the double lap joint. In addition a new expression was developed for the bending moment factor. Hart-Smith argued that it was better to model the overlap as two separate regions as this allowed better application of the connecting loads between free substrate and overlap. The expression derived for the moment factor varies from unity at low loads to zero with increasing loads, see Fig. 5.8. Thus at higher loads the moment and hence the adhesive stress predicted by Hart-Smith are a little lower than those predicted by Goland and Reissner. Finally, the case of imbalanced joints is considered although explicit solutions for the imbalanced joint cannot be derived. Unlike the double lap joint analysis the thermal strains are only considered qualitatively. Tsai and Morton (1994) compare the bending moment factors of Goland and Reissner (1945), Hart-Smith and a more recent approach by Oplinger (1994). All three are only applicable to joints with balanced substrates and the general conclusion, based on comparison with numerical studies, is that Hart-Smith and Oplinger give better end moments for short and long overlaps respectively. However, with regard to adhesive stresses the Goland and Reissner approach is accurate enough for short and long overlaps. Renton and Vinson (1977) made the next significant contribution to the analysis of single lap joints. The effect of shearing in the substrate is included and the substrate is modelled as a generally orthotropic system, to extend the application to composite materials. The analysis is formulated for different substrate thicknesses but balanced single lap joint boundary conditions, based on the Goland and Reissner (1945) formulation, for the overlap bending moment are used. Thermal strains are included in the formulation and the adhesive layer is modelled as a separate block, which enables the adhesive shear stress to drop rapidly to zero at the end of the overlap, which is a free surface. When compared with the Goland and Reissner solution for a typical joint configuration the maximum peel and shear stresses are reduced by around 20% and 40% respectively. As with previous analyses the adhesive shear and peel stress have been assumed constant across the thickness of the adhesive. Further advances to single lap joint analyses have been made by Ojalvo and Eidinoff (1978) and Delale et al. (1981). The former incorporate a fuller description for adhesive shear strain which allows for a linear variation across the adhesive thickness. Substrate shearing has not been included. Balanced

102

Adhesive bonding

joints are considered and the adhesive shear and peel stresses both reach a peak at the end of the overlap. For typical joint parameters the shear and peel stresses are increased and decreased respectively over the values given by Goland and Reissner (1945). It was shown that shear stress could exhibit significant variation across the overlap at the joint ends. Delale et al. include the adhesive longitudinal stress in addition to the shear and peel stresses, however, these adhesive stresses are assumed to be constant across the adhesive thickness and so the shear stress does not become zero at the overlap ends. The formulation is derived for unbalanced orthotropic lap joints and substrate shearing has been incorporated. The boundary conditions used do not incorporate the bending moment factor and thus it is not possible to make a comparison between this and other solutions. Bigwood and Crocombe (1989, 1990) generalised the solution of Goland and Reissner (1945) for an arbitrarily loaded overlap and went on to incorporate a full non-linear representation of the adhesive layer where both the peel and shear stresses contribute to adhesive yield and a hyperbolic tangent model was used to represent the stress strain curve. The governing equations were solved numerically in an incremental manner. Good correlation was found between their analyses and non-linear FE solutions. This was extended further (Crocombe and Bigwood, 1992) to include plastic deformation in the substrates as well as the adhesive. The results were shown to compare well with FE analyses where substrate and adhesive plasticity were also modelled. A number of authors; Allman (1977), Chen and Cheng (1983), Cheng et al. (1991) and Adams and Mallick (1992) have adopted a different approach to obtain a global stress analysis of adhesive joints. They apply the theory of elasticity to the overlap region using appropriate traction boundary conditions (based on the bending moment factor approach) on the loaded faces. Use of equilibrium equations ensures that all stress components are included. Thus tensile, bending and shearing in the substrate are included. However, the assumption that shear stresses do not vary across the adhesive results in the longitudinal adhesive stress being zero. Adhesive peel stress, however, is assumed to vary linearly across the adhesive thickness. Equilibrium equations are applied to the upper and lower substrates and the adhesive layer, ensuring continuity of tractions along common interfaces. It is found that all stresses can be found in terms of a few independent functions in the longitudinal direction, x. Differential equations are then found and solved for these unknowns by applying the variational principle of complementary energy. Chen and Cheng (1983) compare their results with those from Goland and Reissner (1945) for cases where the adhesive layer is relatively flexible (this falls within the bounds of validity of Goland and Reissner's approach). There is good correlation between the two analyses apart from the very end of the overlap where the shear stress drops rapidly to zero and the peel stresses on the loaded and unloaded interfaces increase and decrease respectively. Cheng et al.

Stress analysis

103

(1991) and Adams and Mallick (1992) extend the work of Chen and Cheng by formulating the problem to include different substrates. The former develop a new bending moment factor for unbalanced joints whilst the latter include thermal stresses and both single and double lap joints solutions. In addition longitudinal stresses are included in the adhesive and all stresses vary across the adhesive thickness. This results in the entire stress field being expressed in terms of four independent functions, rather than the two as in the other approaches. Extensions to the traditional closed form lap joint analyses have also continued. Yang and Pang (1996) include asymmetric laminates and all three adhesive stress components are derived using a Fourier series approach. Tsai et al. (1998) present a different way to model substrate shear, allowing it to vary linearly through the substrate thickness. Only adhesive shear stress is considered and it is shown that including the substrate shear will smooth and reduce the shear stress distribution along the overlap. Sawa et al. (2000) modelled an unbalanced single lap joint by considering the two substrates and the adhesive layer as three separate strips, subjected to common but unknown traction along the interfaces and appropriate tractions on the other faces. They defined a separate Fourier series stress function for each strip and finally obtained 48 simultaneous equations for each term included in the series. Solution of this is likely to be as complex as carrying out FE analysis (section 5.4). They found the same sort of trends in adhesive stress as were found in the simpler closed form solutions but with the inclusion of sufficient terms were able to model the bimaterial stress singularity reasonably. Crocombe (2002a) extend their earlier work, where non-linear adhesive behaviour has been incorporated, to consider the effect of environmental degradation. By assuming typical moisture dependent modulus and yield stress relationships it is possible to couple moisture diffusion and stress analysis and hence determine the limit state load that can be sustained by a joint after a given exposure history. This approach is shown schematically in Fig. 5.10.

Figure 5.10 Integrating environmental degradation into a closed form analysis.

104

Adhesive bonding

5.3.2 Other joint configurations There is a vast amount of literature on the closed form global analysis of other configurations of adhesive joint. Presented here are a few key papers just to give some insight into this area. The peel test This is illustrated schematically in Fig. 5.11(a). From an energistic approach, when the flexible strip is elastic, this test provides a very simple measure of the fracture energy of the adhesive-substrate system. The stress analysis is more complex and is particularly necessary when the strip behaves in an inelastic manner. Kaelble (1960) represented the adhesive as a combination of tensile and shear springs (as in many of the lap joint models), the flexible strip was assumed to be elastic and the bonded region subject only to small displacements. By considering equilibrium of a small element in the overlap region equations for peel and shear, stresses were derived and were found to exhibit the same distribution as illustrated in Fig. 5.2. Boundary conditions were obtained by applying large bending theory to the unbonded portion of the flexible strip. This early work was limited by not including plasticity in the flexible strip and thus not being able to account for the proportion of the fracture energy dissipated in plastic deformation in the non-linear bending and subsequent unbending that often occurs in the peel test. Kim and Aravas (1988) and Kinloch et al. (1994) are amongst those who addressed this issue. Both developed moment curvature relationships for an elasto-plastic material during loading and subsequent unloading. The former derived expressions for energy dissipated in terms of the rotation at the root of the peel front, however the rotation was not determined. The latter closed this loop by incorporating the root rotation, using a beam on an elastic foundation approach. In this way the adhesive fracture energy was determined in terms of the measured peel load.

Figure 5.11 Illustrating the peel, scarf and stepped joint configurations.

Stress analysis

105

Scarf and stepped joints Such joints are illustrated in Fig. 5.11(b) and (c) and are generally used to obtain a higher bond strength, to utilise the full capacity of thicker substrates. The increased complexity of these joints means that numerical solutions are generally required to solve the governing equations. Only a selection of the existing literature is presented here. In principle an idealised balanced scarf joint will achieve a uniform stress distribution. Hart-Smith (1973c) extended his shear lag analysis of lap joints to include a tapered substrate. Non-linear adhesive shear stress behaviour was included. It was found that for long overlaps the maximum shear stress occured at the overlap end where the less stiff substrate was loaded and that this stress could be expressed as the product of the average shear stress and the substrate stiffness ratios. Gleich et al. (2000) analysed scarf joints that are eccentric due to a finite substrate tip thicknesses. Elastic shear and peel stresses were included, the analysis was restricted to balanced substrates and a numerical solution was used to solve the resulting differential equations in a way similar to Bigwood and Crocombe (1989). It was shown that the maximum shear stress increases from the ideal scarf value as the tip thickness increased. Mortensen and Thomsen (1997) extended the analysis of the scarf joint to include unbalanced and asymmetric composite substrates in either plate or beam configuration. Adhesive non-linearity has also been included, coupling the peel and shear stresses into a pressure sensitive equivalent stress. Like Gleich et al. (2000) a numerical solution was required and only example analyses rather than parametric studies have been presented. Helms et al. (1997) extended the analyses of scarf joints to include composite substrate shear, in addition to bending and tension. Again, the complexity of the analysis required a numerical solution of the governing differential equations. With enough steps the stepped lap joint approaches the scarf joint. However it is somewhat easier to manufacture than the scarf joint. Like a lap joint, once a step reaches a certain length no further reduction in adhesive stresses can be achieved, rather, the addition of further steps is required. The analysis of a stepped joint is essentially the simultaneous analysis of multiple lap joints with the loading boundary conditions changing with each step. Hart-Smith (1973c) completes his quartet of analyses with the stepped lap joint. Only tension in the substrate and shear in the adhesive has been included but adhesive plasticity has been incorporated into this formulation. Grant (1976) extended Goland and Reissner's (1945) approach from single to stepped lapped joints and also incorporated unbalanced substrates. Initially the deformed shape of the whole joint was found by the application of bending theory. This provided the loading boundary conditions at the beginning and end of the overlap region. Each step was then solved in terms of the (unknown) upper substrate loads at the end of that step. This required three further boundary conditions for each step and these were obtained by imposing displacement and slope continuity across the step

106

Adhesive bonding

boundaries. Mortensen and Thomsen (1997) used a similar approach to their scarf analysis, and solved using a numerical scheme, rather than the quasi-closed form approach of Grant. Their approach however included adhesive plasticity.

5.3.3 Design stress analyses The analyses discussed above have generally been joint specific and not available in a form suitable for general structural design of adhesive joints. General structural analysis packages can certainly be used for this purpose and will be discussed in section 5.4. However, there are a number of approaches and packages that have been developed specifically for adhesive joints and some of these will be discussed briefly below. Bigwood and Crocombe (1989) simplify the general overlap analyses (discussed above) by uncoupling the shear ( P,  V,  M) and peel (V, M) equations and deriving expressions for the maximum stresses for a given load (axial P, shear V or moment M) in terms of joint parameters. P ˆ

V ˆ

1 P 2…1 ‡ 2 † p 2 1 V … 1 ‡ 2 †

0:5

0:75

; V ˆ

; V ˆ

3V 31 M ; M ˆ ; 4t1 t1 …1 ‡ 2 †0:5 1 M

… 1 ‡ 2 †0:5

The parameters i and i were termed the shear and peel compliance factors and are a measure of the relative adhesive to substrate, extension and bending stiffness respectively. This simplification is completely correct for the case of balanced substrates. By using simple superposition the maximum stresses for any combination of end loadings can be found. Simple rules for minimising the adhesive stresses can be seen by considering these formulae. These include: reducing the stiffness of an unloaded substrate, increasing the stiffness of the loaded substrate and increasing the flexibility of the adhesive layer. It can be seen that the overlap length does not appear in these expressions. This is because as long as the overlap exceeds a certain length the adhesive stress distribution, which occurs at the end of the overlap, see Fig. 5.2, will not change. Software packages, based on a number of the analyses discussed above, are available. The full general overlap elastic analysis of Bigwood and Crocombe (1989) is available in a user accessible Excel workbook. ESDU provide a suite of five data items for determining stresses in adhesive joints. Items 80011 and 78042 provide data in the form of charts for double lap joints, the former provides peak peel and shear stresses and the latter an estimate of joint strength based on an inelastic shear lag analysis. Item 92041 provides software for single lap joints giving either elastic shear and peel stresses or inelastic shear lag shear stresses. Items 80039 and 79016 provide software for multi-step lap joints, the

Stress analysis

107

Figure 5.12 Building blocks used in SAAS.

former giving elastic shear and peel stresses whilst the latter provides inelastic shear stresses. All the analyses above are restricted to just one overlap region, either for a specific form of joint (single, stepped, etc.) or for generalised loading. In order to provide design analysis for a wider range of bonded structures, capable of modelling non-linear adhesive behaviour, Crocombe (2002a) developed a package known as SAAS. The versatility of the approach was achieved by implementing FE principles, however, a rule-based FE meshing algorithm made this transparent to the user who simply designs and loads the structure and views the results. Structures are constructed by clipping together the basic building blocks shown in Fig. 5.12. Another approach to facilitate FE was GLUEMAKER, developed by TWI. This is essentially a pre-processor for commercial FE codes such as ABAQUS. This facilitates the model generation for a range of overlap joints. By coupling this with ABAQUS the full power of a commercial FE code is available, however some familiarity with the FE code is likely to be required.

5.4

Finite element analyses of adhesive joints

Finite element analysis is a general stress analysis technique that can be applied to any structure including adhesively bonded joints. The region to be analysed is divided into small sub-regions known as finite elements. The technique is based on the theorem of stationary potential energy which states that when a structure is loaded it will take up the deformed shape that minimises the energy of the system. This deformation is found in terms of the displacements of nodes, which are discrete points usually located on the element boundaries. This only approximates the deformed shape and it is important to ensure that the type of elements and the number of nodes used provide a sufficiently accurate approximation. The reader should consult texts such as Astley (1992) and Zienkiewicz and Taylor (1989) for further details on the FE method. Using the FE method it is possible to model aspects of an adhesive joint that could not be included in the closed form analyses discussed above. Some of the main additional aspects that can be incorporated into FE include:

108 · · · · ·

Adhesive bonding

complex geometric configurations complex material responses coupled multi-physics problems local bi-material stress singularities high-speed dynamic events.

These will each be considered in separate sections below. The literature in this field is vast and the sections below include only a small selection.

5.4.1 Complex geometric configurations This is probably the first area where FE made a real contribution to the understanding of the mechanics of adhesive joints. A fillet of adhesive often results when pressure is applied to the joint during the manufacturing process. This has been termed the spew fillet by Adams and Peppiatt (1974). They, and Crocombe and Adams (1981) showed that a fillet of increasing size transferred more load outside the overlap and so reduced the peak adhesive stress at the end of the overlap. For the joint configuration analysed, it was found that the maximum stress was reduced by 15% for a fillet as small as 1 mm and by more than 50% for a full size fillet. FE modelling has also allowed the variation of stress across the adhesive thickness to be investigated, often assumed constant in the closed form solutions discussed above. Crocombe and Adams (1981) investigated this for the single lap joint at the global stress level (local stress distributions around stress concentrations and singularities are discussed in section 5.4.4). The stresses were found to be essentially constant across the thickness, except for the region at the end of the overlap. A parametric study of the variation of adhesive stresses in this region showed that the variations were greater for lower substrate-adhesive stiffness ratios. Closed form analyses suggest that adhesive stresses decrease with decreasing unloaded substrate stiffness. Thus, including an internal or external substrate taper at the end of the overlap, Fig. 5.13, should enhance joint strength. This is particularly important for composite substrates where high peel stresses may result in interlaminar failure. Adams et al. (1986) consider such configurations in some detail. It was found that, by including an internal taper and an external fillet, predicted joint strengths could be increased by a factor of more than three. These predictions correlated well with experimental joint tests. Stress singularities caused by sharp substrate corners were discussed in section 5.2.3 and will be considered in more detail in section 5.4.4. However, in practice the substrate corner will have some degree of irregularity (rounding) which will remove the singularity locally. This is an aspect that was considered by Adams and Harris (1987). For small degrees of rounding it was found that the singular fields were obtained up to a distance equivalent to the degree of rounding, from the substrate. For cases where the rounding was of the same

Stress analysis

109

Figure 5.13 Internal and external taper.

order as the substrate thickness, increased joints strengths were predicted, based on adhesive plastic energy density, that correlated well with measured joint strengths. An extension of the work on tapering the substrate to improve joint performance has been undertaken by Rispler et al. (2000) and Kaye and Heller (2002). Both have used automated optimisation techniques to determine the most appropriate joint profile. The former used an evolutionary code EVOLVE, which continually iterated and removed elements in the fillet in order to reduce adhesive stresses. A number of different joint configurations were considered and significant stress reductions were found. The resulting fillet shape that evolved naturally was not dissimilar to that considered by Adams et al. (1986). Kaye and Heller (2002) allowed both adhesive and substrate thickness to change in a free form way to minimise the deviation of the adhesive von Mises stresses from the average values. The optimisation routine in the commercial FE code NASTRAN was used. Four different single and double lap joint configurations were analysed and different regions were allowed to evolve. Although such techniques produce optimum configurations, the problem of manufacturing such complex shapes needs to be considered. Other forms of geometric complexity can be readily incorporated into FE models. For example Nakagawa et al. (1999) have studied thermal stress distributions around voids in adhesive butt joints and found that stresses around defects in the centre of the joint are more significant than those near the free surface of the adhesive. Liu and Sawa (2001) modelled combined rivet-bonded single lap joints and investigated the effect of rivet tightening force. They showed that for thin substrates bonded and rivet-bonded gave similar strengths whilst for thicker substrates the rivet-bonded joint was superior. Crocombe et al. (2002c) use FE analyses to determine the substrate strains accurately to enable them to optimise the location of backface strain gauges used to monitor the initiation of fatigue damage in adhesively bonded single lap joints. All the analyses above are two dimensional, with an assumption made that the stress or strain in the out of plane direction is zero (known as plane stress or plane strain respectively). It is generally reckoned that the constraint of much stiffer, surrounding substrates places most of the adhesive in a state of plane strain. Li et al. (2001) have considered the effect of plane stress and strain and state that the

110

Adhesive bonding

choice makes very little difference to the adhesive stress and the bending moment factor for a single lap joint. This would be true only for elastic behaviour as plasticity is much more restricted under a state of plane strain than plane stress. They also conclude that at high loads it does not matter whether the constrained ends of the substrate in the FE model are allowed to rotate or not. The limitations of modelling three-dimensional bonded joints in two dimensions have been considered by Czarnocki and Piekarski (1986) and Richardson et al. (1993), amongst others. The first of these showed that, although the adhesive stress distribution varies across the width of the joint, failure is most likely to occur on the centre plane of the joint and not along its edges. This justified the use of two-dimensional plane strain analyses. Richardson et al. (1993) went on to consider how to modify the load applied to a two-dimensional model to best replicate the stress state on the centre plane of a three-dimensional model. They showed that, for the configuration considered, simply applying the average load on two-dimensional models resulted in errors in the adhesive stresses as high as 20%. A method for determining modified loads was presented. Adams et al. (1994) analyse steel lap joints in three dimensions and show that the transverse anti-claustic curvature in the loaded substrate, adjacent to the overlap end, reduces the peel stresses at the edges of the joint and increases them on the mid-plane. One problem in undertaking three-dimensional analyses is the number of elements required to obtain an accurate solution. Bogdanovich and Kizhakkethara (1999) have applied a technique known as sub-modelling, where the results from a coarse mesh of the global model are applied as boundary conditions to a much finer mesh of a localised region of particular interest. Composite-composite double lap joints were modelled in three dimensions where they showed that convergence of stresses along certain paths in the joint are not guaranteed. They also use the same approach in two dimensions to study in some detail the stresses in the vicinity of a spew fillet. Abdel Wahab et al. (2003) extended FE modelling to the structural level when they analysed the response of composite bonded I beams. Three different analysis techniques were presented and the three-dimensional FE method used a submodelling approach to analyse the stresses around a crack in the adhesive. FE modelling also plays an important role in the assessment of bonded composite repair techniques. Odi and Friend (2002) presented an overview of the field and apply three FE modelling techniques ranging from twodimensional plane stress modelling, through the use of shells, to full threedimensional modelling. They showed that each technique has advantages and disadvantages. Charalambides et al. (1998) modelled co-cured repairs of damaged CFRP plates, representing the composite either as a homogeneous orthotropic material or as a assemblage of separate orthotropic plies. In general, good agreement was found between predicted failure paths and loads. Actis and Szabo (2003) discussed FE modelling techniques for bonded and fastened

Stress analysis

111

repairs. In particular the development of parametric models and their inclusion in FE libraries for subsequent recall was presented. Such models have associated p-type meshing, where convergence of the solution is achieved by increasing the underlying polynomial order of the element rather than increasing the number of elements in the model (known as h-type meshing).

5.4.2 Complex material responses Most of the closed form analyses discussed in section 5.3 assumed a linear elastic material response. Even the most advanced only included a simple nonlinear material representation. All but the most brittle of adhesives exhibit a complex time and temperature dependent non-linear response. Often substrates also exhibit non-linearity. In order to simulate the actual response of a bonded structure it is necessary to include such material behaviour in the analyses. These responses can be included in FE analyses using such material models as plasticity (irrecoverable deformation), hyperelasticity (recoverable deformation) and visco-plasticity or elasticity (time-dependent irrecoverable or recoverable deformation). These models are now available in many FE packages. A good introduction to material non-linearity in FE analysis can be found in texts ranging from introductory (Crocombe, 2002b) through to detailed treatments (Owen and Hinton, 1986). Elasto-plasticity The earliest method of modelling the non-linear behaviour was through the use of plasticity. Adams et al. (1978), Crocombe and Adams (1981) and Harris and Adams (1984) were amongst the first to treat ductile adhesives as elasto-plastic materials. They recognised that, unlike metals, yielding of adhesives (polymers) is dependent on the level of hydrostatic stress. To accommodate this they modified the conventional von Mises yield surface. They were able to predict the strengths of lap joints manufactured with brittle adhesives using a linear adhesive response and the strength of lap and peel joints manufactured with ductile adhesives using a non-linear adhesive response. Crocombe and Adams incorporated substrate plasticity in their analysis of the peel test. High- and lowyield strength flexible aluminium strips were tested and modelled and the important role of plastic energy dissipation was quantified. Harris and Adams also included substrate plasticity to model the three different aluminium alloys used in their experimental work. Subsequent work in this area has refined the criterion used for failure and developed other models for plasticity. Crocombe (1989) used a non-linear analysis and showed that the slower progress of the plastic zone through the overlap region for thinner adhesive layers (see Fig. 5.14) explained why joints with thinner bondlines often have a higher strength. Crocombe et al. (1990) have

112

Adhesive bonding

Figure 5.14 Progress of plastic zone in lap joints with thick and thin bondlines.

carried out non-linear analyses of eight different configurations of joint and investigate the relevance of a range of failure criteria. Sawa and Suga (1996) and Guild et al. (2001) are amongst those who use an elasto-plastic model in conjunction with fracture mechanics to model the failure path and process in bonded joints. The former consider crack growth along the interface of double lap joints whilst the latter consider the crack deflection induced by a film on the adhesive mid-plane in composite double lap joints with a reverse taper and fillets. Both claim good correlation between predicted and experimental results. Duncan and Dean (2003) have extended the yield functions used in the work above. They found that using an extended Drucker-Prager plasticity model resulted in different hydrostatic sensitivities when different stress states were used to derive the data. This was attributed to cavitation of the rubber particles and led to a new flow model. Methods for determining the material constants were found and good correlation was then obtained between FEA and experimental data for butt tension adhesive joints. Hyperelasticity More flexible adhesive systems such as acrylics and polyurethanes exhibit extensive deformation with no discernible yield point. Such responses are better modelled using large strain elasticity (hyperelasticity). Pascal et al. (1994) and Pearson and Pickering (2001) have both presented ways of obtaining MooneyRivlin material parameters that fit the measured high strain material response. These data are then used in FE modelling work. On the basis of this work Pascal

Stress analysis

113

et al. (1994) concluded that crack propagation first appears in an opening mode and not along the interface in their single lap joint. Duncan and Dean (2003) assessed a range of hyperelastic models and tests for generating the model parameters. Following extensive comparison between FE and nine different sets of experimental data they conclude that first-order hyperelastic models showed most promise and that data could be generated from uniaxial tensile tests alone without recourse to other forms of loading. Time dependent behaviour Time dependent (visco) behaviour of adhesives becomes particularly important when temperatures approach the glass transition temperature (TG). These temperatures are often significantly reduced when the adhesive has been exposed to moisture. Reddy and Roy (1991) presented a summary of FE joint analysis and discussed in some detail their non-linear integral visco-elastic material model based on the work of Schapery (1969). It provided a good fit to the material data but was complex to implement and not available in commercial FE packages. Popelar and Liechti (1997) modified the visco-elastic free volume approach of earlier workers to include distortional effects and showed good agreement with experimentally observed local deformation measured using Moire techniques. Su and Mackie (1993) undertook FEA of thick adherend shear tests using a creep material model for two adhesive systems and showed that peak adhesive stresses are reduced with time. The material data was obtained from flexural creep tests on the adhesive. Crocombe (1995a) showed that isochronous stress-strain data, derived from creep compliance, could be used to predict the rate dependent response of single lap joints. Later, Crocombe and Wang (1998) used creep material models to analyse the creep crack growth in bulk adhesive compact tension specimens (Fig. 5.15). Material data was obtained from tensile creep tests on the adhesive and it was shown that the experimental creep crack growth could be predicted using a critical accumulated creep strain ahead of the crack tip. Crocombe (1999) used a creep model to predict the response of adhesive butt joints subjected to sustained loading. It was found that the critical strain increased with decreasing applied load and this trend was also noted in the associated experimental work. An alternative approach for modelling rate dependence is to use elasto-plastic models where the hardening curve is a function of strain rate as well as strain. Over the past few years such models have become available in commercial FE codes and Yu et al. (2001) and Zgoul and Crocombe (2003) have applied them to adhesive joints with some success. In order to model more complex time dependent loading, such as cyclic creep-recovery, unified visco-plastic models have been developed, e.g., Chiu and Jones (1995) and Crocombe et al. (2001). The former model has given good predictions of cyclic shear data but is fairly

114

Adhesive bonding

Figure 5.15 Measured and predicted creep crack growth.

complex and does not account for hydrostatic sensitivity in the adhesive. The model developed by Crocombe et al. overcame these issues and was implemented in commercial FE codes. The rate dependent response of single lap joints (Crocombe, 1995a) was modelled very successfully using the new unified model. Damage modelling The last area of material complexity to be considered in this section is the modelling of damage. This approach enables the complete response of structures up to the final point of failure to be modelled in a single analysis without the need for additional post-processing of FE results. A more descriptive title is progressive damage modelling. This is an emerging field and the techniques for modelling damage can be broadly divided into either local or continuum approaches. In the latter the damage is modelled over a finite region. However, discussion here is limited to the local approach, where the damage is confined to zero volume lines and surfaces in two and three dimensions respectively. This is often referred to as the cohesive crack or cohesive zone approach. Crocombe et al. (1995b) were amongst the first to apply this approach to adhesive joints. They used the cohesive zone approach (which they called stress controlled separation) as a common way to model failure in both cracked and uncracked configurations. The responses of the cracked specimens were unaffected by the shape of unloading in the cohesive zone and it was found that using a stress a little lower than the ultimate tensile strength gave the best results for the noncracked configurations. Swadener and Lietchi (1998) investigated shielding in mixed mode interfacial fracture, combining the cohesive zone model with hydrostatic and rate dependent plasticity. Liechti et al (2000) also used the cohesive zone

Stress analysis

115

approach to evaluate the delamination fracture energy of polyimide films from aluminium substrates from circular blister test results, incorporating plasticity in the films. Rahul-Kumar et al. (1999) applied the technique to adhesive joints, specifically modelling crack growth in T-peel and compressive shear tests. Blackman et al. (2003) applied cohesive zone models to a range of bonded configurations using both analytical and FE techniques. They investigated the uniqueness and physical significance of the maximum (tripping) stress used in the cohesive zone model. Loh et al. (2003a) developed a moisture dependent interfacial rupture element that is based on the cohesive zone model. This was then used to model progressive failure in environmentally degraded specimens. This will be referenced again in section 5.4.3 on coupled multi-physics problems.

5.4.3 Coupled multi-physics problems Coupled multi-physics problems refer to situations where there are two or more governing sets of equations acting simultaneously. Often the solutions to the equations are dependent on each other or are coupled. In the field of adhesive joints the most common coupled problem encountered is a stress-diffusion problem. Another coupling encountered is thermal-stress problems. The coupling in thermal-stress problems is really only one way and thus can be described as a sequentially coupled problem, that is, the results of the thermal analysis affect the stress analysis but not vice versa. Apalak et al. (2003) described a coupled thermal stress problem where heat is transferred through a bonded tee joint structure and removed by forced convection through an air flow. The non-uniform temperature distribution is found initially and this is used as a boundary condition for the subsequent thermal stress analysis. Crocombe (1997) outlined a framework for modelling the degradation caused by exposure to a wet environment. This framework was based on coupling in the stress-diffusion domains. To date, most of the subsequent modelling work (e.g., Crocombe, 1997; Hambly, 1998; Loh et al., 2002, 2003a) has been sequentially coupled with a diffusion analysis providing the moisture distribution from which appropriate moisture dependent material data is used in the stress analysis. Crocombe and Hambly considered the situation where the adhesive was degraded more than the interface, resulting in cohesive failure. The work of Loh et al. addressed the more common situation where the interface became the weak link and interfacial failure was modelled using moisture dependent interface rupture elements. Figure 5.16 shows the application of this technique to single lap joints that were aged in a hot, wet environment. Stable crack growth was predicted with increasing load as the damage spread inwards along a progressively less damaged interface. It is known that stress affects the diffusion process and a limited amount of modelling has been fully coupled, Hambly et al. (1998). A similar approach has been used by Ashcroft et al. (2003) in analysing

116

Adhesive bonding

Figure 5.16 Stable crack growth predicted for a degraded single lap joint.

the fatigue response of composite bonded lap strap joints that had been exposed to various ageing environments. Roy et al. (2000) and Loh et al. (2003b) have developed models for anomalous moisture distribution and implemented them in FE procedures for application to complex shapes.

5.4.4 Local bi-material stress singularities Although bi-material singularities have been studied for some time (Bogy, 1968) one of the first applications to adhesive joints was not presented until much later by Groth and co-workers (e.g., Groth, 1988a). They predicted the strength of unfilleted single lap joints by determining the generalised stress intensity factor H for the peel stress component of the singular bi-material stress field y ˆ Hrÿ The intensity H was found by calculating the singularity strength () from a knowledge of the material properties and using this in conjunction with the FE displacement results obtained from a FE model with a very fine mesh around the overlap end. Groth (1988b) pointed out that the embedded corner considered by Adams and Harris (1987), has two singularity strengths although these can sometimes be approximated by a single term. In fact the more general expression for the singular stress field, given below and written more generally in polar coordinates (r,) based on the singular point (see Fig. 5.6) makes this more clear. X Hkij …†rÿk ij …r,† ˆ Here, k are the eigenvalues of a characteristic transcendental equation. Towse et al. (1999) used a similar local FE model to investigate the mesh sensitivity of a failure criterion based on Weibull statistics. They varied the singularity strength by changing the included angle of the embedded substrate wedge and showed that the maximum Weibull shape parameter that gives a FE

Stress analysis

117

mesh insensitive prediction increases with reducing singularity strength. Lefebvre and Dillard (1999) considered an adhesive wedge on a flat substrate. They investigated the conditions under which the general singular stress equation has a single root and hence where the generalised stress intensity factor H can be used to characterise the stress field and serve as a possible criterion for failure. They found that as long as the adhesive modulus was at least ten times less than that of the substrate, all wedge angles up to 90ë provide a single root. FE analyses of epoxy-aluminium wedges that were used in tests to characterise fatigue initiation, were then carried out and estimates of the plastic zone at the singularity tip were found to be small. Gleich et al. (2001) carried out a similar study to the earlier work of Groth (1988a) and calculated the singularity strength and intensity for a range of adhesive thicknesses. They found that the intensity increased with increasing adhesive thickness (although this is counter to the trends from closed form analyses) and observed that this would account for decreasing joint strength with increasing adhesive thickness. Akisanya and co-workers have made significant contributions to this particular field. Their work has addressed parallel and angled butt joints and mechanical and thermal loading, Akisanya (1997) and Qian and Akisanya (1998). By combining experimental and numerical modelling they showed that failure at a given scarf angle occurs when the free edge intensity factor reaches a certain value for a given scarf angle. They went on to propose a mixed mode criterion that combined results for all scarf angles considered. Mohammed and Liechti (2001) considered both bi-material corners and joints and developed analyses for work hardening plastic, as well as elastic materials. They conclude that it is not generally possible to use generalised stress intensity factors as failure criteria for geometries that have different corner angles.

5.4.5 High-speed dynamic (impact) loading Finite element analyses for impact events generally need to include inertia effects, as the forces to accelerate the structure cannot generally be ignored. This is usually achieved by using an explicit FE code where the load is applied incrementally over very small time steps. A good introduction to such methods is given by Owen and Hinton (1986). Harris and Adams (1985) measured impact and static material properties for four adhesives. These properties were then used in a static FE analysis of various configurations of lap joints made (and tested) with these adhesives. They stated that joint strength predictions were consistent with experimental results. Adams and Harris (1996) assessed the block impact test for adhesives and used FE to investigate the problems with misalignment of the impactor. They pointed out that the analysis is only indicative as dynamic effects have not been included in this preliminary assessment.

118

Adhesive bonding

Sato and Ikegami (2000) presented analyses of single lap, tapered lap and scarf joints. They measured and developed Voigt visco-elastic models which they incorporated in their FE analyses, which used time steps of 0.1 s. Impulses were applied at one end of an otherwise freely supported joint and measured; predicted substrate strains were in good agreement for the tapered and scarf joints. Blackman et al. (2000) used FE to predict the response of the impact wedge peel test and state that the failure behaviour can be successfully modelled. Kihara et al. (2003) described impact test equipment they developed to measure the maximum impact strength of an adhesive joint. They undertook two-dimensional FE analysis using ABAQUS with a time step size of about 0.5 s. The stress waves were transferred through the adhesive layer to unsupported substrates. Good correlation was reported between measured and modelled transient strains thus confirming the validity of the two-dimensional approach. They also showed that the predicted average shear stress was in reasonable agreement with experimental data from a gauge on the adjacent substrate. Sawa and co-workers have carried out significant investigations (numerical and experimental) into the impact loading of adhesive lap joints and butt joints with T shaped adherends, e.g., Higuchi et al. (1999, 2002). Dynamic, elastic three-dimensional FE analyses were carried out using DYNA3D. Strain gauged joints were also tested experimentally under impact conditions to validate the modelling and reasonable agreement was found. One end of the joints was fixed and the impact load was applied, by falling weight, to the other end. Parametric FE studies have been undertaken assessing the effect of substrate material and thickness, adhesive thickness and overlap length. For lap joints, it was found that the maximum adhesive stress (which occurred near the edge of the interface) increased with increasing substrate modulus and decreasing overlap length, adhesive thickness and substrate thickness. It is interesting that the first of these trends is quite contrary to the findings from static analyses, where an increase in substrate modulus would decrease, not increase, the maximum adhesive stress.

5.5

Future developments

Future developments in stress analysis for adhesive joints are likely in two broad fields (i) enhanced accessibility and (ii) improving the realism of the simulation. To enhance accessibility the analysis tools need to become more intuitive to use and need to provide a better interpretation of the results. The adhesive technologist or designer needs to be able to easily construct and modify bonded connections in structures and to know how they will respond to various inputs. Existing FE analysis with progressive damage modelling will provide this but this requires specialist knowledge to implement. Easy construction requires a movement away from joint-specific analyses to a more free form approach. In order to know the loading, the bonded connection or joint must be placed in the

Stress analysis

119

context of the whole structure. Probably the best way to achieve this will be through the use of a sub-modelling approach as discussed in section 5.4.1. However, this needs to be simplified so that the technologist can implement it, ideally the FE model creation will be transparent to the user. This could be achieved by the extension of some of the techniques outlined in section 5.3.3. In fact this development will be mirrored elsewhere as software vendors seek to give software more `intelligence' to support the user. Understanding the response of a bonded joint requires the provision of joint strength rather than adhesive stress distributions. Before progressive damage modelling reaches this level of automation, this can be achieved by embedding appropriate failure criteria enabling the software to provide the user with factors of safety of the complete bonded structure. Improving the realism of the simulation will still require the skills and knowledge of specialists. Some of our current limitations are due to limited computer processing power and these are likely to be addressed with advances in computing technology. Thus in the future it should be possible to undertake dynamic analyses of detailed FE models as a matter of routine. To take full advantage of this, however, will require (i) reliable unified material models which properly couple in the time-temperature-moisture domains, (ii) similar data defining the interface, and (iii) integrated damage response for these models. Moving even further ahead it is appropriate to move away from the usual homogeneous continuum material description to one that can account for variations in the micro and molecular structures. The ideal of predicting the response at the continuum level from a molecular model is just beginning to be achieved. This is likely to continue to provide a fertile ground for future development as people work across the length scales.

5.6

References

Abdel Wahab MM, Crocombe AD, Beevers A and Ebtehaj K, 2002, Int. Journal of Adhesion and Adhesives, 22, pp 61±73. Abdel Wahab MM, Ashcroft IA and Crocombe AD, 2003, J Strain Analysis for Mech Des, in press. Actis RL and Szabo BA, 2003, Comp and Maths with Applic, 46(1), 1±14. Adams RD and Harris JA, 1987, Int J Adhesion and Adhesives, 7, 69±80. Adams RD and Harris JA, 1996, Int J Adhesion and Adhesives, 16(2), 61±71. Adams RD and Mallick V, 1992, J Adhesion, 38, 199±217. Adams RD and Peppiatt NA, 1974, J Strain Analysis, 9, 185. Adams RD, Coppendale J and Peppiatt NA, 1978, Adhesion, 2, 105. Adams RD, Atkins RW, Harris JA and Kinloch AJ, 1986, J Adhesion, 20, 29±53. Adams RD, Gregory DA and Panes GA, 1994, Proc EURADH94, 228±231, Mulhouse, SFA. Akisanya AR, 1997, J Strain Anal for Eng Des, 32(4), 301±311. Allman DJ, 1977, Quart J Mech and Appl Math, 30, 377±386.

120

Adhesive bonding

Anderson TL, 1995, Fracture Mechanics, Fundamentals and Applications, CRC Press. Apalak MK, Apalak ZK, Gunes R and Karakas ES, 2003, Int J Adhesion and Adhsives, 23(2), 115±130. Ashcroft IA, Abdel-Wahab MM and Crocombe AD, 2003, Mech. Adv Matls & Structs, 10, 227±248. Astley RJ, 1992, Finite Elements in Solids and Structures, Chapman and Hall. Bigwood DA and Crocombe AD, 1989, Intl J Adhesion and Adhesives, 9, 229±242. Bigwood DA and Crocombe AD, 1990, Int J Adhesion and Adhesives, 10-1, 31±41. Blackman BRK, Kinloch AJ, Taylor AC and Wang Y, 2000, J Matl Sci, 35(8), 1867± 1884. Blackman BRK, Hadavinia H, Kinloch AJ and Williams JG, 2003, Int J Fracture, 119(1), 25±46. Bogy DB, 1968, J Applied Mechanics, 35, 460±466. Boresi AP, Schmidt RJ and Sidebottom OM, 1993, Advanced Mechanics of Materials, John Wiley and Sons. Bogdanovich AE and Kizhakkethara I, 1999, Composites B, 30(6), 537±551. Charalambides MN, Kinloch AJ and Matthews FL, 1998, Composites A, 29(11), 1383± 1396. Chen D and Cheng S, 1983, J Applied Mechanics, 50, 109±115. Cheng S, Chen D and Shi Y, 1991, J Engineering Mechanics, 118(9), 1962±1973. Chiu WK and Jones R, 1995, Int J Adhesion and Adhesives, 25(3), 131±136. Crocombe AD, 1989, Int J Adhesion and Adhesives, 9(3), 145±153. Crocombe AD, 1995a, Int J Adhesion and Adhesives, 15(1), 21±27. Crocombe AD, 1997, Int J Adhesion and Adhesives, 17(3), 229±238. Crocombe AD, 1999, Int J Matls and Prod Tech, 14(5±6), 411±429. Crocombe AD, 2002a, Paper presented at 2nd World Congress Adhesion and Related Phenomena, The Adhesion Society (USA), Florida, 17±19. Crocombe AD, 2002b, How to Tackle Non-linear Finite Element Analysis, HT19, NAFEMS, 2002. Crocombe AD and Adams RD, 1981, J Adhesion, 13, 141±155. Crocombe AD and Bigwood DA, 1992, J Strain Anal for Mech Des, 27(4), 211±218. Crocombe AD and Wang G, 1998, J Adhes Sci and Tech, 12(6), 655±675. Crocombe AD, Bigwood DA and Richardson G, 1990, Int J Adhesion and Adhesives, 10(3), 167±178. Crocombe AD, Richardson G and Smith PA, 1995b, J Adhesion, 49, 211±244. Crocombe AD, Yu XX and Richardson G, 2001, J Adhes Sci and Tech, 15(3), 279±302. Crocombe AD, Ong CY, Chan CM, Wahab, MA and Ashcroft IA, 2002c, J Adhesion, 78, 745±776. Czarnocki P and Piekarski K, 1986, Int J Adhesion and Adhesives, 6(3), 157±160. Delale F, Erdogan F and Aydinoglu MN, 1981, J Composite Materials, 15, 249±271. Duncan B and Dean G, 2003, Int J Adhesion and Adhesives, 23(2), 143±151. Gleich DM, Van Tooren MJL and De Haan PAJ, 2000, J Adhes Sci Tech, 14(6), 879±893. Gleich DM, Van Tooren MJL and Beukers A, 2001, J Adhes Sci and Tech, 15(10), 1247± 1259. Goland M and Reissner E, 1945, J Applied Mechanics, 66, 17±27. Grant P, 1976, Report SOR(P)109, British Aircraft Corp, Warton. Groth HL, 1988a, Int J Adhesion and Adhesives, 8(2), 107±113. Groth HL, 1988b, Int J Adhesion and Adhesives, 8(1), 55±56.

Stress analysis

121

Guild FJ, Potter KD, Heinrich J, Adams RD and Wisnom MR, 2001, Int J Adhesion and Adhesives, 21(6), 445±454. Hambly HO, 1998, `The Strength of Adhesively Bonded Joints Degraded by Moisture', PhD thesis, University of Surrey. Hambly HO, Crocombe AD and Pan J, 1998, 5th Intl Conference on Structural Adhesives in Engineering, Bristol, Inst of Materials, 217±222. Harris JA and Adams RD, 1984, Int J Adhesion and Adhesives, 4(2), 65±78. Harris JA and Adams RD, 1985, Proc Inst Mech Engrs, 199-C2, 121±131. Hart-Smith LJ, 1973a, NASA report CR112235, Langley Research Centre. Hart-Smith LJ, 1973b, NASA report CR112236, Langley Research Centre. Hart-Smith LJ, 1973c, NASA report CR112237, Langley Research Centre. Helms JE, Yang C and Pang SS, 1997, J Eng Matls and Tech, 119(4), 408±414. Higuchi I, Sawa T and Okuno H, 1999, J Adhesion, 69(1±2), 59±82. Higuchi I, Sawa T and Suga H, 2002, J Adhes Sci and Tech, 16(12), 1547±1702. Kaelble DH, 1960, Trans Soc Rheology, IV, 45±73. Kaye RH and Heller M, 2002, Int J Adhesion and Adhesives, 22(1), 7±22. Kihara K, Isono H, Yamabe H and Sugibayashi T, 2003. Int J Adhesion and Adhesives, 23(4), 253±259. Kim KS and Aravas N, 1988, Int J Solids and Structures, 24(4), 417±435. Kinloch AJ, Lau CC and Williams JG, 1994, Int J Fracture, 66(1), 45±70. Lefebvre DR and Dillard DA, 1999, J Adhesion, 70(1±2), 119±138. Li G and Lee-Sullivan P, 2001, Int J Adhesion and Adhesives, 21(3), 211±220. Liechti KM, Shirani A, Dillingham RG, Boerio FJ and Weaver SM, 2000, J Adhes, 73(2± 3), 259±297. Liu JM and Sawa T, 2001, J Adhes Sci and Tech, 15(1), 43±61. Loh WK, Crocombe AD, Wahab MA and Ashcroft IA, 2002, Eng Fract Mech, 69, 2113± 2128. Loh WK, Crocombe AD, Wahab MMA and Ashcroft IA, 2003a, J Adhesion, in press. Loh WK, Crocombe AD, Wahab MMA and Ashcroft IA, 2003b, submitted to Int J Adhesion and Adhesives. Mohammed I and Liechti KM, 2001, Int J Solid and Struct, 38(24±25), 4375±4394. Mortensen F and Thomsen OT, 1997, Composite Structures, 38, 281±294. Nakagawa F, Sawa T, Nakano Y and Katsuo M, 1999, J Adhes Sci and Tech, 13(3), 309± 323. Odi RA and Friend CM, 2002, J Rein Plas and Comp, 21(4), 311±332. Ojalvo U and Eidinoff HL, 1978, AIAA, 16(3), 204±211. Oplinger DW, 1994, Int J Solids and Structures, 31(18), 2565±2587. Owen DRJ and Hinton E, 1986, Finite Elements in Plasticity, Theory and Practice, Pineridge Press. Pascal J, Darqueceretti E, Felder F and Pouchelon A, 1994, J Adhes Sci and Tech, 553± 573. Pearson I and Pickering M, 2001, Finite Elements in Analysis and Design, 37(3), 221± 232. Popelar CF and Liechti KM, 1997, J Eng Matls and Tech, 119(3), 205±210. Qian Z and Akisanya AR, 1998, Acta Mater, 46(14), 4895±4904. Rahul-Kumar P, Jagota A, Bennison SJ, Saigal S and Muralidhar S, 1999, Acta Mater, 47(15±16), 4161±4169. Reddy JN and Roy S, 1991, Adhesive Bonding (ed. Lee LH), Plenum Press, 359±394.

122

Adhesive bonding

Renton WJ and Vinson JR, 1977, J Applied Mechanics, 101±106. Rice JR, 1988, J Applied Mechanics, 55, 98±103. Richardson G, Crocombe AD and Smith PA, 1993, Int J Adhesion and Adhesives, 13(3), 193±200. Rispler AR, Tong L, Steven GP and Wisnom MR, 2000, Int J Adhesion and Adhesives, 20(3), 221±232. Roy S, Xu WX, Park SJ and Liechti KM, 2000, J Applied Mechanics, 67(2), 391±396. Sato C and Ikegami K, 2000, Int J Adhesion and Adhesives, 20(1), 17±25. Sawa T and Suga H, 1996, J Adhes Sci and Tech, 10(12), 1255±1271. Sawa T, Liu J, Hakano K and Tanaka J, 2000, J Adhes Sci and Tech, 14(1), 43±66. Schapery RA, 1969, A&S Report 69-2, Purdue University, W. Lafayette. Su N and Mackie RI, 1993, Int J Adhesion and Adhesives, 13(1), 33±40. Swadener JG and Liechti KM, 1998, J Applied Mechanics, 65(1), 25±29. Towse A, Potter KD, Wisnom MR and Adams RD, 1999, Int J Adhesion and Adhesives, 19(1), 71±82. Tsai MY and Morton J, 1994, Int J Solids and Structures, 31(18), 2537±2563. Tsai MY, Oplinger DW and Morton J, 1998, Int J Solids and Structures, 35(12), 1163± 1185. Volkersen O, 1938, Luftfahrtforschung, 15, 41±47. Volkersen O, 1965, Construction Metallique, 4, 3. Yang C and Pang SS, 1996, J Eng Matls and Tech, 118(2), 247±255. Yu XX, Crocombe AD and Richardson G, 2001, Int J Adhesion and Adhesives, 21(3), 197±210. Zgoul M and Crocombe AD, 2003, Int J Adhesion and Adhesives, in press. Zienkiewicz OC and Taylor RL, 1989, The Finite Element Method, 4th edn, Vol. 1, McGraw-Hill.

6

Environmental (durability) effects J COMYN

6.1

Introduction

The factors in the natural environment that can attack and deteriorate adhesive joints are oxygen, UV, water and salt spray. Oxygen and UV in combination are more damaging than separately, and they cause chemical degradation of adhesives. This is usually a problem only where one of the adherends is transparent. This problem can be reduced by selecting adhesives with natural resistance, such as acrylics in the case of pressure-sensitive adhesives or by employing stabilising additives. Water, both as liquid or vapour, attacks all adhesive joints, and this is a major problem which limits the use of adhesives. Water enters adhesives and alters their properties, but this is a lesser problem than attack by water on the adhesive interface. With structural adhesive joints to metals, loss of joint strength is minimised by selection of a suitable metal pre-treatment, or the use of a coupling agent. Water is a problem because of its ubiquity and its extreme properties. It has a high permittivity which will be significant if ion-pairs contribute to the interfacial forces. Also it has high surface tension which will particularly weaken van der Waals forces between adhesive and metallic substrates.

6.2

Additives to reduce photo-oxidative degradation

Oxygen can be absorbed by polymers with the formation of hydroperoxide (±OOH) groups, which can later decompose to give free radicals, which so provide active centres for polymer degradation. Antioxidants are additives that act as radical scavengers. Here hindered phenols are examples, and their action is illustrated in Fig. 6.1, by the case of butylated hydroxytoluene (BHT). The BHT molecule loses a hydrogen atom which combines with the radical R but itself becomes a radical, which is prevented from reacting by the bulky ± C(CH3)3 groups. BHT is a very effective radical-scavenger, but has the drawback of being volatile. Loss by evaporation (Comyn, 1998) can be eliminated by

124

Adhesive bonding

Figure 6.1 Antioxidant and UV stabilisers for adhesives.

using larger molecules which contain the same moiety; these are effectively a number of BHT molecules linked together. 2-Hydroxybenzophenones are examples of UV-stabilisers and when they absorb a UV photon the molecule enters an excited state which involves reorganisation of the hydrogen bond and some carbon-carbon double bonds. It returns to the ground state by losing the excess energy as heat. This is shown in Fig. 6.1. Glass-fibre reinforced polyester can transmit UV, and joints bonded to galvanised steel with a moisture-cured isocyanate adhesive have been exposed to a variety of ageing conditions (Ramani et al., 2000). Joints were weakened in a weatherometer and the mode of failure moved to the polyester-adhesive interface; in unexposed joints failure was cohesive or at the steel-adhesive interface. When exposure was in a QUV there was no change in strength or mode of failure, as the UV did not penetrate.

Environmental (durability) effects

6.3

125

Behaviour of structural joints to metals in wet surroundings

6.3.1 Effect of humidity There are many cases in the literature (for example, Falconer, MacDonald et al., 1964; Gledhill and Kinloch, 1974 and Cotter, 1977) of adhesive joints with metallic adherends and rigid adhesives being weakened by exposure to wet surroundings, and a common feature is the shape of the plot of joint strength against time. Joint strength falls most rapidly at the beginning, and eventually slows down to a very low or zero rate. Although the shapes of curves are similar, there are variations in the initial rates of strength loss and in the fraction of strength which is lost. Experiments by Brewis, Comyn et al. (1980a, 1980b, 1980c, 1981, 1987b) exposed joints with aluminium adherends and a range of structural adhesives at 50 ëC and 100% relative humidity (r.h.), and control specimens were either stored in the laboratory or at 50 ëC and 50% r.h. The patterns which emerged are fairly typical, and they are illustrated in Figs 6.2 and 6.3 and listed below. 1. 2. 3.

On exposure to air at 100% r.h. and 50 ëC joint strengths initially fall, typically by 40±60%, but then tend to level out. Little or no weakening takes place when joints are aged at 50% r.h. and 50 ëC. When joints which have been exposed at 100% r.h. for 5,000 h are then stored for a further 5,000 h at 50% r.h., a significant part of the strength is recovered. These are the triangular points in Figs 6.2 and 6.3.

Figure 6.2 Strengths of joints in aluminium alloy bonded with a nitrile-phenolic adhesive on exposure to wet air at 50 ëC, m 50% r.h. q 100% r.h. joints which have been exposed at 100% r.h. for 5,000 h are then stored for a further 5,000 h at 50% r.h. D Brewis, Comyn et al. (1987b). Crown copyright.

126

Adhesive bonding

Figure 6.3 Strengths of joints in aluminium alloy bonded with a nitrile-phenolic adhesive on exposure to wet air at 50 ëC, m 50% r.h. q 100% r.h. joints which have been exposed at 100% r.h. for 5,000 h are then stored for a further 5,000 h at 50% r.h. D Brewis, Comyn et al. (1987b). Crown copyright.

4. 5.

The use of a primer with a nitrile phenolic adhesive had a marked improvement on dry and humid aged strengths. The amount of failure at the adhesive-metal interface increased with time of exposure at 100% r.h.

Although joints are weakened by exposure to air of high humidity (e.g., 80± 100% r.h.), it has been frequently observed that joints can withstand exposure at lower humidities (e.g., 50% r.h. or less) for long periods without weakening. For example, DeLollis (1977) has referred to some epoxide-aluminium joints which showed no loss of strength after exposure to laboratory humidity for up to 11 years. In experiments on epoxide adhesives, Brewis et al. (1980a, 1980c, 1981) found no significant weakening of joints after exposure for 10,000 hours at about 45% r.h. and 20 ëC. Figure 6.4 shows that joints with a range of adhesives (Ashcroft, Digby and Shaw, 2001) are not significantly weakened on exposure to a natural hot/dry climate for up to six years. Here the average temperature was 25 ëC and the average 55% r.h. Such information led to the proposal from Gledhill, Kinloch and Shaw (1980) that there must be a critical concentration of water in the adhesive and corresponding relative humidity in the surroundings, which demarcates conditions under which weakening will occur from those under which it will not. In a joint which is absorbing water, there may be an outer zone where the critical water concentration is exceeded, and this zone can be regarded as a crack in the bondline which can be dealt with by fracture mechanics. The hypothesis

Environmental (durability) effects

127

Figure 6.4 Strengths of unstressed double lap joints on exposure to a hot/dry climate for up to six years. Ashcroft, Digby and Shaw (2001), Crown copyright.

was tested with some butt joints bonded with an epoxide immersed in water at 20, 40, 60 and 90 ëC and also in air at 20 ëC and 55% r.h. All the water-immersed joints became weaker, and it could be shown using the fracture mechanics approach that the strengths of the joints could be correlated if the critical concentration of water in the adhesive was 1.35%. Brewis, Comyn et al. (1990) attempted to locate the critical conditions for some aluminium joints bonded with an epoxide adhesive. Surface preparation of the aluminium alloy adherends was by sand-blasting; this was chosen because of the poor durability of sand-blasted joints, a factor which was thought would give rapid results and which would be particularly sensitive to changes in relative humidity. Joints stored for up to 1,008 hours do not weaken with increasing r.h. A slight weakening was evident after 2,016 hours, and this became greater after 5,040 and 10,080 hours. After 10,080 hours the locus had a kink at 65% r.h. (Fig. 6.5). This corresponds to a critical concentration of water in the adhesive of 1.45%, a value which is very similar to that of Gledhill, Kinloch and Shaw.

6.3.2 Surface treatment Surface treatment of metals is the most effective way of optimising the resistance of joints to water. A graphic illustration of the effect of surface treatment on the wet-durability of adhesive joints to aluminium was given by Butt and Cotter 1976, and their data are shown in Fig. 6.6. The surface

128

Adhesive bonding

Figure 6.5 Dependence of joint strength upon relative humidity, after 10,080 hours exposure. Brewis, Comyn et al. (1990).

treatments employed were etching in chromic-sulfuric acid, alkaline etching, solvent degreasing and phosphoric acid anodising. Before exposure the different treatments gave identical joint strengths but exposure at 43 ëC and 97% r.h. produced differences. Here the degreased adherends performed worst and those

Figure 6.6 Effect of high humidity (97% r.h. at 43 ëC) on the strength of aluminium joints bonded with an epoxide-polyamide adhesive. After Butt and Cotter (1976). Surface pre-treatments are m chromic-sulfuric acid etch, o alkaline etch (commercial formulation), n solvent degrease, l phosphoric acid anodise.

Environmental (durability) effects

129

etched in chromic-sulfuric acid did best. In this case anodisation in phosphoric acid gave a poor performance, but this is unusual. The surface pre-treatment of aluminium alloys for adhesive bonding has been reviewed by Critchlow and Brewis (1996). A total of 41 treatments were considered, and these included mechanical, chemical and electrochemical methods. In some cases a further step in the treatment process included the use of chemicals such as primers, coupling agents of hydration inhibitors. It was reported that phosphoric acid anodisation was the best surface treatment for optimum wet-durability, but noted that treatment should be matched to a suitable primer and adhesive. The literature in the period 1976±1991 on the wet-durability of aluminium joints bonded with epoxide adhesives has been reviewed (Armstrong, 1997); a test programme was also performed in which wedge-test samples were immersed in water at ambient temperatures for up to five and a half years. Surface treatments were anodisation in chromic acid, etching in phosphoric acid and abrasion. A comments was that `one should not talk of the durability of adhesives so much as the durability of surface preparation methods'. The superior performance of anodisation was noted. Immersion in solutions is impractical for treating large structures when repairs are needed. Bergan (1999) has described a technique for phosphoric acid anodising which involves containing the acid in place using a vacuum bag. Critchlow and Brewis (1995) have also reviewed surface treatments for the titanium alloy Ti-6Al-4V, and reported that overall the most effective methods for improving wet-durability are anodising in sodium hydroxide or chromic acid solutions. At the time of reporting plasma spraying was showing promise.

6.3.3 Natural and accelerated ageing A comparison of ageing joints in natural and laboratory conditions has been undertaken by Ashcroft, Digby and Shaw (2001); they commented that their data are probably unique. The aluminium adherends were etched in chromic acid and bonded with a total of eight epoxide or phenolic adhesives, which were exposed to the natural weathers shown in Table 6.1. Laboratory ageing was at 20 ëC and 60% r.h. or at 35 ëC and 85% r.h. The mode of failure was dominantly cohesive, Table 6.1 Natural weathers used by Ashcroft, Digby and Shaw (2001) Location Hot/wet Hot/dry Temperate

Australia Australia UK

Average conditions Temperature/ëC r.h./% monthly rainfall/mm 23 25 10

85 55 78

297 39 49

130

Adhesive bonding

but the amount of interfacial failure increased with both natural and laboratory ageing. This was often accompanied by metallic corrosion, particularly in hot/ wet conditions. A conclusion was that there is no simple method of determining the longterm durability from accelerated tests, and that excessive temperatures and humidities will trigger degradation mechanisms which are not representative. In general, accelerated tests tend to overestimate the reduction of joint strength.

6.3.4 Salt water Common salt can seriously weaken and corrode adhesive joints. In some tests involving double cantilever beam specimens in aluminium, McMillan (1981) found that exposure to 5% salt spray for three months was more damaging than exposure to semi-tropical conditions for three years. Fay and Maddison (1990) have described the effect of salt-spray on joints in steel. Their results, which are given in Fig. 6.7, show that surface treatments with the proprietary products Accomet C and EP2005, or a silane coupling agent, give better protection than with oily or degreased surfaces.

6.3.5 Stress Joints tend to weaken more rapidly if they are stressed during exposure. Davies and Fay (1993) have reported time to failure for joints, both stressed and unstressed, with mild steel and zinc-coated steel adherends. Results for zincnickel coated steels are shown in Fig. 6.8 showing that all unstressed joints survived for two and a half years and that survival of the others decreases with increasing stress. Fay and Maddison (1990) measured times to failure at 100% r.h. and 42±48 ëC for joints in steel with a number of surface treatments, and

Figure 6.7 Effect of salt spray on strength of steel lap joints. Fay and Maddison (1990).

Environmental (durability) effects

131

Figure 6.8 Average failure times of joints in zinc-nickel coated steel under stress, on exposure to tropical conditions. Davies and Fay (1993).

bonded with a toughened epoxide adhesive. Results, which are shown in Table 6.2, show that failure is hastened by increasing stress and that time to failure can be much increased by selection of an appropriate surface treatment. Parker (1993) has described durabilities of stressed and unstressed joints in clad aluminium alloy BS 2L73 exposed to hot-wet, hot-dry and temperate climates for up to eight years. A variety of surface treatments were used. Joints were stressed to either 10 or 20% of their dry strength. Failure was hastened by increasing stress. At 20% stress the order of effectiveness of surface treatments was phosphoric acid anodise > chromic acid anodise > chromic acid etch. The effect of cyclic stresses on the durability of aluminium-epoxide joints has been observed by Briskham and Smith (2000), using a range of surface treatments. Joints were immersed in water at 55 ëC and stress levels were about 0.15 or 1.2 MPa and the frequency was 2 Hz. Here the best performing treatment was phosphoric acid anodisation. Treatment with an aminosilane coupling agent was the poorest, which perhaps was not expected as this performed better than all the other methods with unstressed joints. Joints with phosphoric acid anodisation consistently failed in a cohesive manner, while all the other methods showed some interfacial failure.

132

Adhesive bonding

Table 6.2 Times to failure for stressed joints exposed at 100% relative humidity and 42±48 ëC. Fay and Maddison (1990) Surface treatment

Load/kN

Time to failure/days

Degreased

0.4 1.0 2.0

72 44 25

86 44 28

86 44 28

Oiled

0.4 1.0 2.0

62 24 0

66 25 0

70 25 0

Accomet C (proprietary)

0.4 1.0 2.0

254 89 18

>1121 103 18

>1121 110 19

Silane coupling agent GPMS

0.4 1.0 2.0

>1121 124 26

>1121 126 28

>1121 128 28

EP 2005 (proprietary)

0.4 1.0 2.0

96 62 12

99 63 12

125 64 12

Reference has been made in section 6.3.3 to work by Ashcroft, Digby and Shaw on ageing joints in natural and laboratory conditions. Their programme included exposing stressed joints, and their results for joints in a hot/wet climate after six years are shown in Fig. 6.9. It can be seen that all stressed joints with EP adhesive are reduced to zero strength, whilst most of the others maintain a significant fraction of their strength.

6.3.6 Alloy type The intention in cladding aluminium alloys with the pure metal is to reduce corrosion, however, it has been shown that clad alloys can be inferior in salt spray tests. The matter has been reviewed by Brewis (1983). The data in Table 6.3 are due to Minford (1981) for the exposure of joints with three different aluminium alloys, and three different surface treatments to high humidity and salt-spray. Alloy X5085-H111 is more durable than the others when the metal has not been treated or is only degreased. Poole and Watts (1985) assessed the durability of some bonded aluminium alloys using the Boeing wedge test. They analysed the metal surfaces by x-ray photoelectron spectroscopy (XPS) but found no significant correlation between composition and durability. Kinloch and Smart (1981) used XPS to analyse the surfaces of an aluminium alloy used in butt-joints, some of which had been exposed to water. It appeared that the amount of magnesium oxide on the

Environmental (durability) effects

133

Figure 6.9 Effect of stress on double lap joints exposed to hot/wet climate for six years. Ashcroft, Digby and Shaw (2001), Crown copyright.

surface may be an important factor in influencing durability, a high concentration of MgO being associated with poor durability.

6.4

Water and adhesives

6.4.1 Water diffusion into adhesive bondlines All adhesives absorb water, and water uptake data for a number of structural adhesives are collected in Table 6.4. Such data are obtained by measuring the weight of water absorbed by an immersed film, and include the diffusion coefficient D and the weight absorbed at equilibrium ME. This means that adhesive layers in joints will absorb water and its vapour, and transmit it to the interface. This cannot be prevented by sealing the edges with a paint or lacquer, as these also absorb water. The data can be used to calculate the rate at which water will enter joints, and water concentration profiles within them (Comyn, 1983). Metallic adherends are impermeable to water, but water would enter by diffusion through permeable adherends such as fibre reinforced plastics and wood. The rate of the initial fall in strength of exposed joints is controlled by the rate of water diffusion in the bondline. This is demonstrated in Fig. 6.10 where joint strengths are compared with the amount of water to have entered a joint. Although the scales of the two ordinates have been adjusted to give a best fit, there is nevertheless an excellent comparison between the points, which are for measured joint strengths, and the line which is the calculated water level.

Table 6.3 Effect of alloy type and surface treatment on durability of joints bonded with a one-part epoxide adhesive, Minford (1981) Exposure conditions

None 3 months 23.9 ëC/85% r.h. 3 months 51.7 ëC/100% r.h. 3 weeks 35 ëC/5% salt spray

Joint strength/MPa A

Alloy 2036-T4 B

C

A

Alloy 6151-T4 B

C

A

12.7 2.6 0.14 0

13.3 2.2 0 0

14.7 15.2 4.8 7.6

17.6 9.3 6.3 0.55

18.5 10.7 7.2 1.0

17.4 16.6 7.6 14.4

15.5 12.8 8.7 4.7

Surface pretreatments are A ± mill finish, B ± vapour degrease, C ± chromate-phosphate conversion coating.

Alloy X5085-H111 B C 15.6 13.0 10.5 3.7

14.5 13.2 8.5 8.3

Environmental (durability) effects

135

Table 6.4 Water uptake properties of structural adhesives Adhesive Nitrile-phenolica Vinyl-phenolica FM1000 epoxide-polyamideb

Acrylic adhesives toughened withc Chlorosulfonated polyethylene Nitrile rubber

Wood adhesivesd Urea-formaldehyde Melamine-formaldehyde Phenol-resorcinol-formaldehyde Epoxides, DGEBA with the following hardenerse Di(1-aminopropylethoxyether) Triethylene tetramine 1,3-diaminobenzene Diaminodiphenylmethane

t (ëC)

D (10ÿ12 m2 sÿ1)

ME (%)

25 50 25 50 1 25 50

3.3 4.7 1.8 2.3 0.075 1.1 3.2

1.50 4.5 3.5 8.6 (20.4) (15.8) (15.5)

23 37 47 23 37 47

0.64 1.20 0.94 0.19 0.28 0.66

0.73 0.82 3.27 1.72 2.99 3.89

25 40 25 40 25 40

0.25 0.49 0.21 0.39 1.1 1.8

7.4 8.2 27.2 41.0 13.9 14.4

25 45 25 45 25 45 25 45

0.13 0.46 0.16 0.32 0.19 0.97 0.0099 0.006

5.0 4.7 3.8 3.4 2.3 3.1 4.1 1.6

Polyimides based on pyromellitic dianhydride with the following hardenersf 1,4-diaminobenzene 22 0.015 7.4 22 0.56 3.3 4,40 -oxydianiline Note: Bracketted values of ME are not true equilibrium values, but maximum uptake observed in systems which then lost weight. Sources: aBrewis, Comyn et al. (1987a), bBrewis, Comyn et al. (1980c), cBianchi, Garbassi et al. (1990), dBrewis, Comyn et al. (1987b), eBrewis, Comyn et al. (1980a), fMoylan, Best et al. (1991).

136

Adhesive bonding

Figure 6.10 Comparison of joint strength (experimental points) with calculated water uptake (line) by joints bonded with DGEBA-DAPEE epoxide adhesive. Comyn, Brewis et al. (1979).

Furthermore, linear relationships have been observed between strength and water content for a number of joints with aluminium adherends with a range of structural adhesives by Brewis, Comyn and their coworkers (1980a±c, 1981, 1987a). An exception was the epoxide-polyamide adhesive FM1000 where there was a clear deviation from the straight line at high water content, and this was accompanied by the onset of corrosion. Parker (1988) has shown a linear relationship between joint strength and the square root of exposure time to humid conditions, for some titanium alloy joints bonded with a modified epoxide adhesive, so demonstrating that the decline in strength is controlled by the diffusion of water. Some joints had been exposed for 11 years. The diffusion of water in structural adhesives obeys the Arrhenius equation, which means that the rate of diffusion increases strongly with temperature. The expected consequence of this is that joints will weaken more rapidly as the temperature rises. This was shown to be the case by Gledhill and Kinloch (1974), and Gledhill, Kinloch and Shaw (1980). Here butt-joints in mild steel, prepared by degreasing and grit-blasting, were bonded with an epoxide adhesive and immersed in water.

6.4.2 Reversible and irreversible processes Once within a joint, there are several possible ways by which water may cause weakening. These have been reviewed by Comyn (1983) and include the following. · altering the properties of the adhesive in a reversible manner, such as plasticisation · altering the properties of the adhesive in an irreversible manner, such as causing it to crack, craze or hydrolyse

Environmental (durability) effects

137

· attacking the adhesive-adherend interface · causing swelling stresses. After exposing some electrically conducting adhesives to saturated air at 85 ëC for up to 50 days, and then drying some at 150 ëC, (Xu and Dillard, 2003) carried out a number of mechanical and thermal tests and measured water absorption, which indicated that the reversible process of plasticisation and the irreversible ones of further crosslinking and thermal degradation were taking place. Recovery of joint strengths clearly indicates that reversible processes are involved, and the increasing amounts of interfacial failure with exposure show that interfacial phenomena are more important then bulk properties of the adhesive.

6.4.3 Hydrolysis Bowditch (1996) has commented that `structural adhesives are selected with essentially hydrolysis resistant chemistry and so chemical attack is not generally an important degradation mechanism' and that swelling of an adhesive in water would be associated with absorption, making such materials be seen as unsuitable.

6.5

Water and adhesive interfaces

6.5.1 Oxide stability A widely held view is that surface treatments which confer most wet-durability do so by producing an oxide layer which is durable. Certainly such methods replace the adventitious oxides on milled metals. With etching or anodising in chromic or phosphoric acid solutions, the new oxide layer is thicker and has a honeycomb structure. This issue has been reviewed by Kinloch (1987). Ahearn and Davies (1989) have shown that durability can be enhanced by some phosphoric acid compounds can act as hydration inhibitors.

6.5.2 Physical adsorption and work of adhesion The measurement of contact angles which liquids make on solid surfaces can be used to obtain the surface energy components of the solids, and these in turn can be used to calculate the work of adhesion for two solids in contact. The latter calculation can be for two surfaces in an inert medium such as dry air, or in water or saturated vapour. All this assumes that the mechanism of adhesion is physical adsorption. If the work of adhesion is positive, then the bond is stable. Conversely a negative work of adhesion indicates instability. The parameter which has created most interest in the literature is the work of adhesion in the presence of water, as

138

Adhesive bonding

Table 6.5 Values of work of adhesion for various interfaces in dry air and in water Work of adhesion/mJ mÿ2

Interface

Epoxide/steel Epoxide/aluminium Epoxide/silica Epoxide/carbon fibre composite

Air

Water

Interfacial debonding in water?

291 232 178 88±90

±255 ±137 ±57 22±44

Yes Yes Yes No

this can be used to predict joint durability. Kinloch (1983) has compared work of adhesion of adhesive interfaces in air and in water with their tendency to debond interfacially in an unstressed condition. Some data are shown in Table 6.5. The fact that interfacial debonding occurs only when the thermodynamic work of adhesion is negative is very strong evidence of the validity of thermodynamics in predicting the durability of adhesive bonds. Because metals have high energy oxide surfaces, the work of adhesion in the presence of water will be negative, i.e., water may displace the adhesive from the substrate. That such phenomena are due to liquids attacking the interface rather than just swelling the adhesive was demonstrated by Orman and Kerr (1971) who showed that whilst ethanol swells and reduces the tensile strength of an epoxide adhesive, it has little effect on the strength of aluminium joints bonded with the same adhesive. In contrast, water has minimal effect on the tensile strength of the adhesive, but causes large weakening of aluminium joints. What underlies this is the extreme properties of water, and in particular its high value of the polar component of surface free energy. The value is 51.0 mJ mÿ2 and that for ethanol is 5.4 mJ mÿ2.

6.5.3 Chemical bonds Chemical bonds are inherently strong, and would usefully contribute to the durability of adhesive joints. Such bonds might be ion-pairs or covalencies. The force F‡ÿ of two ions separated by distance r is given by eqn 6.1, where z1 and z2 are the valencies of the ions, e is the electronic charge, 0 is the permittivity of a vacuum and r is the relative permittivity of the medium. F‡ÿ ˆ

z1 z2 e2 40 r r2

6:1

Epoxide adhesives have low values of r (about 4 or 5) and phenolics are probably similar, whilst that for water is about 80. Hence a small amount of water entering an adhesive would increase r and lower F‡ÿ , not to zero, but to a fraction of its original value. Complete removal of water would restore F‡ÿ to its original value.

Environmental (durability) effects

139

Table 6.6 Comparison of experimental and calculated falls in joint strengths on ageing at 100% relative humidity at 50 ëC. Chromic acid etched adherends (Comyn, Brewis and Tredwell, 1987) Adhesive

Modified epoxide BSL 312 Epoxide nylon FM1000 Epoxide DGEBA/DAPEE Nitrile phenolic 1 Nitrile phenolic 2 Nitrile phenolic 2 with primer Vinyl phenolic

Fall in joint strength % Experimental

Calculated

50 78 40 54 37 14 45

36 68 45 40 20 18 56

The relative permittivities of mixtures of water with organic solvents are approximately linear with composition. If this is the case for water-adhesive mixtures and the relative permittivity of the adhesive reasonably represents that surrounding the interfacial ion-pairs, then strength reductions can be calculated. The results of the calculation, using r ˆ 5 for adhesives and r ˆ 80 for water are compared with actual falls in strength in Table 6.6 (Comyn, Brewis and Tredwell, 1987). Agreement between the two columns of figures in Table 6.6 is considered to be good. The ion-pair approach allows partial weakening of joints in the presence of water, with recovery when the joints are dried out. This is in contrast to the physical adsorption theory which predicts the reduction in strength to zero as water displaces adhesive from the metal oxide, and no recovery as a glassy adhesive would have insufficient molecular mobility for it to re-establish intimate contact with the substrate. Silane coupling agents are widely used to improve the wet-durability of adhesive joints, and the common view is that interfacial covalent bonds are produced. They have the general structure R±Si(OR0 )3, where R is a group that can react with the adhesive of liquid resin and R0 is usually methyl or ethyl. A graphic illustration of the effectiveness of silane coupling agents was provided by Comyn, Groves and Saville (1994) for some joints of glass bonded to lead alloy with an epoxide adhesive. Some of the glass specimens had been treated with 3-aminopropyl triethoxysilane (APES). The joints had been exposed to warm, wet air (100% r.h. at 50 ëC). Joint strengths fell to zero without the silane, but fell by a moderate amount and then tended to level out when APES is used. In fact after 96 days exposure, most of the joints without the silane had fallen apart. Some interfaces between amorphous silica and aluminium were found (Turner and Boerio 2002) to be strong and resistant to water at 60 ëC, and because of this it was proposed that the Si±O±Al and Si±O±Ti covalent linkages

140

Adhesive bonding

were formed at the interface. A number of workers have provided spectroscopic evidence for the formation of Si±O±metal linkages between silane coupling agents and metals or metal oxides. These include Gettings and Kinloch (1977) who found Si±O±Fe and Si±O±Cr with stainless steel, Davis and Watts (1996) who found Si±O±Fe on iron and Naviroj, Koenig, and Ishida, (1985) who identified Si±O±Al and Si±O±Ti on powdered metal oxides.

6.6

Other fluids

There is little information available on the behaviour of joints in fluids other than water. Lap joints in aluminium bonded with polysulfide, silicone and fluorosilicone sealants have been immersed in jet-fuel, water and waterantifreeze (monomethyl ether of diethylene glycol) mixtures (Comyn, Day and Shaw, 1997). The adherends were solvent-degreased and some were coated with an epoxide-chromate primer. It was seen that joints with polysulfide were not weakened in jet-fuel or water at room temperature after 160 days, and that performance was not improved by the use of phosphoric acid anodisation or silane primers. Joints were much weakened in antifreeze. Here the best guide to durability was the amounts of liquids absorbed by the sealants; specifically the polysulfide absorbed 60% by weight of antifreeze and the silicone 87% of jet-fuel. The fluorosilicone absorbed very little of any of the fluids and its joints were not weakened. When the adhesive was a rubber modified epoxide (Comyn, Day and Shaw, 2000), strengths after immersion were much increased by the use of phosphoric acid anodisation and the coupling agents 3-glycidoxypropyl trimethoxysilane and 3-aminopropyl triethoxysilane. All joints were weakened by immersion in antifreeze but not in jet-fuel or waterantifreeze.

6.7

Timber joints

The durability of wood joints bonded with urea-formaldehyde (UF), melamineurea-formaldehyde (MUF), phenol-formaldehyde (PF) and resorcinol-phenolformaldehyde (RPF) adhesives has been reviewed by Dinwoodie (1983). Correlation between accelerated tests and natural weathering is not good but best durability is obtained with adhesives containing phenol, resorcinol or melamine, and worst durability with unmodified UF adhesives.

6.8

Future trends

The current state of the technology is that metals can be bonded with structural adhesives, and these will survive for long periods in outdoor conditions. They will be weakened and the extent of the loss will be about one-quarter to one-half the dry strength. The main problem is that the surface treatments have many

Environmental (durability) effects

141

stages and are thus costly, and chromium compounds are expensive, carcinogenic and present a disposal problem. Indeed it seems that only industries such as aerospace and Formula 1 can afford them. The thrust of development will be towards simpler and friendlier surface treatments that can be widely used in manufacture, and adhesives where the surface treatment is built in. The latter path has already been trodden in that many adhesives and sealants contain silane coupling agents.

6.9

Further information

The most comprehensive treatment of wet-durability is Kinloch's book (1983). In this Minford (1983) has compared wet durabilities of joints with phenolic and epoxide-based adhesives. Joints in aluminium have been reviewed by Brewis (1983), Mahoon (1983) has dealt with titanium and Brockmann (1983) with steel. Hartshorn (1986) has tabulated references to outdoor weathering trials with structural adhesive joints. Reviews which have provided updates are those by Critchlow and Brewis (1995 and 1996) which respectively deal with the surface treatment of titanium and aluminium, and that by Armstrong (1997). The paper by Ashcroft, Digby and Shaw (2001) which compares natural and laboratory ageing over six years is noteworthy.

6.10 References Ahearn J S and Davies G D (1989), J Adhes, 28, 75. Armstrong K B (1997), Int J Adhes Adhes, 17, 89. Ashcroft I A, Digby R P and Shaw S J (2001), J Adhes, 75, 175. Bianchi N, Garbassi F, Pucciariello R and Romano G (1990), Int J Adhes Adhes, 10, 19. Bergan L (1999), Int J Adhes Adhes, 19, 199. Bowditch M R (1996), Int J Adhes Adhes, 16, 73. Brewis D M (1983), Durability of Structural Adhesives, Kinloch A J Ed, Applied Science Publishers, London, Ch 5 p 215. Brewis D M, Comyn J and Tegg J L (1980a), Polymer, 21, 134. Brewis D M, Comyn J and Tegg J L (1980b), Int J Adhes Adhes, 1, 35. Brewis D M, Comyn J, Cope B C and Moloney A C (1980c), Polymer, 21, 1477. Brewis D M, Comyn J, Cope B C and Moloney A C (1981), Polymer Eng Sci, 21, 797. Brewis D M, Comyn J and Phanopoulos C (1987a), Int J Adhes Adhes, 7, 43. Brewis D M, Comyn J and Tredwell S T (1987b), Int J Adhes Adhes, 7, 30. Brewis D M, Comyn J, Raval A K and Kinloch A J (1990), Int J Adhes Adhes, 10, 27. Briskham P and Smith G (2000), Int J Adhes Adhes, 20, 33. Brockmann W (1983), Durability of Structural Adhesives, Kinloch A J ed, Applied Science Publishers, London, Ch 7 p 281. Butt R I and Cotter J L (1976), J Adhes, 8, 11. Comyn J (1983), Durability of Structural Adhesives, Kinloch A J ed, Applied Science Publishers, London, Ch 3, p. 85.

142

Adhesive bonding

Comyn J (1998), Plast Rubber Compos Process Appl, 27, 110. Comyn J, Brewis D M, Shalash R J A and Tegg J L (1979), Adhesion, 3, 13. Comyn J, Brewis D M and Tredwell ST (1987), J Adhes, 21, 59. Comyn J, Day J and Shaw S J (1997), Int J Adhes Adhes, 17, 213. Comyn J, Day J and Shaw S J (2000), Int J Adhes Adhes, 20, 77. Comyn J, Groves C L and Saville R W (1994), Int J Adhes Adhes, 14, 15. Cotter J L (1977), Durability of Structural Adhesives, in Developments in Adhesives ± 1, Wake W C ed, Applied Science Publishers, London, Ch 1. Critchlow G W and Brewis D M (1995), Int J Adhes Adhes, 15, 161. Critchlow G W and Brewis D M (1996), Int J Adhes Adhes, 16, 255. Davies R E and Fay P A (1993), Int J Adhes Adhes, 13, 97. Davis S J and Watts J F (1996), Int J Adhes Adhes, 16, 5. DeLollis N J (1977), Natl SAMPE Symp Exhib, 22, 673. Dinwoodie J M (1983), Wood Adhesives; Chemistry and Technology, Pizzi A ed., Ch 1, p. 1. Falconer D J, MacDonald N C and Walker P (1964), Chem Ind, 1230. Fay P A and Maddison A (1990), Int J Adhes Adhes, 10, 179. Gettings M and Kinloch A J (1977), J Mater Sci, 12, 2511. Gledhill R A and Kinloch A J (1974), J Adhes, 6, 315. Gledhill R A, Kinloch A J and Shaw S (1980), J Adhes, 1, 3. Kinloch A J (1983), Durability of Structural Adhesives, Kinloch A J ed, Applied Science Publishers, London, Ch 1, p 1. Kinloch A J (1987), Adhesion and Adhesives, Science and Technology, Chapman and Hall, London, p 376±380. Kinloch A J and Smart N J (1981), J Adhes, 12, 23. Hartshorn (1986), Structural Adhesives, Chemistry and Technology, Hartshorn S R ed, Plenum Press, New York, Ch 8, p 347. Mahoon A (1983), Durability of Structural Adhesives, Kinloch A J, ed, Applied Science Publishers, London, Ch 6, p 255 McMillan J C (1981), Durability of Structural Adhesives, Kinloch A J ed, Applied Science Publishers, London, Ch 4, p 243. Minford J D (1981), Treatise on Adhesion and Adhesives, vol 5, Patrick R L ed, Marcel Dekker, New York, p 45. Minford J D (1983), Durability of Structural Adhesives, Kinloch A J ed, Applied Science Publishers, London, Ch 4, p 135. Moylan C B, Best M E and Ree M (1991), J Polymer Sci, Phys Ed, 29, 87. Naviroj S, Koenig J L and Ishida H (1985), J Adhesion, 18, 93. Orman S and Kerr C (1971), Aspects of Adhesion, 6, 64. Parker B M (1988), J Adhes, 26, 131. Parker B M (1993), Int J Adhes Adhes, 13, 47. Poole P and Watts J F (1985), Int J Adhes Adhes, 5, 33. Ramani K, Verhoff J, Kumar G, Blank N and Rosenberg S, (2000), Int J Adhes Adhes, 20, 377. Turner R H and Boerio F J, (2002), J Adhes, 78, 495. Xu S and Dillard D A, (2003), J Adhes, 79, 699.

7

Non-destructive testing

P CAWLEY

The types of defect encountered in adhesive joints and the non-destructive testing techniques available to detect them are reviewed. Three types of defect: complete disbonds, voids and porosity, poor cohesive strength of the adhesive layer and poor adhesive-adherend interfacial properties can be present. It is shown that a variety of techniques is available for disbond detection, ultrasonics and different types of bond tester being the most commonly used. These techniques are very time consuming if large bond areas are to be tested so there is increasing interest in rapid scanning methods such as transient thermography and shearography. The detection of poor cohesive properties is more difficult but can be achieved with ultrasonic or dielectric measurements. Monitoring interfacial properties is much more difficult and there is currently no reliable test after the joint is made. Much current research is focused on monitoring environmental degradation.

7.1

Introduction

In spite of its potential advantages, the use of adhesive bonding in safety critical structures has been limited by a lack of adequate non-destructive testing procedures; without such procedures, the reliability of a structure cannot be guaranteed. Such testing will usually be performed after manufacture or at stages during manufacture; however, in more stringent applications, inspection during service may also be required. Ideally, the non-destructive test would predict the strength of the bond and its susceptibility to environmental attack. This is very difficult to achieve, partly because a direct measurement of strength cannot be non-destructive, so it is necessary to correlate strength with other properties such as bond area, interfacial stiffness, etc. Also, the stress distribution in a typical adhesive joint is far from uniform so the strength is much more sensitive to the integrity of some areas of the joint than to others. Measurement of bond area, stiffness and so on, do not necessarily give good correlations with strength. Changes in these properties do, however, give an indication that a joint may be defective.

144

Adhesive bonding

There are three main types of defect which occur in practice: · disbonds, voids and porosity · poor cohesive strength, i.e., a weak adhesive layer · poor adhesive-adherend interfacial properties. Disbonds can be produced by the absence of adhesive, the failure of the adhesive to bond to one of the adherends or by in-service degradation. These are usually straightforward to detect (though time consuming if large areas are to be covered) unless the gap is filled with liquid or the sides are pressed together by compressive loading. Voids or large gas bubbles in the adhesive are caused either by a lack of adhesive or by the presence of foreign matter on, or even in, the adherends. Porosity of the adhesive is similar to voiding except that the size of the bubbles can be much smaller. It is usually caused by volatiles or gases trapped in the adhesive. A major problem can occur with composite adherends if these are not adequately dried before bonding as absorbed moisture can vaporise during the cure cycle to produce bubbles in the adhesive. Voiding and porosity can be detected using standard ultrasonic techniques but the test setup becomes more critical as the degree of porosity to be detected is reduced. No reliable non-destructive test for the properties of the adhesive-adherend interface has been developed, despite a huge amount of research effort; the reasons for this are discussed briefly later. Standard practice in the aerospace industry1,2 is to test the adherend surface prior to bonding on the grounds that failures due to poor interfacial properties, either on initial loading or after environmental attack, are generally a result of inadequate surface preparation. Great care must therefore be taken to ensure that surface contamination does not occur between the time of this test and the bonding operation. Provided the adherend preparation has been satisfactory, the interfacial strength of a joint should be greater than its cohesive strength. This is desirable since cohesive strength is more predictable than interfacial strength and hence can be used in design calculations. Variations in the physical properties of a particular adhesive are primarily due to changes in the cure cycle. If, for example, the cure temperature is too low then insufficient cross-linking of the polymer takes place and an adhesive of incorrect modulus results. It is possible to measure the cohesive properties of the adhesive using ultrasonic techniques, very accurate results being obtained in the laboratory. However, the test is relatively time consuming and tricky to set up so in practice, if the cohesive properties are to be checked, destructive tests are often performed on specimens manufactured under the same conditions as the actual structure. Since most industrial inspection aims to detect disbonds, rather than to measure the cohesive or interfacial properties, this chapter is chiefly devoted to the different techniques available for this purpose. Most testing is done using conventional ultrasonics or by specialist bond testers and these are covered in

Non-destructive testing

145

the next two sections. However, these methods are slow if large bond areas are to be covered so section 7.4 discusses alternative techniques that offer the possibility of rapid scanning. Cohesive property measurement is discussed briefly in section 7.5 and the challenges of interfacial property determination and monitoring of environmental degradation are covered in section 7.6.

7.2

Conventional ultrasonics

7.2.1 Basis of the technique The monitoring of ultrasonic echoes in the time domain forms one of the most widely used methods of non-destructive testing for bonded joints and composites. The method is commonly used for the detection of disbonds, voids and porosity in adhesive joints. An incident pulse of ultrasound will be reflected and transmitted (assuming normal incidence, and hence no refraction), at each interface of the joint. The amplitudes of the reflected (R) and transmitted (T) pulses for a unit amplitude incident signal are dependent on the acoustic impedances of the materials on either side of the interface and are given by: Rˆ

Z2 ÿ Z1 Z1 ‡ Z2

7:1

Tˆ

2Z2 Z1 ‡ Z2

7:2

where Z is the acoustic impedance given by the product c;  is the density, c is the speed of sound and subscripts 1 and 2 denote the materials on the incident and remote sides of the interface respectively. If a defect is assumed to contain air or any other low-density substance then it will have a very low acoustic impedance relative to the adhesive or adherend. Therefore a pulse incident at the defect is practically totally reflected leaving negligible energy to be transmitted through the defect. Measurement of the reflected or transmitted energy may therefore be used to indicate the presence of a defect. The simple ray model of a pulse of ultrasound being specularly reflected at a defect is valid when the defect diameter is around one wavelength or larger. Porosity typically comprises pores which are orders of magnitude smaller than the wavelength. Therefore porosity does not give a simple specular reflection, but scatters energy in all directions, so attenuating the propagating pulse. Due to the severe impedance mismatch between solid materials and air, it is difficult to propagate ultrasound from a transducer through air to the test structure. It is therefore vital that there is a satisfactory coupling medium between the transducer and test piece. This is often achieved by immersing the test piece and transducer in a water bath. The ultrasound then propagates across the water filled gap (typically 25±100 mm depending on the transducer)

146

Adhesive bonding

Figure 7.1 Schematic of water jet (squirter) coupling (from ref. 3).

into the test piece. Alternatively, the transducer can be held in contact with the test structure, coupling being provided by a thin layer of gel. Both methods tend to have problems since the immersion technique is often impractical for large components and buoyant honeycombs. The contact technique is slow when large areas need to be examined, and can be sensitive to contact pressure. A further alternative is a water jet transducer or `squirter' in which the ultrasound propagates along a water jet which surrounds the transducer, as shown in Fig. 7.1. There have also been recent developments in air-coupled transducers4 and wheel probes in which coupling is provided by a tyre made from a low attenuation rubber.5 Techniques which monitor ultrasonic reflection or transmission can detect small disbonds with a high degree of reliability. However, the change produced by the defect is more subtle if the coupling agent, water or fuel is allowed to penetrate the defect. The presence of the liquid reduces the reflection coefficient and the defect becomes more difficult to detect. When the technique is used in production control, liquid ingress can usually be prevented. However, when joints with an unknown history are examined, the results need to be interpreted with care. Disbonds are also more difficult to detect if the faces are pressed together by compressive loading6 and problems can also occur if the bondline thickness is large, leading to high attenuation in the adhesive.7

Non-destructive testing

147

7.2.2 Test configurations Through transmission The through transmission technique monitors the amplitude of an ultrasonic pulse transmitted through the joint. The amplitude is reduced to virtually zero when a disbond larger than the beam width is present, smaller amplitude reductions being seen in the case of porosity or disbonds and voids whose plan area is lower than the beam width. The technique uses separate transmitting and receiving transducers positioned either side of the structure to be tested. Alignment of one transducer above the other is important and can present difficulties when large components are tested. Alignment of the transducer axis perpendicular to the surface to be tested, however, is not as critical as with other techniques. Through transmission is often used for the production inspection of large structures such as aircraft fuselage and wing sections, coupling generally being achieved with squirters. The method is also particularly suited to the inspection of honeycomb structures. Using a pulse-echo technique (see below), only the bonding of the top face to the core can be tested reliably, whereas using through transmission, both top and bottom bonds between skins and core can be inspected in a single test. However, through transmission is not generally suitable for in-service inspection since access to both sides of the structure is often difficult or impossible. Also, while it gives an indication that a defect is present, no information about its depth within the structure can be obtained. Pulse-echo The pulse-echo technique generally uses a single transducer as both the transmitter and receiver. Provided the ultrasonic pulses generated by the transducer are short enough, the individual echoes from each interface can be resolved, their position and amplitude being used to detect the presence of a defect. A large fraction of ultrasound will be reflected at a defect owing to its large reflection coefficient; echoes from features behind the defect will also be reduced or disappear. It is also possible to use a single transducer in a `double through transmission' configuration. This is achieved by placing a reflector plate beneath the structure in an immersion tank as shown in Fig. 7.2 and monitoring the echo from the reflector plate. As in through transmission, this signal is virtually completely attenuated by disbands larger than the beam, smaller amplitude reductions being produced by voids, porosity and smaller disbonds. In all pulse echo testing, alignment of the transducer axis perpendicular to the surface of the structure or reflector plate is important if the reflected signal is to be received back at the transducer.

148

Adhesive bonding

Figure 7.2 Schematic of double through transmission inspection.

7.2.3 Ultrasonic transducers and data presentation Effect of frequency Ultrasonic transducers are usually characterised by their centre frequency, most conventional testing being done with transducers in the range 1±10 MHz. The pulse length obtained is generally of the order of 3±5 cycles of oscillation at the centre frequency when excited by a voltage spike from a typical ultrasonic test set. If the reflections from two successive interfaces in a joint are to be resolved (i.e., not overlap), the pulse duration should not exceed the travel time from the first interface to the second and back. Therefore if the interfaces are close together, a short pulse duration, implying high frequency, is required. This is illustrated in Fig. 7.3 which shows successive reflections from aluminium plates in water using a 10 MHz transducer. In Fig. 7.3(a) the plate is 3.2 mm thick and the front (F) and successive back face reflections (B1, B2, B3, B4) can easily be separated. However, when the plate thickness is reduced to 1.5 mm (Fig. 7.3(b)), the reflections overlap. The need to use higher frequencies to obtain resolution can conflict with the need to employ a lower frequency in order to limit the attenuation of the signal as it passes through the structure. The attenuation is small in metal adherends but can be significant in thick composites, or in the adhesive if the bondline thickness is large. The reflections from an adhesive joint are much more complex than those from a plain plate (which is equivalent to a single adherend). The series of reflections is shown schematically in Fig. 7.4, some pulses being positive-going and others negative depending on whether the reflection is from a high-low or low-high impedance interface (see eqn 7.1). In this figure the RF pulses of the form of Fig. 7.3 are shown rectified and enveloped to improve clarity; this is generally done in ultrasonic test sets where

Non-destructive testing

149

Figure 7.3 A-scans from aluminium plate in water using 10 MHz ultrasonic transducer (a) plate thickness 3.2 mm; (b) plate thickness 1.5 mm (from ref. 3).

the signal is also usually rectified so the phase differences are not seen. If the adhesive thickness is of the order of 100 m, a transducer frequency of over 30 MHz is required to resolve the echoes from the top and bottom adhesiveadherend interfaces so in most cases where 1±10 MHz transducers are used, the echoes of Fig. 7.4 will merge together as shown in Fig. 7.5(a). (This test was

150

Adhesive bonding

Figure 7.4 Schematic of A-scan from good joint (from ref. 3).

done in the double through transmission arrangement of Fig. 7.2 so there is a second series of echoes from the joint produced by reverberation in the water path between the joint and the reflector plate.) In this case it is impossible to monitor the amplitude of a particular echo (e.g., that from the top adhesiveadherend interface) to detect the presence of a disbond. However, disbonds can still be detected from the ringing seen in the signal. If the joint is well bonded, energy is transmitted into the adhesive and through into the bottom adherend. Energy is dissipated in the adhesive, and in immersion testing is also radiated into the water behind the joint. Therefore the reverberating echoes from the joint die away as shown in Fig. 7.5(a). If the joint is disbonded, the signal reverberates in the top adherend and the decay rate is much lower, as shown in Fig. 7.5(b). This change in decay rate can therefore be used for disbond detection, but the signal processing is more complicated than simply monitoring the amplitude of an echo. In through transmission and double through transmission testing there is no need to separate echoes from closely spaced reflectors so there is less advantage in using high-frequency transducers. Disbond detection is therefore more straightforward than in pulse-echo. The signal attenuation produced by porosity is a result of scattering from the pores and this increases with frequency. Porosity is usually measured in either through transmission or double through transmission so the frequency must be chosen to give a measureable indication at the limit of acceptability. The minimum plan diameter of a discrete disbond that can be detected is of the order of a wavelength so this increases as the frequency decreases. The compression wave speed in aluminium adherends is

Non-destructive testing

151

Figure 7.5 A-scans from (a) good and (b) disbonded joints (after ref. 3).

about 5 mm/s so at a frequency of 1 MHz the minimum diameter is about 5 mm, and this decreases to 0.5 mm at 10 MHz. Depending on the inspection requirements, this may influence the minimum frequency that can be used. Data presentation Several methods of displaying the ultrasonic reflections are available, the most common being A and C-scans. The simplest presentation is an A-scan which shows the amplitude of the signal as a function of time (or distance, if a value for the velocity of sound in the medium is known), as shown in Figs 7.3±7.5. In manual testing, an A-scan is obtained at each test point and is interpreted by the operator. If the amplitude of the through transmitted signal (or of a particular echo in pulse echo testing) is monitored at each point on the surface of the testpiece, a C-scan can be produced. Measurements at each point are taken using a scanning mechanism, which produces a plan of the defect positions but gives no information on their depth. The example of Fig. 7.6 shows a C-scan of an

152

Adhesive bonding

Figure 7.6 C-scan of joint after environmental attack showing large edge disbonds and smaller disbonds remote from edges (from ref. 8).

adhesive joint sample roughly 100 mm square that had been subjected to environmental attack. Large edge disbonds can clearly be seen, together with groups of small disbonds remote from the edges. Corrosion of the aluminium on one side of the sample that was unbonded is also evident. The automatic scanning mechanisms required to produce C-scans usually employ immersion or water jet coupling whereas A-scan devices often use the contact technique. Portable scanning frames to facilitate C-scans on site are also available.9

7.3

Bond testers

The signals obtained in through transmission and double through transmission testing are easy to interpret. However, if only single-sided access is possible, it is necessary to use pulse-echo inspection where signal interpretation can be more problematic as discussed above. This is particularly so with honeycomb structures where the skins tend to be thin so high frequencies are required to resolve the echo from the skin-core bondline. The signal obtained also varies rapidly with position depending on whether the test is carried out in the middle of a cell or above a cell wall. Therefore scanning to produce a C-scan map is generally required to obtain reliable results. This can be inconvenient, particularly for field testing. Ultrasonic bond testers are designed to overcome these signal interpretation problems.

Non-destructive testing

153

A further disadvantage with ultrasonic testing is the need for a couplant between the transducer and the structure; again this is a particular problem in field testing. Sonic bond testers have been designed to overcome this problem, as well as producing signals that are easy to interpret. However, they are much less sensitive than the ultrasonic testers, as discussed below.

7.3.1 Ultrasonic bond testers The active element of an ultrasonic transducer is a piezoelectric disc. The transducer centre frequency is the first through thickness resonance frequency of the disc, which occurs at a frequency where the disc thickness is half a wavelength. Hence, tˆ

 2

7:3

and c ˆ f

7:4

so c 7:5 2t where t is the disc thickness, c is the speed of sound in the disc, f is the frequency and  is the wavelength. If the transducer is coupled to a structure such as a plate or joint, the effective thickness of the system is increased and the frequency of the fundamental resonance is decreased. One class of bond tester effectively measures the ultrasonic impedance of a system comprising the transducer coupled to the joint. These instruments generally operate at a single frequency selected to be below the first resonance frequency of the transducer coupled to a good joint. If a disbond is present, the effective thickness of the structure coupled to the transducer is decreased so the resonance frequency increases. Therefore the operating frequency is further from the resonance and the impedance increases. The extent of the impedance change can be related to the depth of the disbond in the structure. The second class of ultrasonic bond tester measures the first resonance frequency of the transducer coupled to the joint. The resonance frequency is lowest for a good joint and increases if disbonds are present. Again, the depth of disbond in a multi-layered structure can be related to the resonance frequency. The best known instrument in this class is the Fokker Bond Tester Mk II.3 f ˆ

7.3.2 Sonic bond testers The layer(s) above a disbond can be regarded as a plate supported around the edges of the disbond. Since the plate can deflect, the local stiffness of the

154

Adhesive bonding

Figure 7.7 Minimum detectable defect diameter as a function of defect depth for coin-tap and mechanical impedance methods (from ref. 10).

structure in the direction normal to the surface is lower above the defect than in good areas. There is a family of sonic vibration methods which use different techniques to measure this reduced stiffness, the best known being the coin-tap test and the mechanical impedance method.10 The stiffness of the plate formed by the layer(s) above the defect is proportional to the cube of the defect depth and is inversely proportional to the square of the defect diameter. Therefore deep defects are difficult to detect unless they are large. Figure 7.7 shows a graph of minimum detectable defect diameter versus depth for solid aluminium and carbon fibre composite adherends in bonded joints, and for carbon fibre skins on honeycomb. With 1 mm thick aluminium adherends, the minimum detectable disbond diameter is about 5 mm, but the minimum detectable size increases to around 30 mm when the adherend thickness is 3 mm. Before specifying one of these methods it is therefore necessary to be sure that its sensitivity is high enough. In spite of their poor sensitivity, the techniques are widely used, particularly for field testing where the advantages of simple interpretation of results, portable equipment and lack of couplant are particularly valuable. They are also used to check the skincore bonding on honeycomb structures with thin, porous skins where couplant cannot be used. Here, the thin skins mean that the poor sensitivity at large depths is not a problem.

7.4

Rapid scanning methods

When large bond areas are to be inspected, point-by-point scanning using conventional ultrasound or bond testers becomes very time consuming so alternatives in which an image of an area of structure is obtained in a `single shot' become increasingly attractive. Radiography is very commonly used in

Non-destructive testing

155

non-destructive testing, but it is of little use in adhesive joints since the direction of disbonds is usually parallel to the surface and so is normal to the easiest direction to send an X-ray beam. This means that there is very little change in absorbtion due to the disbond so it is not detectable. The problem is particularly severe with metal adherends since the X-ray absorbtion in the adherends is then much greater than in the adhesive so the absence of adhesive makes minimal difference to the overall absorption in the joint. The most promising rapid scanning methods that are commercially available as fully engineered systems, or are approaching this level of development, are transient thermography, shearography and laser ultrasound. Unfortunately, complete transient thermography and shearography systems typically cost over £100k and laser ultrasound is considerably more expensive. The principles of transient thermography and shearography are reviewed here; details of laser ultrasound can be found in ref. 11.

7.4.1 Transient thermography Transient thermography is based on the effects that occur when a material is subjected to a rapid pulse of heat on one of its external surfaces. Initially, the heat pulse causes the surface temperature to be raised, and shortly afterwards the surface begins to cool as the heat pulse diffuses into the material. Thus the process can be viewed as a `wavefront' of heat that flows from the exposed surface into the material. For a perfectly homogeneous material, the `wavefront' of heat passes through uniformly. However, where there are defects such as delaminations or disbonds, these create a higher thermal impedance to the passage of the `wavefront'. Physically, when the defects are close to the surface, they restrict the cooling rate due to the diffusion process, so producing `hot spots'. When the surface is viewed by a thermal imager, temperature differences arising from the presence of the defect appear shortly after the deposition of the heat pulse. Similarly, on the opposite side of the structure, the defect appears as a `cold spot' because the defect impedes the passage of heat to this surface. These effects are shown schematically in Fig. 7.8. The temperature rise on the heated surface is governed by the amount of energy deposited and the speed of application, combined with the thermal properties of the surface material. However, from this point on, provided that the pulse is short enough, the ensuing diffusion process is totally controlled by the material itself. The contrast observed at either surface due to the presence of defects is a function of the defect size, its depth from the observed surface, the initial surface temperature rise and the thermal properties of the material. While these parameters change from one specimen to another, the testing technique is always to record the temperature variation of either surface directly after the thermal transient has been applied. The contrast due to the presence of defects may be seen over timescales ranging from sub-millisecond to several seconds

Figure 7.8 Schematic representation of the transient thermography method: (a) through transmission (double-sided); (b) reflection (single-sided) (from ref. 11).

Non-destructive testing

157

depending on the material properties and thickness. In many applications, the useful information is obtained within 500 ms so it is necessary to use a system which can acquire many sequential images over this time window. The equipment required to perform transient thermography falls into two categories: the heat source and the thermal imaging/analysis system. The source of heat must have a sufficiently fast rise time to provide a rapid temperature rise since it is the steepness of the temperature gradient that provides the contrast between defective and non-defective areas. In metals and carbon composites, this rapid temperature rise can conveniently be provided by discharging several kilojoules of energy from a bank of capacitors through Xenon flash tubes which are directed at the area of interest. When poor conductors such as glass fibre composites are to be inspected, the rate of heating produced by hot air blowers is frequently adequate. Major strides in thermal imaging equipment have been made in recent years. The early work on the method was done using a thermal imaging camera whose output was recorded on video tape, the analysis being performed on the recorded image using slow-motion replay and freeze-frame facilities. (The technique was once called pulse video thermography.12) Now the images are stored digitally and automatic processing routines are available.13 In general, the sensitivity of the method is reduced as the depth of the defect from the surface monitored by the thermal imaging camera is increased. The sensitivity is best expressed as the defect diameter/depth ratio required for the defect to be detectable. This sensitivity is material specific and must be determined practically using specially fabricated test plates, flat bottomed holes of different diameters and depths frequently being the most convenient type of defect to consider. Typical values of the ratio of minimum detectable defect diameter to defect depth are 2±4 for metals.14 The performance in carbon fibre composite may be somewhat poorer than this because the composite has higher conductivity in the plane of the fibres than in the through thickness direction. The heat therefore tends to diffuse parallel to the surface, rather than through the thickness, and so is less affected by the presence of disbonds or delaminations that run parallel to the surface. However, there is a wide variety of sensitivities reported in the literature on carbon fibre composites.14 In general, the best results are obtained if the maximum possible amount of energy is deposited on the surface of the structure, though care must be taken to avoid thermal damage.

7.4.2 Shearography Shearography is an optical method which uses speckle shearing interferometry to measure displacement gradients at the surface of a structure. The speckle patterns produced with the component in stressed and unstressed states are subtracted, differences revealing changes in displacement gradient. These are generally more rapid in damaged regions. Laser speckles are produced whenever

158

Adhesive bonding

a surface whose roughness is of the order of one wavelength of light or greater is illuminated with highly coherent light. The light scattered from any moderately distant point consists of many coherent wavelets, each arising from a different element of the surface. The optical path differences between these various wavelets may differ by several wavelengths and the interference between these wavelets results in a granular pattern of intensity that is termed speckle.15 Shearography uses a laser operating in the visible light range to illuminate the target area of the structure. In practical applications16 the light is fed from the laser to an expansion lens via a fibre optic cable as shown in Fig. 7.9(a), the target being viewed by a video camera via an image shearing lens. The operation of the system is shown schematically in Fig. 7.9(b). A thin glass wedge covers one half of the lens aperture. Without the wedge, rays scattered from a point P on the object and received by the two halves of the lens will converge to a single point in the image plane. The glass wedge is a small angle prism which deviates

Figure 7.9 (a) Shearography equipment; (b) schematic representation of shearing process (from ref. 11).

Non-destructive testing

159

the rays passing through it so in the presence of the wedge, the rays from the point P are mapped onto two points, P1 and P2, in the image plane. Hence, two sheared images of the whole object are produced and these images interfere with each other to produce an interference pattern. This reference pattern is stored in the computer. When the object is deformed, the interference pattern will be modified, and the monitor displays the image formed by subtraction of the reference pattern from the current image. Fringes on this display map the derivative of the deformation. The most common loading arrangement is to use vacuum stressing.16 A system for use in a production environment has been developed which will cover a 1 m2 field of view, and hand-held units for field applications have also been produced.16 Pressure drops of 20±100 mm Hg are sufficient in most applications and this can be achieved without sophisticated sealing arrangements. It is also possible to use thermal stressing17 and the technique can be used to produce time-averaged images of the displacement gradients produced by vibration excitation.18 Shearography detects defects from the changes in the displacement gradient which they produce when the structure is loaded. For a given loading, these changes are dependent on the stiffness of the plate formed by the layer(s) above the defect in the same way as the changes monitored by the sonic bond testers discussed above. Therefore the sensitivity of shearography will be highly dependent on the depth and diameter of the disbond.

7.5

Cohesive property measurement

A variety of ultrasonic techniques can in principle be used to measure the cohesive properties of the adhesive. A study in the early 1990s19 showed that one robust method was to measure the amplitude of the reflection coefficient from the top adhesive-adherend interface and the transit time (T) of the signal across the adhesive layer as shown in Fig. 7.10. These measurements require the reflections from the top and bottom of the adhesive layer to be resolved so transducer centre frequencies above 30 MHz are required for typical aerospace bond geometries where the bondline thickness is of the order of 100 m. Measurement of the reflection coefficient requires the amplitudes of the front face reflection (F) and the top adhesive-adherend interface reflection (B1) to be measured and compared to the amplitudes obtained in a test over an unbonded adherend. The test yields the thickness of the adhesive layer and the longitudinal bulk wave velocity which can be related to the elastic modulus at the test frequency. Rokhlin20 has developed a more sophisticated test using measurements of the response of the joint at normal and oblique angles of incidence. This can be used to obtain the longitudinal and shear moduli and attenuations of the adhesive layer, as well as its thickness and density; it has also been used to monitor

160

Adhesive bonding

Figure 7.10 High frequency A-scan used to measure cohesive properties of bonded joint (from ref. 19).

interfacial changes during environmental attack. A joint comprising an adhesive between two conducting adherends can be viewed as a capacitor whose capacitance is a function of the dielectric properties of the adhesive which are known to vary during cure. Therefore dielectric measurements can be used to monitor the average progress of cure across the joint area.21

7.6

The interface problem and monitoring environmental degradation

The adhesive-adherend interface is particularly important with aluminium adherends because the surface preparation is a critical factor in the susceptibility of the joint to environmental attack. A `microcomposite' interlayer is formed comprising the oxide layer which may be penetrated by adhesive or primer. Inspection of the interlayer is difficult because it is frequently only of the order of 1 m thick,22 compared with an adhesive layer thickness of the order of 100 m, and access is possible only via the adherends that are typically over 1000 m thick. This problem has been the subject of intensive research for over 20 years and a review of early work in the field is given by Thompson and Thompson.23 Ultrasonic methods have generally been regarded as potentially the most useful and the bulk of the research effort has been concentrated in this field. Four basic categories of ultrasonic technique have been investigated.24 Calculations based on likely interphase properties24 indicated that the measurement of oblique-incidence reflection coefficients is likely to be the most promising technique in practice, since its sensitivity to the interphase characteristics is at least as good as that of the other methods and the reflection coefficients are relatively insensitive to small changes in the bulk adherend and

Non-destructive testing

161

adhesive properties. Unless there have been major production errors such as gross contamination, since the adhesive-adherend interlayer is so thin, there is little prospect of determining its properties in the as-made condition in order to, for example, check that the appropriate surface preparation has been applied.25 It may, however, be possible to monitor the onset of environmental attack and this has been the subject of extensive recent work at Imperial College,8 Ohio State University20 and University of Toronto.26, 27 It is straightforward to detect disbonds as they advance from the edges of the joint where the adhesive and the adhesive-adherend interface are exposed to the environment, and there is also some evidence of disbond formation in areas remote from the edges8 (see Fig. 7.6). Some work has shown evidence of changes prior to complete disbonding,20 though this has not been evident in other studies. This may be a function of the type of adhesive, the environment and the loading. Much work remains to be done in this area.

7.7

Conclusions

The types of defect encountered in adhesive joints and the non-destructive testing techniques available to detect them have been reviewed. A variety of techniques is available for disbond detection, ultrasonics and different types of bond tester being the most commonly used. These techniques are very time consuming if large bond areas are to be tested so there is increasing interest in rapid scanning methods such as transient thermography and shearography. The detection of poor cohesive properties is more difficult but can be achieved with ultrasonic or dielectric measurements. Monitoring interfacial properties is much more difficult and there is currently no reliable test after the joint is made. Much current research is focused on monitoring environmental degradation.

7.8

References

1. Schliekelmann R J, `The nondestructive testing of adhesive bonded metal-to-metal joints', Nondestructive Testing, 5, 79±86, 1972. 2. Kim D M and Sutliff E F, `The contact potential difference (CPD) measurement method for prebond nondestructive surface inspection', SAMPE Quarterly, 9, 59±63, 1978. 3. Guyott C C H, Cawley P and Adams R D, `The non-destructive testing of adhesively bonded structure: a review', J Adhesion, 20, 129±159, 1986. 4. Farlow R and Hayward G, `Real-time ultrasonic techniques suitable for implementing non-contact NDT systems employing piezoceramic composite transducers', Insight, 36, 926±935, 1994. 5. Drinkwater B W and Cawley P, `An ultrasonic wheel probe alternative to liquid coupling', Insight (formerly Brit J NDT), 36, 430±433, 1994. 6. Brotherhood C J, Drinkwater B W and Guild F J, `The effect of compressive loading on the ultrasonic detectability of kissing bonds in adhesive joints', J Nondestructive

162

Adhesive bonding

Evaluation, 21, 95±104, 2002. 7. Allin J M, Cawley P and Lowe M J S, `Adhesive disbond detection of automotive components using first mode ultrasonic resonance', NDT&E International, 2003 (in press). 8. Vine K, Cawley P and Kinloch A J, `The correlation of non-destructive measurements and toughness changes in adhesive joints during environmental attack', J Adhesion, 77, 125±161, 2001. 9. Smith R A, `Impact of the portable scanner on NDT ± a revolution?', Insight, 40, 635±639, 1998. 10. Cawley P, `Low frequency NDT techniques for the detection of disbonds and delaminations', Brit J NDT, 32, 454±461, 1990. 11. Cawley P, `The rapid nondestructive inspection of large composite structures', Composites, 25, 351±357, 1994. 12. Hobbs C P, Kenway-Jackson, D and Milne J M, `Quantitative measuremement of thermal parameters over large areas using pulse video thermography', Proc SPIE, Vol 1467, Thermosense XIII, 264±277, 1991. 13. Shepard S M, `Introduction to active thermography for non-destructive evaluation', Anti-Corrosion Methods and Materials, 44, 236±239, 1997. 14. Hobbs C P, Kenway-Jackson D and Judd M D, Proceedings of the International Symposium on Advanced Materials for Lightweight Structures, ESTEC, Noordwijk, March 1994 (ESA-WPP-070). 15. Tiziani H J, `Physical properties of speckles', in Speckle Metrology, RK Erf (ed.), Academic Press, New York, pp 5±9, 1978. 16. Newman J W, `Shearographic inspection of aircraft structure', Materials Evaluation, 49, 1106±1109, 1991. 17. Newman J W, `Production and field inspection of composite aerospace structures with advanced shearography', Proc 22nd International SAMPE Tech Conf, Boston, Ma, pp 1243±1249, 1990. 18. Toh S L, Shang H M, Chau F S and Tay C J, `Flaw detection in composites using time average shearography', Optics and Laser Technology, 23, 25±30, 1991. 19. Dewen P N and Cawley P, `Ultrasonic determination of the cohesive properties of bonded joints by measurement of reflection coefficient and bondline transit time', J Adhesion, 40, 207±227, 1993. 20. Rokhlin S I, Baltazar A, Xie B, Chen J and Reuven R, `Method for monitoring environmental degradation of adhesive bonds', Materials Evaluation, 60, 795±801, 2002. 21. Banks W M, Hayward D, Joshi S B, Li Z-C, Jeffrey K and Pethrick R A, `High frequency dielectric investigations of adhesive bonded structures', Insight, 37, 964± 968, 1995. 22. Davies R J and Kinloch A J, `The surface characterisation and adhesive bonding of aluminium', Adhesion (ed. K.W. Allen) Vol 13, pp 8±22, Elsevier (London), 1989. 23. Thompson R B and Thompson D O, `Past experiences in the development of tests for adhesive bond strength', J Adhesion Sci. Technol, 5, 583±599, 1991. 24. Cawley P, Pialucha T P and Lowe M J S, `A comparison of different methods for the detection of a weak adhesive/adherend interface in bonded joints', Review of Progress in Quantitative NDE, Vol 12, D O Thompson and D E Chimenti (eds), Plenum Press, New York, pp 1531±1538, 1993. 25. Cawley P, Pialucha T P and Zeller B D, `The characterisation of oxide layers in

Non-destructive testing

163

adhesive joints using ultrasonic reflection measurements', Proc Royal Soc Lond, Series A, Vol 452, pp 1903±1926, 1996. 26. Moidu A K, Sinclair A N and Spelt J K, `Adhesive joint durability assessed using open-faced peel specimens', J Adhesion, 65, 239±257, 1998. 27. Moidu A K, Sinclair A N and Spelt J K, `Nondestructive characterisation of adhesive joint durability using ultrasonic reflection measurements', Research in Nondestructive Evaluation, 11, 81±95, 1999.

8

Impact behaviour of adhesively bonded joints C SATO

8.1

Introduction

Adhesively bonded joints in actual products may sometimes be subjected to impact loading. For instance, the structures of recent passenger cars can be impacted in case of crash and they have many adhesively bonded joints such as weld bonded steel frames and skins. The door panels and engine bonnets are also bonded adhesively with hemming techniques, and even for joints of plastic parts (including bumpers) adhesion is employed. In order to prove the crashworthiness of the structures, the evaluation and estimation of the impact strength of the joints are indispensable in many cases of its design, particularly where the joints are used at vitally important parts and human safety is strongly involved in them. Mobile phones are another typical case in which many joints are bonded with adhesives or pressure-sensitive adhesives and they are often subjected to impact loading caused by dropping. It is therefore necessary that the durability of the joints against impact loading has to be ensured in the process of design. Many procedures to assess the impact performance of materials have been devised because of interest in a wide variety of applications and some of them have been adopted as standard. The methods using a pendulum hammer, the socalled Charpy or Izod tests, are the most usual techniques for the impact test of relatively low impact velocity. A standard, ASTM Block Impact Test (ASTM D950-78) is a variation of such pendulum hammer methods modified to be suitable for the evaluation of adhesive joints, and it is the most usual and popular one. Using this method, the energy needed to break the specimen can be determined from the gap between the initial height of the pendulum hammer before the test and its maximum height after the collision. The data obtained on absorbed energy are useful only to compare quantitatively the performance of adhesives; they cannot be applied directly to actual joint design. More suitable methods are therefore required to evaluate the impact strength of joints and design them for actual use. Impact phenomena of materials are quite different from static ones in two ways. One is the change of material properties due to high strain rate. The other

Impact behaviour of adhesively bonded joints

165

is the presence of a stress wave that propagates in elastic bodies as waves would do on the surface of water. The former needs complicated experimental setups to assess material properties under high rate loading. The latter requires difficult analysis to determine the stress distribution and its variation with respect to time. FEM simulations are almost always necessary to calculate stress wave propagation in bodies of complicated shape. Therefore, if prediction of stress states in adhesively bonded joints and their strength is required, the two difficulties have to be solved simultaneously. Not much research on impact phenomena of adhesively bonded joints has been conducted so far because the conditions are hard to replicate. Since highperformance sensors and data logging systems are essential, impact experiments have been expensive compared to static ones. Stress wave simulation needs high-powered computers and long calculation times. However, recently, electric devices such as high-speed DC-coupling strain amplifiers are getting cheaper thanks to the progress of electronics. The price of computers has also decreased drastically, even more than the strain amplifiers and everybody can afford a PC that has the capability to carry out three-dimensional finite element analysis. Thus, the obstacle has been removed recently. The review shows how to estimate stress distribution in adhesively bonded joints in the case of impact, to evaluate the strength of the joints, and also to design the joints rationally.

8.2

Experimental method for impact test of adhesives and adhesively bonded joints, and characteristics of adhesives under high rate loading

8.2.1 Pendulum test As mentioned in the introduction, a kind of pendulum hammer test, the block impact test (ASTM D950-78) has been the only standard to test adhesively bonded joints under impact loading for long time. In the test, a small block is adhered to a larger block which is fixed to the base of the testing machine and an impact load is applied to the small block due to the collision of the pendulum hammer as shown in Fig. 8.1. At impact, the joint is subjected to high rate shear loading and is fractured. After the pendulum hammer hits the specimen, its velocity decreases. We can calculate the impact energy absorbed by the joint specimens from the difference of the pendulum height between the initial position before the test and its maximum height after the collision. Strictly speaking, the `absorbed' energy is a mix of several different kinds of energies, such as thermodynamic potentials of the surface made by the fracture, i.e., surface energy, energy absorbed by the plastic or viscoelastic deformation of the adhesive and adherends, kinetic energy of the small block which flies

166

Adhesive bonding

Figure 8.1 ASTM block impact test (ASTM D950-78).

away after the fracture caused by the impact, and the energy absorbed by the pendulum hammer or the frame of the test rig as acoustic waves propagate into them. The `surface energy' and the energy absorbed by the plastic and viscoelastic dissipation of the adhesive are of interest here and they have to be separated from the other types. Elimination of the kinetic energy of the small block is easy because it can be calculated based on the mass of the block and the hammer velocity, which can be estimated from the final height of the pendulum. The acoustic dissipation seems to be negligible in terms of quantity. If very stiff adherends and a brittle adhesive are used, the modified `absorbed' energy extracted from obtained data is similar to the energy creating new surfaces. Therefore, the methodology of fracture mechanics is applicable and fracture toughness based on energy balance Gc, in other words, critical energy density per area, can be obtained although the criterion of initial and subsequent fractures cannot be distinguished. In addition, the mode of fracture is always in a mixed state of Modes I and II. If an instrumentation technique is used instead of conventional pendulum testers, a load-displacement curve can be additionally obtained. With this technique, a load sensor and a displacement sensor are installed with a pendulum tester. The load sensor, which is usually a piezoelectric loadcell, is inserted between the pendulum and a tooth of the hammer. The displacement sensor (rotational and angular displacement types are often used) is attached to the axis of the pendulum. It has a great advantage because we can obtain important data, such as maximum loads applied to the specimens, maximum displacement of the small block (from which maximum shear strain of the adhesive layer could be estimated), and more precise absorbed energy.

Impact behaviour of adhesively bonded joints

167

Figure 8.2 Three loading cases for block impact test specimens and the stress distributions in the adhesive layer for each case (ref. 1).

Adams and Harris calculated the stress distribution in the specimen of the block impact test using the Finite Element Method in some cases of changing loading points.1 Figure 8.2 shows the loading condition they assumed and the stress variations in the adhesive layer calculated by FEM. In the figure, Case 1 indicates uniformly distributed applied load, where precise alignment of the specimen and impactor should be adjusted appropriately. The reality, however, is that such conditions are difficult to realise and the impactor should collide at the top edge (Case II) or bottom edge (Case III) of the small block more or less. Unfortunately, the stress distribution in the adhesive layer greatly depends on the loading cases and Case II showed high peer stress and relatively low shear stress at the edge of the

168

Adhesive bonding

adhesive layer due to the bending moment. On the other hand, Case III, collision of the impactor tooth to the vicinity of the struck end of the adhesive layer, caused low peer stress and high shear stress. Thus, Adams and Harris concluded that the main information that could be derived from this test was a quantitative comparison of the ability of various adhesives to withstand high loading rates and the transfer of the results to a practical situation was suspect. Nevertheless, the results are convenient for estimating approximately the maximum stress occurring in the joints. In some cases, we can evaluate the strength of the joints with the value of stress shown in the research although carefully prepared specimens and precisely adjusted test-rigs have to be employed. In the experiments, strength assessment of adhesives was carried out using four different types of epoxy adhesives, MY750 (Ciba-Geigy) consisted of a diglycidyl ether of bisphenol A with anhydride hardener HY906 and a tertiary amine catalyst DY062, AY103 (Ciba-Geigy) plasticised with an amine hardener HY956, ESP105 (Permabond) which is a single part toughened epoxy, and toughened MY750 modified with CTBN, i.e. a synthetic rubber carboxylterminated butadiene-acrylonitrile. Figure 8.3 shows the results of the experiments. The absorbed energy of unmodified MY750 was obviously smallest and that made sense because the adhesive was brittle. However, AY103, the plasticised epoxy also showed low absorbed energy, whereas it was greater than the result of unmodified MY750. The rubber modified epoxy (CTBN in Fig. 8.3) and the single part toughened epoxy ESP105 showed much higher absorbed energy compared with unmodified MY750 and AY103. Thus, the results indicate the importance of ductility for adhesives in order to withstand impact loading. Pendulum testers are available not only for the impact blocks but also for other shapes of specimens including lap joints. Harris and Adams carried out the impact tests of single lap joints having aluminium alloy adherends bonded with

Figure 8.3 Energy absorbed in block impact test for various adhesives (ref. 1).

Impact behaviour of adhesively bonded joints

169

Figure 8.4 Test rig for the impact testing of lap joints using pendulum tester (ref. 2).

epoxy adhesives.2 In the experiments, special clamping jigs were used (as shown in Fig. 8.4) by which the single lap specimens were fixed and impact loading was applied to them. Instrumentation devices, a loadcell and a displacement sensor were installed in the testing set-up, so that the magnitude of loads could be measured and stored in a transient recorder together with the data of displacement. The variation of obtained load with respect to time was quite smooth until the fracture of the specimen as shown in Fig. 8.5, and the end displacement, which could be recognised as the elongation of the specimen, rose almost linearly and that indicated almost constant velocity during impact loading. The failure loads of the specimens are shown in Fig. 8.6. In the figure, (a), (b) and (c) denote the kinds of adherends, 2L73, BB2hh and BB2s respectively, where 2L73 is a high-strength aluminium alloy, BB2hh is an aluminium alloy for automobile structures in `half hard' state, and BB2s is in `soft' state. The adherends were bonded with the same series of the adhesives, MY750, AY103, CTBN and ESP105 used in the block impact tests mentioned above. The results are very interesting because the failure loads of the specimens bonded with a variety of adhesives were not so different, although the absorbed energies of the block impact tests were very scattered. CTBN showed the best ability to bear both static and impact loads, and ESP105 was in second place. The impact failure loads of the joints bonded with the unmodified MY750, AY103 and ESP105 were slightly greater than the static strength of them, whereas CTBN showed opposite results. Though fracture mechanics is a useful tool to analyse joint strength, it is still rare to apply the technique to adhesive joints impacted. Kinloch and Kodokian reported the fracture toughness (GIC) of aluminium joints bonded with epoxy

170

Adhesive bonding

Figure 8.5 Typical dynamic recording of the load and displacement response under impact of a lap joint, which consists of BB2hh adherends bonded with AY103 adhesive (ref. 2).

adhesive measured using an instrumented Charpy tester.3 In the experiment, they used the configuration of a single-edged notched three-point bending (SENB) adhesive joint specimen, and calculated the fracture toughness GIC of the specimens and the dependency of the rate in the toughness was clarified. Another kind of pendulum test using a fling wedge was recently adopted as an impact wedge-peel test. In the test, two thin metal plates bonded adhesively are used as a specimen. The specimen has a bonded part and a debonded part, and a wedge is inserted in the debonded part. To pull the wedge by the impact of pendulum, impact load was applied to the specimen as shown in Fig. 8.7. The method can be thought of as a modified version of the Boeing wedge test for impact loading. Blackman et al. carried out more precise experiments of the IWP test using a fast hydraulic testing machine and a piezoelectric loadcell.4 Figure 8.8 shows the experimental setup and Fig. 8.9 shows typical variation of load and displacement to time. In this case, steel plates bonded with XB5315 (Ciba Polymers) adhesive were employed as a specimen and tested at 2 m/s at 20 ëC. Since the hydraulic testing machine was used, the displacement increased with constant rate and this is an advantage compared to pendulum tests. The load showed peaks at the initial stage of loading and subsequently the stable fracture

Impact behaviour of adhesively bonded joints

171

Figure 8.6 Static and impact joint strength with each adherend: (a) 2L73, (b) BB2hh, (c) BB2s (ref. 2).

172

Adhesive bonding

Figure 8.7 Configuration of impact wedge-peel (IWP) specimen (ISO11343, ref. 4).

Figure 8.8 Experimental setup of IWP test (ref. 4).

Impact behaviour of adhesively bonded joints

173

Figure 8.9 Typical variation of load and displacement of IWP test versus time (ref. 4).

of the specimen continued under lower loads. However, unstable crack growth occurred at ÿ40 ëC because the adhesive became brittle. They also conducted tests of tapered double cantilever beam specimens under high-rate loading using the same materials employed in the IWP tests and obtained the fracture toughness of the specimens. They also compared the fracture toughness with the wedge cleavage force.

8.2.2 Drop-weight tester Another traditional impact test method is the drop-weight test. Aisaka et al. conducted the impact tests on single lap joint specimens which had GFRP (Glass Fibre Reinforced Plastics) adherends bonded with an epoxy adhesive using a drop-weight impact tester.5 Usui carried out impact fatigue tests using almost the same setup of drop-weight, but the specimen was configured differently, not with single lap joints but with a modified impact block set upright and made of steel bonded with an epoxy adhesive AW106 (Ciba-Geigy) hardened with plyamide-amine HV953U.6 He tried to change the wave shape of the impact load by means of inserting wave-forming buffers made of plastic or rubber plates between the specimens and the drop-weight, and obtained three different durations of loading, Case A (load duration was 0.7 ms), B (2 ms) and C (9 ms). Figure 8.10 shows the accumulated rate of failure of the joints with respect to the number of repeated impacts and applied impact loads, where m denotes the coefficient of Wible slope. The results show that the joints became weaker and broke under smaller stress as the duration increased. Therefore, the fatigue strength of the joints subjected to repeated impacts seemed to depend on both the magnitude of stress and the loading duration.

174

Adhesive bonding

Figure 8.10 Low cycle impact fatigue results of adhesive joints under three conditions of loading duration (ref. 6).

8.2.3 Split Hopkinson bar (Kolsky bar) These pendulum tests and drop-weight tests are easy to use, but a high strain rate cannot be realised. A strain rate of up to about 102/s can be obtained. When a higher strain rate is necessary, the split Hopkinson bar technique is often used because the strain rate of 102/s±103/s can be realised easily and using special experimental setups the much higher strain rate of up to 104/s could be obtained. A Typical Hopkinson bar equipment is used for impact compression tests, and it has two steel bars (input bar and output bar) between which a specimen is inserted as shown in Fig. 8.11. A striker, which is accelerated usually with a gas gun, collides with the end of the input bar to cause a stress wave in the bar and the wave reaches and transmits the specimen as an impact load. The applied load

Impact behaviour of adhesively bonded joints

175

Figure 8.11 Basic configuration of split Hopkinson bar apparatus for compressive impact tests of materials.

and its variation with respect to time are measured with strain gauges stuck to the surface of the output bar. Deformation of the specimen can also be calculated from the velocities of the edges of the input bar and the output bar. The velocities of the bars can be given from the data of a transmitted wave, an incident wave and a reflected wave obtained with strain gauges attached to the output bar and the input bars respectively. Since a relatively higher strain rate can be realised easily and the deformation of specimens can be measured without any additional displacement sensor, the Hopkinson bar method is the usual technique for impact tests. The ingenious apparatus was first introduced by Kolsky, and sometimes was called the Kolsky bar.7 For the strength evaluation of adhesively bonded joints subjected to impact loads, a characteristic of the Hopkinson bar, that it is suitable only for compression tests, is sometimes an obstacle because tensile impact tests are very often required. Therefore, compression loads should be transformed into tensile loads by any means. Yokoyama used the split Hopkinson bar technique shown in Fig. 8.12 to apply a tensile load to butt joints bonded with a cyano-acrylic adhesive.8 In the experiment, the tensile stress wave was caused by the collision of a striker tube against a loading block, fixed at the end of an input bar as shown in Fig. 8.12, and finally the tensile wave was applied to the specimen as a tensile impact load. Yokoyama also conducted an impact shear test of adhesive joints using a compressive setup of split Hopkinson bar and joint specimens having adherends of a pin and a collar, in other words, a pin-and-collar specimen shown in Fig. 8.13.9 The same configuration of the specimen was also used for impact tests by Bezemer et al.10 Instead of a split Hopkinson bar, a drop-weight tester and a compressive air gun were used. The load applied to the specimen was monitored during the drop-weight tests and, with the air gun, the velocity of a projectile was measured. From the data of both load and projectile velocities, absorbed energies were calculated in each case. Yokoyama used a cyano-acrylate

Figure 8.12 Schematic diagram of the tensile split Hopkinson bar apparatus for the test of butt joints bonded adhesively with cyano-acrylate resin under tensile impact (ref. 8).

Impact behaviour of adhesively bonded joints

177

Figure 8.13 Configuration of assembled pin-and-collar specimen and the setting way (ref. 9).

adhesive Aron-Alpha #201 (Toagosei Co. Ltd) and Bezemer used three kinds of adhesive; a polyurethane adhesive KoÈmmerling KoÈrapur 649, an epoxy adhesive Araldite LY 5052 (Ciba-Geigy), and an anaerobic sealing product Threebond 1132. The results of Yokoyama's work show that the fracture loads of cyanoacrylate adhesive joints under impact are much higher than those of static tests and the critical impact stresses are about twice those of static ones. Whereas Bezemer explained the strength of the joint specimens in terms of absorbed energy, the results of the epoxy and the polyurethane tests showed quantitative agreement with Yokoyama's critical stress of cyano-acrylate adhesive because the failure energies under impact were slightly higher than static ones. In the results by Bezemer, only the anaerobic adhesive, which is very brittle and not suitable for impact conditions, had a different tendency to weaken under impact.

8.2.4 Other special methods Since the adhesive layer in practical joints is usually subject to the combination of shear and tensile stresses even under impact loading, tests for combined high rate loading are important to obtain the strength criterion of joints, but are not easy to carry out. Sato and Ikegami measured the strength of butt joints of steel tubes, (bonded with a epoxy resin Scotch Weld 1838 (3M) hardened with polyamide resin), when subjected to combination impact loads of tension and torsion using a clamped Hopkinson bar equipment.11 Figure 8.14 is a schematic illustration of how the equipment works. First in the experiment, the Hopkinson bar, strain energy store bar in other words, was fixed by a clamping system at the end. The

178

Adhesive bonding

Figure 8.14 Schematic illustration of stress wave propagation in the clamped Hopkinson bar for the combined impact loading of tension and torsion (ref. 11).

other end of the bar was loaded statically in the direction of tension, torsion or their combination using hydraulic actuators. When the clamp released, the strain energy stored in the Hopkinson bar was transformed into a stress wave and propagated into the adhesive layer. The strength of the butt joints under combined impact loading are shown in Fig. 8.15 with approximation curves fitted by von Mises and Tresca laws. The tensile strength, approximately 80 MPa, is larger than the shear strength of about 50 MPa. In the figure, there are four curves plotted from the tensile strength or the shear strength, and the curve of von Mises law plotted with the tensile strength seems to be the best approximation of all the data points. The failure criteria suitable for the joints, therefore, are described by a formula as follows: 2 ‡ 3 2 ˆ 2y

8:1

Impact behaviour of adhesively bonded joints

179

Figure 8.15 Impact strength of tubular butt joints of steel pipe bonded with an epoxy adhesive subjected to combined impact loading of normal and shear stress and approximation curves fitted with von Mises and Tresca laws (ref. 11).

where  and  are applied normal and shear stress, and y is the tensile strength of the butt joint. However, in the adhesive layer, hydrostatic constraint must occur due to the difference of elastic constants between the adhesive and the adherends. True fracture criteria of the adhesive layer, therefore, may be slightly different from the results. The strength of the joints measured under impact loading was much higher than their static strength, almost double in fact, as shown in Fig. 8.16. However, the shapes of failure loci were the same in each of the two cases and they could be plotted by quadratic curves. Therefore, even under the different conditions of static and impact loading, the same failure criteria, in this case quadratic criteria, could be available. Cayssials and Lataillade used an inertia wheel apparatus to carry out impact tests of lap shear specimens which consisted of galvanised steel sheet bonded with two kinds of epoxy adhesive such as an unmodified DGEBA hardened with dicyandiamide and a modified version of the resin that included block copolymer modifiers and fillers.12 The configuration of the lap shear specimen is shown in Fig. 8.17. The inertia wheel apparatus (Fig. 8.18) had a heavy steel

180

Adhesive bonding

Figure 8.16 Comparison between impact strength and static strength of the tubular butt approximated with curves plotted by quadratic criteria (ref. 11).

wheel, on which a small hammer was fixed, and when the motorised wheel reached the nominal tangential velocity of rotation, the end of the specimen swung and was impacted by the collision of the hammer with an anvil fixed to the specimen end. The system has the advantage that its high inertia means that the hammer's velocity does not change even during the deformation of the

Figure 8.17 Lap joint specimen for tensile impact test using inertia wheel apparatus (ref. 12).

Impact behaviour of adhesively bonded joints

181

Figure 8.18 Configuration of inertia wheel apparatus and triggering system (ref. 12).

specimen. As a result, the yield stress and yield strain of the adhesive depended on the strain rate; the yield stress increased and the yield strain decreased as the strain rate increased.

8.3

Stress distribution and variation in adhesively bonded joints subject to impact load

Correct stress analysis is necessary to estimate the strength of adhesively bonded joints. Much research on the stress distribution in the joints has been carried out since the theory first emerged, Volkersen's shear lag model.13 Since the stress distribution in the joints depends on the dimensions, kinds of materials and particularly their configuration, suitable configuration to reduce stress concentration, has been pursued. Stress analysis using the finite element method has a history of over 30 years and almost all joint configurations have already been investigated with the method. For instance, Adams et al. conducted FEM analysis of lap joints and compared them with the closed forms of Volkersen or Goland-Reisner models. The other research treated the joints considering the elastoplastic or viscoelastic properties of adherends and adhesives. However, dynamic analysis of the joints is still rare because it is more difficult than static analysis. The reason for the difficulty is that dynamic analysis needs repeated calculations and consumes computer resources.

182

Adhesive bonding

Figure 8.19 Load balance in single lap joint.

Here, we try to make a model of a single lap joint subjected to impact loading in order to calculate the dynamic response of stress distribution in the adhesive layer. The Volkersen model, which is the most simple one for lap joints, is used here. As shown in Fig. 8.19, the condition of force equilibrium, the compatibility of deformation and the constitutive relations of the materials by each element can be described with the following formulae: t1 w

@1 ÿ wA ˆ 0; @x

1 ˆ

@u1 ; @x

E1 1 ˆ 1 ;

2 ˆ

t2 w

@u2 ; @x

E2 2 ˆ 2 ;

@2 ‡ wA ˆ 0 @x

8:2

u 1 ÿ u2 tA

8:3

GA A ˆ A

8:4

A ˆ

where t1, t2 and tA are the thickness of adherend1, adherend2 and adhesive layer, and u1, u2 and w indicate the particle displacements of adherend1, adherend2 and the width of the joints. The other symbols , , , , E and G denote normal strain, normal stress, shear strain, shear stress, Young's modulus and shear modulus respectively, and the suffixes, 1, 2 and A indicate adherend1, adhrend2 and adhesive layer. Equation 8.1 can be modified into a dynamic model adding inertia force of each element as follows:

Impact behaviour of adhesively bonded joints

183

@1 @ 2 u1 @2 @ 2 u2 ÿ wA ˆ t1 w1 2 ; t2 w ‡ wA ˆ t2 w2 2 8:5 @x @t @x @t where 1 and 2 are the density of adherend1 and adherend2. Simplifying the formula to reduce the variables, assuming that the adherends have the same thickness t, Young's modulus and density denoted as E and , we obtain the final formula describing dynamic Volkersen model as follows: t1 w

@ 2 A 2GA  @ 2 A

ÿ ˆ A @x2 ttA E E @t2

8:6

The formula is a kind of partially differential equation, a so-called `telegram equation' and it could be solved fully analytically in some cases of simple boundary and initial conditions, but that is not so easy. The dynamic responses of stress distribution in the adhesive layer of a single lap joint subjected to step loads is shown in Fig. 8.20. The result was calculated numerically using the discrete element method based on the dynamic Volkersen model. It can be seen that stress near the input side of the step load increases initially and the stress at the opposite side increases with a delay for the interval of stress wave propagation between both the sides, but the stresses become equal to each other and become similar to the result of the static Volkersen model. The analytical results shown above are obtained based on the Volkersen model which ignores the adherend deflection caused by the bending moment due to the offset of the adherends. The deformation cannot be ignored if thin adherends have to be used. The other model of lap-joints was presented by Goland and Reisner.14 The bend of the adherends was considered in the model. Transforming the model to a dynamic one seems to be economical and natural. However, with the modified model it is difficult to obtain a closed form or to calculate numerically with the discrete element method because of its

Figure 8.20 Analytical result of dynamic Volkersen model.

184

Adhesive bonding

Figure 8.21 Stress distribution and variation with respect to time for single lap joint calculated by dynamic finite element method (ref. 15).

Impact behaviour of adhesively bonded joints

185

complexity. Simulation using the finite element method without any model is easier than either of these. Sato et al. investigated the deformation and stress distribution in single lap joints, taper lap joints and scarf joints subjected to impact load using the finite element method.15 In the analysis, the adhesive layer was treated as a viscoelastic material and the impact load had the shape of a step function. Figure 8.21 shows a result of the single lap joint of aluminium adherends bonded with an epoxy adhesive. The thickness of the adherends is 4 mm and the overlap length is 100 mm. Here, the stresses in the adhesive layer are divided by the applied stress for the end of the adherends, so that the value has to be multiplied by 25 to know the stress concentration factors of popular definition. In the analytical results, very sharp and impulsive stress concentration occurs at the edge of the load input side and consequently other stress peaks can be seen at the opposite edge during the initial period. That is very different from the results of the dynamic Volkersen model. After the transient process, the stresses of both the edges increase gradually due to the bending moment caused by the stress wave propagation through a `cranked path' in the offset bonded joint. Higuchi et al. conducted dynamic analysis of butt joints of cylindrical steel rods subjected to tensile impact loads using the three-dimensional finite element method and showed stress variation in the adhesive layer with respect to time and the presence of stress singularity at the circumferential edge of the adhesive layer.16

8.4

Actual joint design considering impact load

Though adhesively bonded joints are used quite often at positions subjected to impact in actual applications, design considering impact strength has been rare until now because stress analysis and strength evaluation under the conditions are still difficult to carry out. Therefore, it is possible to show only a few examples. Figure 8.22 shows the configuration of a hybrid side-impact beam built inside car doors to protect passengers from severe side impacts. The beam is an impact-absorbing member which consists of a hollow aluminium bar having a rectangular cross-section filled with hard urethane and carbon composite plate bonded adhesively onto one side of the bar with a ductile epoxy adhesive. Previously, steel beams were used for the purpose. The hybrid beam is a recent alternative which reduces weight. Aluminium alloy is very useful in reducing the weight of cars because its specific bending stiffness is higher than conventional steels. However, whereas the density is much lower than that of steel, the cross-sectional area of the member must be increased to give the same strength as steel, and the maximum strain on both the surfaces, above and below, increases. In other words, the aluminium members can break more easily than steel ones under bending deformation, and they are apt to fail in the manner of brittle fracture rather than

186

Adhesive bonding

Figure 8.22 Configuration of a hybrid side-impact beam and one-third models.

steel. The fact leads to smaller absorption energy for bending loads. Bonding carbon composite on the aluminum members has a big advantage to prevent the fracture of aluminium alloy and makes the absorption of energy increase about three times as much as that of monolithic beams. Figure 8.22 also shows one-third models of the hybrid beams prepared for preliminary tests in the developments. Strength prediction of the adhesive joints between the aluminium extrusions and the carbon composite plates was conducted in the design process as follows. First, stress distribution in the adhesive layer was simulated using the finite element method. Dynamic simulation has to be conducted, and a commercial FEM code, LS-Dyna, was used in this case. There are singular stress points at the edges of the adhesive layer, but they are neglected and fine mesh was not used near the edges. The quadratic criteria shown in Fig. 8.16 were applied to the stress distribution to predict whether fracture of the joint occurred or not. Equivalent failure stress based on the criteria was calculated from the failure stresses at the edges of a single lap specimen which consisted of an aluminium alloy adherend and a CFRP adherend. The failure stress was evaluated from the stress distribution simulated by the static finite element method and the maximum load of the specimens actually tested under tensile static loading. To make the stress situations of both the hybrid beams and the single lap joints closer, mesh size of the single lap joint was the same as the size of the hybrid beams in the simulations. In the above-mentioned procedure, the dynamic strength of the joints in the hybrid beams was predicted from the static tests of the single lap joints, and was

Impact behaviour of adhesively bonded joints

187

not strictly precise. However, the prediction must underestimate actual strength because the impact strength of adhesively bonded joints was higher than the static ones mentioned above. Therefore, it might be said that the prediction of strength is safer than reality and there is no problem in actual use. If more precise estimation is required, impact tests of the single lap joints have to be carried out.

8.5

Future trends and further information

Commercial packages of finite element codes have recently become more powerful and more affordable. Even using PC-based FEM packages, the dynamic simulation of three-dimensional elastoplastic problems can be done quite easily within a short time if the mesh size is not too fine. These trends will continue during the next few decades. Therefore, the difficulty of stress evaluation has decreased and will eventually disappear, and then anybody will be able to carry out such stress analysis without hindrance if they are trained appropriately. The acceptable failure criteria of adhesively bonded joints under impact loading is still an uncertain subject and more progress is required. To define this, not only the data for each adhesive but also the establishment of a proper testing method, by which the impact strength available for rational design of the joints can be measured, is essential. Fortunately, instead of conventional pendulum testers, the novel testing method of impact wedge-peel was adopted as a standard by ISO (ISO 11343), and it has spread into many industries. Using this method, the energy absorption and fracture toughness of the joints are measured more precisely. Further investigations about how to use the information subsequently, should be conducted in the future.

8.6

Conclusion

As adhesives have been used in more hostile conditions including impact loading, making sure of their durability becomes more important. Due to the progress of adhesives in terms of ductility, more durable adhesives such as rubber-toughened epoxy have been available and can be used for the conditions of serious impact loading under which the use of adhesives was avoided previously. Thus the engineering of adhesives including evaluation methods are becoming important.

8.7

References

1. R D Adams and J A Harris, `A critical assessment of the block test for measuring the impact strength of adhesive bonds', Int J Adhesion and Adhesives, 16, 2, 61±71, 1996.

188

Adhesive bonding

2. J A Harris and R D Adams, `An assessment of the impact performance of bonded joints for use in high energy absorbing structures', Proc Instn Mech Engrs, 199, C2, 121±131, 1985. 3. A J Kinloch and G A Kodokian, `The Impact resistance of structural adhesive joints', J Adhesion, 24, 2±4, 109±126, 1987. 4. B R K Blackman, A J Kinloch, A C Taylor and Y Wang, `The impact wedge-peel performance of structural adhesives', J Material Science, 35, 8, 1867±1884, 2000. 5. K Aisaka, T Hukuda and T Fujii, `Impact strength of FRP adhesive joints', Reinforced Plastics (in Japanese), 33, 2, 59±65, 1987. 6. Y Usui and O Sakata, `Impact fatigue strength of adhesive joints', Precise Machinery (in Japanese), 48, 4, 498±502, 1982. 7. H Kolsky, `An investigation of the mechanical properties of materials at very high rates of loading', Proc Phys Soc Series B, 62, 676±700, 1949. 8. T Yokoyama, `Experimental determination of impact tensile properties of adhesive butt joints with the split Hopkinson bar', J Strain Analysis, 38, 3, 233±245, 2003. 9. T Yokoyama and H Shimizu, `Evaluation of impact shear strength of adhesive joints with the split Hopkinson bar', JSME Int J Series A, 41, 4, 503±509, 1998. 10. A A Bezemer, C B Guyt and A Volt, `New impact specimen for adhesive: optimization of high-speed-loaded adhesive joints', Int J Adhesion and Adhesives, 18, 4, 255±260, 1998. 11. C Sato and K Ikegami, `Strength of adhesively-bonded butt joints of tubes subjected to combined high-rate loads', J Adhesion, 70, 1, 57±73, 1999. 12. F Cayssials and J L Lataillade, `Effect of the secondary transition on the behaviour of epoxy adhesive joints at high rates of loading', J Adhesion, 58, 3±4, 281±298, 1996. 13. O Volkersen, `Die Niet Kraft Verteilung in Zug beanspruchten', Luftfahrt Forschung, 15, 41±47, 1938. 14. M Goland and E Reisner, `Stresses in cemented joint', ASME J Appl Mech, 11, 1, A17±27, 1947. 15. C Sato and K Ikegami, `Dynamic deformation of lap joints and scarf joints under impact loads', Int J Adhesion and Adhesives, 20, 1, 17±25, 2000. 16. I Higuchi et al., 'Three-dimensional finite element analysis of stress response in adhesive butt joints subjected to impact tensile loads', J Adhesion, 69, 1±2, 59±82, 1999.

9

Fracture mechanics of adhesive bonds D A DILLARD

9.1

Introduction

Historically, designs for engineering structures have been dominated by using a strength approach, in which the stresses (or in some cases strains) are compared to some allowable strength of the material. Stresses or strains may individually be compared to a limiting strength, or may be combined into an appropriate metric reflecting the effect of stress interactions, in methods such as the von Mises yield criterion, which has been widely applied to the design of ductile materials (Seely and Smith, 1952). Although such approaches have been widely and successfully applied, problems have arisen in a number of now infamous structural failures, in which pre-existing or service-induced flaws have propagated catastrophically, at times with significant loss of life and property (Broek, 1978). Since traditional strength-based approaches to design normally assume that the materials being used are continua, they are not well suited for applications to systems in which flaws, cracks, debonds, delaminations, damage, or other imperfections are present. Stresses and local strains are greatly increased at the tips of these defects, which often serve as initiation sites for structural failures. Historically a more recent approach to analysis and design, fracture mechanics offers an alternative set of criteria for evaluating the integrity of real structures that may contain flaws, and is making important inroads into design. Providing an overview of fracture mechanics and its applications to adhesively bonded joints, this chapter is laid out in the following sections: first we examine the rationale for a failure criterion that is based on an energy characterization rather than the traditional strength-based approach. This is followed by two sections that highlight alternate, but equivalent, fracture mechanics formulations, the mathematically convenient stress intensity factor and the physically intuitive energy release rate. Building on this latter approach, the remainder of the chapter then examines several aspects of special relevance to adhesive joints. The resistance to debonding is discussed in the section on thermodynamic, intrinsic, and practical adhesion. Effects of mode mixity are

190

Adhesive bonding

then addressed, followed by a section on experimental methods to measure the fracture energy of bonded joints. Applications of fracture mechanics to durability of adhesive joints and to design provide additional perspectives on this approach. Finally a section on recent developments is presented prior to the summary.

9.2

An energy criterion for failure

Applications of strength-based criteria break down when sharp-tipped cracks are present, because mathematically the stresses are predicted to become singular (infinite) as the crack tip is approached. In spite of the singular stresses and strains, however, the energy concentrated in the vicinity of a crack tip must remain finite, suggesting that an energy-based failure criterion would only involve bounded quantities. Rather than being based on a critical stress state at which a continuum would yield or fail, fracture mechanics recognizes that a material's resistance to crack propagation is related to the energy required to separate adjoining material. Since a force moving through a distance produces work, a stress moving through a separation distance during a failure event is equivalent to energy per unit area, the basic quantity of fracture mechanics. Fracture mechanics thus offers an alternative and fundamentally different approach for analyzing and designing engineering components and structures. Although the stresses at a crack tip are mathematically predicted to be infinite for a linear elastic material, in real materials the high stresses will exceed the yield point, resulting in ductile deformations on a local scale, even in relatively brittle materials. This localized energy dissipation is in fact why measured fracture energies of structural materials are often orders of magnitude greater than are expected for perfectly brittle materials. A number of techniques are used to significantly enhance this energy dissipation in engineering materials, including adhesives, to produce tougher, more robust systems. In spite of the presence of localized plasticity at the crack tip, in many test specimens and engineering designs, the material away from the fracture zone remains linear elastic. When this is the case, linear elastic fracture mechanics (LEFM) can serve as the basis for analysis, testing, and design. While allowing for localized plasticity at the crack tip, LEFM assumes that there is no largescale yielding of the bulk of the specimen. For the purposes of this chapter, our attention will be focused on LEFM, which has proven useful for many adhesion problems. There are two basic approaches to linear elastic fracture mechanics, the stress intensity factor approach and the energy release rate approach, both of which are widely employed for analyzing flawed materials. The following two sections describe these methods, demonstrate their equivalence, and highlight their distinct features.

Fracture mechanics of adhesive bonds

9.3

191

The stress intensity factor approach

The concept of a stress intensity factor is attributed to Irwin (1958) and is based on the fact that the stresses ahead of a sharp-tipped crack are proportional to rÿ1=2 , where r is the distance from the crack tip. In spite of the infinite stresses predicted by this model, the stress intensity factor, K, remains finite, allowing the severity of a given crack and loading condition to be characterized with this scaling parameter. Especially convenient for mathematical analysis of stress fields for fracture mechanics problems, this approach has been widely used to determine the stress intensity factor for a wide range of crack configurations (Sih, 1973). Opening and shear components of the stress intensity factor may easily be obtained for plane problems using the complex notation of the Muskhelishvili approach (Muskhelishvili, 1953) or by other methods. In simplest form, the relevant failure criterion could state that failure will occur when the applied stress intensity factor, K, reaches the critical stress intensity factor, Kc, a material property. Fracture may occur in three different loading modes: mode I (opening), mode II (forward shear), and mode III (antiplane or out-of-plane shear or tearing), as shown in Fig. 9.1. Just as strengthbased failure criteria become more involved for multiaxial stress fields, the fracture criterion for mixed mode loading may include contributions from each mode in some appropriate manner.

Figure 9.1 Cracks can propagate under three different modes of fracture: opening, forward shear, and anti-plane shear.

192

Adhesive bonding

9.3.1 Cracks in monolithic materials For an applied opening mode (mode I) stress intensity factor, KI, the stresses in the vicinity of a crack in a homogeneous material are given by (Broek, 1978):       p KI   3 1 ÿ sin sin ‡ T ‡ O… r† x ˆ p cos 2 2 2 2r       p KI   3 y ˆ p cos 1 ‡ sin sin ‡ O… r† 2 2 2 2r       p KI   3 sin cos ‡ O… r† xy ˆ p cos 2 2 2 2r

9:1

where r and  are the polar coordinates illustrated in Fig. 9.2, and T is the Tstress, a non-singular stress that acts parallel to the crack. The first term for each stress component is proportional to rÿ1=2 , becoming singular as the crack tip is approached. Similar expressions can be given when mode II and III loading is present (Broek, 1978). The mode mixity, useful for characterizing fracture in plane problems, is given by:   ÿ1 KII 9:2 ˆ tan KI

Figure 9.2 Stress state at coordinates r and  in a cracked monolithic material.

9.3.2 Cracks at Interfaces The resulting stress state becomes more involved for interfacial cracks, where the crack is assumed to run along an interface between two linear elastic materials with different elastic properties, as shown in Fig. 9.3. When analyzing

Fracture mechanics of adhesive bonds

193

Figure 9.3 An interfacial crack between two dissimilar adherends, along with an element at coordinates r and .

stresses at a bimaterial interface, use of one of the Dundurs parameters (Dundurs, 1969) is often convenient: ˆ

1 …2 ÿ 1† ÿ 2 …1 ÿ 1† 1 …2 ‡ 1† ‡ 2 …1 ‡ 1†

9:3

Here, is a measure of the in-plane areal modulus mismatch (Hutchinson and Suo, 1992); i ˆ …3 ÿ i †=…1 ‡ i † for plane stress, i ˆ 3 ÿ 4i for plane strain; and i and i are the shear modulus and Poisson's ratio, respectively, of the two materials. The elastic mismatch parameter, , is then given by:   1 1ÿ ln 9:4 ˆ 2 1‡ The normal and shear stresses in the near field region are given by the real and imaginary components, respectively, of Hutchinson and Suo (1992): y ‡ ixy ˆ

…K1 ‡ iK2 †ri p 2r

9:5

p where i ˆ ÿ1. The complex stress intensity is given by K ˆ K1 ‡ iK2 , where K1 and K2 arise in a fashion similar to KI and KII in the solution for a homogeneous material, but take on somewhat different meaning because of the possibility for coupling. The eigenvalue exponents may now become complex, and although the real part retains the dominant rÿ1=2 dependence in the near field, the imaginary component, when present, results in oscillatory terms in the stresses and displacements. Mathematically, these lead to interpenetration of the crack faces, although the extent of these physically unreasonable predictions is

194

Adhesive bonding

normally quite small (Anderson et al., 1977), and they are not believed to invalidate the general approach for many practical applications. An additional complication that arises because of the oscillatory nature of the displacements is that the mode mixity becomes a function of the position at which the phase angle is determined (Hutchinson and Suo, 1992):   i ÿ1 Im‰K` Š 9:6 ˆ tan Re‰K`i Š where ` is the relevant length scale, which is sometimes chosen based on the dimensions of the specimen or characteristic material dimensions.

9.4

The energy release rate approach

The energy release rate approach is an alternative but often equivalent approach to fracture mechanics. Griffith laid the foundations for this approach in 1921 (Griffith 1921), several decades prior to Irwin's introduction of the stress intensity factor. The applied energy release rate, G, is the amount of energy per unit crack area available to a growing crack by the applied loading conditions, a relationship that is often expressed as: @…W ÿ U† 9:7 @A for systems in which dissipation is limited to the crack tip region. Here W is the external work, U is the stored elastic energy, and A is the crack area. The resulting failure criterion, in simplest form, states that the crack will propagate when this applied energy release rate reaches the critical value, Gc, also known as the fracture energy, of the material or bonded system. Characterized as the amount of energy required to propagate a crack per unit area, this concept is quite intuitive from a physical standpoint, and has found widespread applications, including to bonded systems. Nonetheless, the stress intensity factor approach and the energy release rate approach can be shown to be equivalent for homogeneous materials using Broek (1978): Gˆ

K2 GI ˆ I E K2 GII ˆ II E

9:8

2 KIII …1 ‡ † E  ˆ E, the Young's modulus, for plane stress conditions, and E  ˆ E=…1 ÿ  2 † where E for plane strain, where  is Poisson's ratio. For fracture at an interface in planar geometries, this equivalence becomes (Hutchinson and Suo, 1992):

GIII ˆ

Fracture mechanics of adhesive bonds

195

…1 ÿ 2 † 2 …K1 ‡ K22 † 9:9 E   1 1 1 1 ‡ where  ˆ 1 E 2 . E 2 E One of the key advantages of using fracture mechanics to analyze cracked material systems is how easily the energy release rate can be determined for many bonded configurations. For systems in which the load and deflection are linearly related, the energy release rate can be shown to be simply (Broek, 1978) Gˆ

1 dC G ˆ P2 9:10 2 dA where P represents the generalized force (e.g. force, moment, or pressure), C is the compliance of the system, relating the generalized displacement (linear displacement, rotation, or displaced volume, respectively) to the generalized force, and A is the crack area. This covers many practical adhesion tests, although nonlinear forms are sometimes encountered, such as for cases involving membrane stretching or peel tests, and alternative forms of eqn 9.10 are needed (Williams, 1984). These simple formulae allow the energy release rate of many geometries to be readily determined; good approximations can often be determined with relatively simple and brief derivations. Suo and Hutchinson (1990) introduced a particularly useful relationship for determining the energy release rate for arbitrary loading of a general bilayer beam (plane stress) or plate (plane strain), illustrated in Fig. 9.4.     1 P21 M12 1 P22 M22 P23 M32 9:11 ‡ 12 3 ‡  ‡ 12 3 ÿ ÿ Gˆ  h H Ah Ih3 2E 1 h 2 E2 H where  1 =E 2 ˆE  ˆ h=H A ˆ …1=† ‡  " #      1 2 1 1
1 1 ÿ
ÿ I ˆ
ÿ ‡ ‡
ÿ ‡ 3   3   3 and


ˆ

1 ‡ 2 ‡ 2 2…1 ‡ †

The phase angle can also be determined, although is more involved (Suo and Hutchinson, 1990). Applications of these relationships readily provide energy release rates and mode mixities for a variety of specimen configurations and loading modes of adhesive joints (Thouless and Yang, 2002) and coatings (Papini and Spelt, 2002).

196

Adhesive bonding

Figure 9.4 Arbitrary loading on a bimaterial laminate, as analyzed by Suo and Hutchinson.

Where large-scale plasticity is involved, LEFM may not be applicable and the J-integral approach, introduced by Rice (1968) may be used. For limited amounts of plasticity, the J-integral simplifies to G as is appropriate for LEFM. The J-integral and other path independent criteria have been used to account for inelastic behavior (Anderson, 1995).

9.5

Thermodynamic, intrinsic, and practical adhesion energy

For perfectly brittle materials, the critical energy release rate, Gc, should simply be the energy required to create the new surfaces. For this idealized, thermodynamically reversible situation, the energy release rate for crack propagation in a monolithic material would be 2 , where is the surface energy of the material. The factor of 2 comes about because of the two surfaces that are created per unit area of crack propagation. For debonding at an interface between two materials, the thermodynamic work of adhesion is given by Wadh ˆ 1 ‡ 2 ÿ 12 , where 1 , 2 , and 12 are the surface energies of materials 1 and 2 and the interface, respectively. The thermodynamic work of adhesion, often measured by contact angle techniques or the JKR (Johnson et al., 1971) method, arises from dispersion or other physi-sorption forces, and is usually of the order of several tens of mJ/m2. Although these thermodynamic energies are extremely useful in establishing the thermodynamics of wetting for an adhesive on a substrate, they are only a very small fraction of the practical adhesion, as measured by a debonding test. The practical work of adhesion, or the apparent fracture energy, is often 3±6 orders of magnitude larger than the thermodynamic surface energy (for cohesion failures) or work of adhesion (for adhesion failures), implying that significant energy is being dissipated through other mechanisms, including plastic or viscoelastic deformation or microcracking of the adhesive. In some geometries such as peel specimens (Kinloch and Williams, 2002), significant dissipation within the adherends can occur as well, increasing the practical adhesion by as much as a hundredfold (Kim and Kim, 1988). Because polymeric adhesives are viscoelastic, the fracture energy can depend strongly on time, rate, and

Fracture mechanics of adhesive bonds

197

temperature. Fracture energy has been observed to correlate with tan , the ratio of the viscoelastic loss to storage moduli, in polymers and adhesive joints (Xu and Dillard, 2003). Fracture energies are large in transition regions, but smaller at very slow propagation rates, where viscous processes are negligible, and at very fast propagation, where molecular mobility is insufficient to effectively dissipate energy. Thus, if fracture tests could be carried out at an infinitesimally slow rate, or in recognition of the time temperature superposition principle (Ferry, 1980), at elevated temperatures, the practical work of adhesion should be reduced substantially. Such tests have been conducted with elastomeric adhesives, and the limiting fracture energies are referred to as the intrinsic adhesion (Gent and Kinloch, 1971), G0. This intrinsic adhesion value is still measured to be several orders of magnitude larger than the thermodynamic work of adhesion due to dissipation associated with chain stretching and rupture. In turn, the fracture energy or practical work of adhesion, measured at typical test speeds, is often several orders of magnitude larger than the intrinsic adhesion. Even though the intrinsic and practical works of adhesion are much larger than the thermodynamic surface energy or work of adhesion, they may be strongly dependent on these values. If an adhesive does not wet the surface well, for example, that bond is likely to have poor practical adhesion, even if the adhesive itself is capable of dissipating considerable energy through plastic or viscoelastic deformation. Empirically, this dependence has been expressed in multiplicative forms such as the following: _ T; . . .†† Gc ˆ Wadh …1 ‡ …a; _ T; . . .†† Gc ˆ G0 …1 ‡ …a;

9:12

or , which are where Gc for a given condition will depend on either appropriate dissipation functions depending on debond rate, temperature, and perhaps other factors. Figure 9.5 schematically illustrates the relationship among thermodynamic, intrinsic, and practical adhesion, along with representative values.

9.6

The effect of mode mixity

For monolithic materials, cracks will kink or turn so that they run in a mode I fashion. Criteria for crack propagation direction based on maximum circumferential stress around the crack tip (Erdogan and Sih, 1963) maximum energy release rate (Palaniswamy and Knauss, 1978) or pure mode I propagation (Goldstein and Salganik, 1974) are in good agreement on this aspect (Akisanya and Fleck, 1992). In bonded joints subjected to arbitrary loading conditions, however, the crack is often constrained within the adhesive layer. As such, the crack can also be made to grow in mode II, mode III, or mixtures of the three loading modes.

198

Adhesive bonding

Figure 9.5 Schematic illustration of energies associated with debonding. The practical work of adhesion or fracture energy, G c, is dependent on rate, temperature, and thermodynamic or intrinsic adhesion levels.

If the fracture energy were totally reversible, the work required to create new surface energy should be independent of the loading mode, but where inelastic dissipation is involved, the work may depend on the manner in which the surface area is created. In relatively brittle systems, for example, systematic increases in fracture energy are observed as the amount of mode II loading (in either direction) is increased. Reasons cited include additional fracture area (as is often evident in hackle patterns, where the adhesive fractures along 45ë planes, resulting in a zig-zag locus of failure), friction between surfaces rubbing together in shear loading, differences in plastic zone size, and shielding effects (Liang and Liechti 1995). In other systems, however, the situation may become different (Duer et al., 1996; Kinloch et al., 1993). Indeed, in some systems, the fracture energy under mode II conditions may actually be higher than in mode I. For example, in scrim cloth supported adhesive layers, shear loading will drive the debond to the interface, out of the vicinity of the scrim layer, which can dissipate considerable energy due to fiber bridging and the tortuous fracture paths (Parvatareddy and Dillard 1999). Similar results have been observed in rubber toughened adhesives (Chen et al., 2002), where shear loading can drive the failure to the interface where fewer rubber particles are present. Thus, although mode I characterization is most popular, fracture energies obtained for this mode may not lead to conservative designs for certain more complex adhesives. In principle, characterization for a range of mode mixities is advisable for a thorough understanding of how a given adhesive system may perform during fracture events.

Fracture mechanics of adhesive bonds

199

In addition to affecting the fracture energy, mode mixity can also affect the locus of failure (Chen and Dillard, 2002). The concept that chains always break at the weakest link is a useful criterion for discrete systems, but does not necessarily apply for continuous systems, where the mode mixity can steer propagating cracks in certain ways. Shear loading tends to steer cracks towards one interface, as observed visually and confirmed by surface analysis (Chen et al., 2002). The T-stress affects the stability of propagating cracks. Tensile residual stresses within the adhesive layer, for example, increase the T-stress, destabilizing crack propagation (Chen and Dillard, 2001). Tensile T-stresses can cause the debond to alternate back and forth between the adherends (Chen and Dillard, 2001; Fleck et al., 1991). Failures have been known to propagate along tougher planes, avoiding more brittle regions because of the applied stress state. In interpreting the failure mode in bonded joints, one should recognize that the actual locus of failure depends on a complex interaction between the stress state and the spatially varying material properties.

9.7

Experimental evaluation of fracture energy

A variety of specimen configurations have been advocated over the years for a wide range of adhesive and coating applications, many of which are summarized by Kinloch (1987). In selecting appropriate test specimens, one may choose specimens that result in loading modes that closely relate to the applications of interest. To measure the fracture energy of an adhesive bond experimentally, precracked specimens are typically loaded under prescribed conditions, allowing the debond to propagate. For mode I, the double cantilever beam (DCB) specimen, illustrated in Fig. 9.6 along with several other beam specimens, is one of the most widely used fracture specimens. In ASTM D3433, the DCB specimens are pulled apart until crack propagation results in the load decreasing. The crosshead is then held at a fixed displacement, and the debond is allowed to propagate slowly and to arrest prior to the next loading cycle. This approach provides both the initiation and arrest values of the fracture energy. Unloading the specimen between cycles is often used to verify that the specimen is not exhibiting gross plastic strain, which would invalidate the LEFM analysis. The specimen is then subjected to a number of loading cycles, allowing one to determine a number of fracture energies for each specimen. In other standards, the crosshead displacement is continued at constant rate until the debond has propagated over the entire specimen length (Blackman and Kinloch, 2001). Such tests are quicker to perform, but do not provide the arrest values of the fracture energy. Regardless of the method used, the crack length should be determined as the test proceeds in order to analyze the data. Data analysis may be conducted by several methods, including the use of a simple beam formula, modified (incorrectly according to Thouless and Yang, 2002) for shear deformations important at small debond lengths such as:

200

Adhesive bonding

Figure 9.6 Illustration of several common beam-type fracture specimens.

 4P2c  2 3a ‡ h2 9:13 2 3 Ew h as advocated for the DCB specimen by ASTM (2001), where Pc is the critical load, h is the adherend thickness, E is the modulus of the adherend, and w is the adherend and bond width. In the corrected beam theory approach (Blackman and Kinloch, 2001), the fracture energy is given by GIc ˆ

3P2c 3 m …a ‡ ^ a†2 9:14 2w where m and ^a are the slope and debond length correction, respectively, obtained by plotting the cube root of specimen compliance as a function of a. This approach has the advantage of being more robust for various observers, and corrects for root rotation and displacement in the adherends, while still relying on simple beam theory. The ^ a is the crack length correction that accounts for these factors, and is the negative of the abscissa intercept. In the experimental compliance method, the compliance is plotted as a function of the observed debond length on log-log scale. This method makes no assumption about the relationship between compliance and crack length, other than that it obeys a power law relationship. After thorough evaluations of several methods, Blackman and Kinloch (2001) advocate the use of several methods to check the validity of the measured fracture energies, but favor the experimental compliance method over other approaches when only one method is used. GIc ˆ

Fracture mechanics of adhesive bonds

201

The critical stress intensity factor or fracture toughness, Kc , and the critical energy release rate or fracture energy, Gc, as determined for monolithic materials are viewed as material properties, although these may depend on rate, temperature, and other environmental factors. For adhesive bonds, however, other factors may influence the measured fracture energy, including the fracture mode (Anderson et al., 1977), the thickness of the bond (Kinloch and Shaw, 1981), and the properties of the adherends (Blackman et al., 2001). Although not always significant, each of these factors should be considered when developing test plans for bonded joints. The magnitudes of measured fracture energies for adhesive bonds range from a few J/m2 to as much as 10 kJ/m2 or more. Toughening mechanisms, such as those induced by phase-separated elastomer or thermoplastic particles, can raise the toughness of brittle systems by an order of magnitude or more. Poor surface preparation may lead to interfacial failures with very low fracture energies. Exposure to environments such as high humidity can result in precipitous loss in fracture toughness for systems with poor resistance to environmental effects.

9.8

Durability

Recognizing that debonding may occur slowly over time rather than catastrophically as addressed so far with the critical energy release rate, fracture mechanics offers a unique means for designing bonds from a durability perspective. Subcritical cracking or debonding can occur for a variety of loading scenarios. Since the pioneering work of Mostovoy and Ripling (1972), many investigators have used a fracture approach to characterize the resistance of adhesive bonds to cyclic fatigue. The life of an adhesive joint may then be predicted using information related to the rate of debonding as a function of the applied energy release rate. Using fracture specimens such as DCBs, debond rate data is collected as a function of the applied energy release rate. The fatigue response typically results in a characteristic sigmoidal curve, illustrated in Fig. 9.7, that is bounded on the upper end by the critical energy release rate, at which rapid crack growth would occur. On the lower side, the response may be bounded by the threshold energy release rate, below which debond propagation does not occur. In between these limiting values, the response can often be fitted by a power law relationship, as first proposed by Paris (Paris et al., 1961). The debond characteristics are then fitted with an appropriate equation which is then integrated from the initial flaw size to the critical flaw size, allowing one to predict the time to failure (Kinloch and Osiyemi, 1993). These powerful techniques allow predictions to be made for very different geometries than those on which the data were collected. This basic approach of using the relationship between debond growth rate and applied energy release rate can also be extended to other time dependent debonding processes. Knowing the debond characteristics, one can predict the

202

Adhesive bonding

Figure 9.7 Illustration of debond rate as a function of applied energy release rate under fatigue loading conditions.

rate of debonding due to viscoelastic debonding or sub-critical debonding in the presence of exposure to a variety of environmental conditions including moisture. Increasing the severity of these environments often shifts the debond rate to lower values of applied energy release rate. Simple, self-loading test geometries such as the wedge specimen (Cognard, 1987), deadload peel tests, and curvature mismatch specimens (Dillard, 1988), have been used with good success to obtain the subcritical debonding rates for a range of structural (Parvatareddy et al., 1998), microelectronic, and pressure-sensitive adhesives, as well as sealants.

9.9

Designing with fracture mechanics

In spite of the power and versatility of fracture mechanics to predict critical and subcritical debonding, likely failure modes, and lifetime predictions in adhesively bonded joints, designers have not universally adopted this approach. In contrast with a strength-based approach, fracture mechanics involves an additional length scale. In structural bonds, the length parameter is often the length of dominant debonds, although in elastomeric adhesive systems, the thickness of the bond can be the important length scale (Gent, 1974). Designing from a fracture mechanics standpoint requires that the size of initial flaws be measured, estimated, or assumed. Non-destructive testing is often used to determine the size, location, and orientation of initial flaws, and in critical applications, this information may be monitored by periodic measurements. If no cracks are seen with the NDE inspection, conservative design practice dictates that flaws, of the size of the detection limits of the NDE equipment, be assumed to exist at the worst possible location and orientation.

Fracture mechanics of adhesive bonds

203

Lacking actual crack information, representative flaws may be inferred by testing bonded joints, analyzing in terms of the known fracture energy of the system, then determining the flaw size (Kinloch and Osiyemi, 1993). As discussed earlier, the energy release rate can readily be calculated for many test specimens using closed form solutions. For analyzing actual bonded structures, however, geometrical, loading, and material complexities make the analysis more difficult. As in many fields of applied mechanics, the finite element method is a powerful technique for accurately analyzing such problems. One of the problems associated with a fracture mechanics analysis, whether for monolithic or bonded materials, is that fracture parameters can be determined only at the tips of actual or assumed cracks, delaminations, or debonds. The need to assume a flaw and the complexity of analyzing a range of potential geometries have limited use of fracture analysis in design.

9.10 Recent developments and current research areas Traditionally, there have been the strength and fracture mechanics approaches to characterizing materials. Proponents of each method have argued their cases, but have been forced to deal with complications. For strength advocates, the singular stress fields present at crack tips and bimaterial corners present many problems. Designers using this approach often ignore the idea of flaws or cracks, and this has led to some serious failures. When promoting fracture mechanics at the exclusion of strength criteria, proponents often assume that a well-defined crack exists, regardless of how physically tenuous this may be. Engineering designers are often less familiar with this approach, and are forced to assume the existence of a flaw in order to conduct an analysis. The relationship between fracture and strength criteria can be seen in the pioneering work of Dugdale (1960) and Barenblatt (1962), in which they included a yielded zone at the tip of a crack to eliminate the stress singularity. Extending this approach more recently, the cohesive zone model offers to bridge this chasm. By invoking a strength criterion and an energy dissipation criterion, this method allows for both aspects of failure to be included in a single model. Instead of considering a single fracture parameter, the fracture energy, this approach adds a cohesive strength, ^, as well. To implement the procedure, traction separation laws, such as shown in Fig. 9.8 are invoked numerically. This method is becoming increasingly popular (Georgiou et al., 2003; Kafkalidis and Thouless, 2002), and when implemented with special elements, can result in cracks or debonds initiating where no flaw exists and propagating in realistic directions (Goyal et al., 2004a, 2004b). Continued work is needed to simplify the design process and allow extension to a range of debonding rates (time dependent to impact conditions), environments, and loading modes. Fracture mechanics has been found to be particularly useful at predicting the path followed by cracks propagating in both monolithic and bonded systems

204

Adhesive bonding

Figure 9.8 Illustration of possible traction-separation law for cohesive zone modeling of debond propagation.

(Chen and Dillard, 2002). Therefore, this is a powerful tool for understanding the locus of failures. Continued studies in this field could provide fruitful information, especially in probing how debonds may change location based on the complex interaction of stress state with spatially varying material properties and defects in layered materials. Several challenges remain in the experimental characterization of fracture properties of bonded joints. The effect of mode mixity on measured fracture energy is still poorly understood, especially in practical engineering adhesives. Whereas mode I toughness is critical in monolithic materials and in some adhesives, these results may not always be conservative. Additional information is needed to ensure conservative designs. The effect of impact loading is important, especially stick-slip behavior, which can result when viscoelastic deformation can blunt slowly moving cracks, increasing the energy required for propagation. The singular stresses present at the tip of a crack are just a special case of a more general elasticity solution for related geometries. Analyzed as wedges of various angles, singular stress fields are often present at re-entrant corners in monolithic materials, and at the terminus of many bonds. These highly stressed regions have been shown to be sites for failure initiation under static (Hattori et al., 1989; Adams et al., 1997) and fatigue (Lefebvre et al., 2002; Lefebvre and Dillard, 1999; Lefebvre et al., 1999) loading. These singular regions are dominated by stress states obeying  / r , where  is the smallest eigenvalue. For bimaterial wedges,  is often algebraically larger than ÿ1/2, which is the case for a sharp crack. As such, the singularity is weaker, but is nonetheless sufficient to dominate failure initiation. These singular regions can thus serve as precursors of cracks (Adams and Harris, 1987; Reedy, 2002). Further work is needed, however, in determining appropriate failure criteria that might allow designers to predict the failure behavior of such bonds. One interesting approach

Fracture mechanics of adhesive bonds

205

is the use of Weibull statistics to estimate when failure will occur (Towse et al., 1999). In this approach, the singular stresses are recognized to occur only over infinitesimal volumes, statistically offsetting one another. In an interesting historic perspective, this closes the loop on the observations of da Vinci several centuries ago that long wires are weaker than shorter wires (Timoshenko, 1953), presumably because of greater flaw probability.

9.11 Conclusions Fracture mechanics offers a powerful tool to characterize failure of both monolithic materials and bonded systems. Based on the concept that all real material systems contain (or may develop) flaws that can significantly alter the resulting stress state, fracture mechanics has proven uniquely appropriate for characterizing the structural integrity of a wide array of materials and structures. Fracture mechanics has been applied to adhesive joints with good success for characterizing the critical and subcritical debonding. Although more sophisticated approaches are available, linear elastic fracture mechanics has found widespread applications for structural adhesives, adhesives used in biomedical and microelectronic areas, coatings, sealants, and pressure-sensitive adhesives. Fracture test methods differ from strength-based specimens in that sharp-tipped debonds are intentionally introduced. The propagation of these debonds under quasi-static, creep, impact, and fatigue loading conditions may be studied to determine material properties that are useful in selecting appropriate adhesive systems and ultimately in the design of bonded joints.

9.12 References Adams R D and Harris J A (1987). `The Influence of Local Geometry on the Strength of Adhesive Joints.' International Journal of Adhesion and Adhesives, 7(2), 69±80. Adams R D, Comyn J and Wake W C (1997). Structural Adhesive Joints in Engineering, Chapman and Hall, London. Akisanya A R and Fleck N A (1992). `Analysis of a Wavy Crack in Sandwich Specimens.' International Journal of Fracture, 55(1), 29±45. Anderson G P, Bennett S J and DeVries K L (1977). Analysis and Testing of Adhesive Bonds, Academic Press, New York. Anderson T L (1995). Fracture Mechanics: Fundamentals and Applications, CRC Press, Boca Raton. ASTM (2001). `D 3433-99: Standard Test Method for Fracture Strength in Cleavage of Adhesives in Bonded Metal Joints.' Annual Book of ASTM Standards, ASTM, West Conshohocken, 225±231. Barenblatt G I (1962). `The mathematical theory of equilibrium cracks in brittle fracture.' Advances in Applied Mechanics, 7, 55±129. Blackman B R K and Kinloch A J (2001). `Fracture Tests for Structural Adhesive Joints.' Fracture Mechanics Testing Methods for Polymers, Adhesives and Composites, A Pavan, D R Moore and J G Williams, eds, Elsevier, Amsterdam, 225±305.

206

Adhesive bonding

Blackman B R K, Kinloch A J and Paraschi M (2001). `The effect of the substrate material on the value of the adhesive fracture energy, G(c): Further considerations.' Journal of Materials Science Letters, 20(3), 265±267. Broek D (1978). Elementary Engineering Fracture Mechanics, Sijthoff & Noordhoff, Alphen aan den Rijn. Chen B and Dillard D A (2001). `The effect of the T-stress on crack path selection in adhesively bonded joints.' International Journal of Adhesion and Adhesives, 21(5), 357±368. Chen B and Dillard D A (2002). `Crack Path Selection in Adhesively Bonded Joints.' Adhesion Science and Engineering ± I: The Mechanics of Adhesion, D A Dillard and A V Pocius, eds, Elsevier Science, Amsterdam, 389±442. Chen B, Dillard D A, Dillard J G and Clark R L (2002). `Crack path selection in adhesively bonded joints: the roles of external loads and specimen geometry.' International Journal of Fracture, 114(2), 167±190. Cognard J (1987). `Quantitative Measurement of the Energy of Fracture of an Adhesive Joint Using the Wedge-Test.' Journal of Adhesion, 22(2), 97±108. Dillard D A (1988). `Stresses between Adherends with Different Curvatures.' Journal of Adhesion, 26(1), 59±69. Duer R, Katevatis D, Kinloch A J and Williams J G (1996). `Comments on mixed-mode fracture in adhesive joints.' International Journal of Fracture, 75(2), 157±162. Dugdale D S (1960). `Yielding in steel sheets containing slits.' Journal of the Mechanics and Physics of Solids, 8, 100±104. Dundurs J (1969). Journal of Applied Mechanics, 36, 650±652. Erdogan V F, and Sih G C (1963). `On Crack Extension in Plates Under Plane Loading and Transverse Shear.' Trans. ASME J. Bas. Eng., 85, 519±527. Ferry J D (1980). Viscoelastic Properties of Polymers, Wiley, New York. Fleck N A, Hutchinson J W and Suo Z G (1991). `Crack Path Selection in a Brittle Adhesive Layer.' International Journal of Solids and Structures, 27(13), 1683±1703. Gent A N (1974). `Fracture Mechanics of Adhesive Bonds.' Rubber Chemistry and Technology, 47, 202±212. Gent, A N and Kinloch A J (1971). `Adhesion of viscoelastic materials to rigid substrates. III Energy criterion for failure.' J. Polymer Sci., (Polymer Physics Edition), 9(4), 659±668. Georgiou I, Hadavinia H, Ivankovic A, Kinloch A J, Tropsa V and Williams J G (2003). `Cohesive zone models and the plastically deforming peel test.' Journal of Adhesion, 79(3), 239±265. Goldstein R V and Salganik R L (1974). `Brittle Fracture of Solids with Arbitrary Cracks.' International Journal of Fracture, 10(4). Goyal V K, Jaunky N R, Johnson E R and Ambus D R (2004a). `Intralaminar and interlaminar progressive failure analyses of composite panels with circular cutouts.' Composite Structures, 64(1), 91±105. Goyal V K, Johnson E R and Davila C G (2004b). `Irreversible constitutive law for modeling the delamination process using interfacial surface discontinuities.' Composite Structures, 65(3±4), 289±305. Griffith A A (1921). `The Phenomena of Rupture and Flow in Solids.' Philosophical Transactions of the Royal Society, A221, 163±198. Hattori T, Sakata S and Murakami G (1989). `A Stress Singularity Parameter Approach for Evaluating the Interfacial Reliability of Plastic Encapsulated LSI Devices.'

Fracture mechanics of adhesive bonds

207

Journal of Electronic Packaging, 111, 243±248. Hutchinson J W and Suo Z (1992). `Mixed-Mode Cracking in Layered Materials.' Advances in Applied Mechanics, 29. Irwin G R (1958). Handbuch der Physik. S. FluÈgge, ed., Springer-Verlag, BerlinHeidelberg, 551±590. Johnson K L, Kendall K and Roberts A D (1971). `Surface Energy and the Contact of Elastic Solids.' Proc. R. Soc. London A., 324, 301±313. Kafkalidis M S and Thouless M D (2002). `The effects of geometry and material properties on the fracture of single lap-shear joints.' International Journal of Solids and Structures, 39(17), 4367±4383. Kim K S and Kim J (1988). `Elasto-Plastic Analysis of the Peel Test for Thin-Film Adhesion.' Journal of Engineering Materials and Technology ± Transactions of the Asme, 110(3), 266±273. Kinloch A J (1987). Adhesion and Adhesives: Science and Technology, Chapman and Hall, London. Kinloch, A J and Osiyemi S O (1993). `Predicting the Fatigue Life of Adhesively-Bonded Joints.' Journal of Adhesion, 43(1±2), 79±90. Kinloch A J and Shaw S J (1981). `The Fracture Resistance of a Toughened Epoxy Adhesive.' J. Adhesion, 12, 59±77. Kinloch A J and Williams J G (2002). `The Mechanics of Peel Tests.' The Mechanics of Adhesion, D A Dillard and A V Pocius, eds, Elsevier, Amsterdam, 273±302. Kinloch A J, Wang Y, Williams J G and Yayla P (1993). `The mixed-mode delamination of fiber composite materials.' Composites Science and Technology, 47(3), 225±237. Lefebvre D R and Dillard D A (1999). `A stress singularity approach for the prediction of fatigue crack initiation in adhesive bonds. Part 1: Theory.' Journal of Adhesion, 70(1±2), 119±138. Lefebvre D R, Dillard D A and Dillard J G (1999). `A stress singularity approach for the prediction of fatigue crack initiation in adhesive bonds. Part 2: Experimental.' Journal of Adhesion, 70(1±2), 139±154. Lefebvre D R, Ahn B K, Dillard D A and Dillard J G (2002). `The effect of surface treatments on interfacial fatigue crack initiation in aluminum/epoxy bonds.' International Journal of Fracture, 114(2), 191±202. Liang Y M and Liechti K M (1995). `Toughening mechanisms in mixed-mode interfacial fracture.' International Journal of Solids and Structures, 32(6±7), 957±978. Mostovoy S and Ripling E J (1972). `Fracturing Characteristics of Adhesive Joints: Final Report of Naval Air Systems Command.' Contract No. N00019-72-0250. Muskhelishvili N I (1953). Some Basic Problems of the Mathematical Theory of Elasticity, Noordhoff. Palaniswamy K and Knauss W G (1978). `On the Problem of Crack Extension in Brittle Solids Under General Loading.' Mechanics Today, 4, 87±148. Papini M and Spelt J K (2002). `The Mechanics of Coatings.' The Mechanics of Adhesion, D A Dillard and A V Pocius, eds, Elsevier, Amsterdam, 303±350. Paris P C, Gomez M P and Anderson W E (1961). `A Rational Analytic Theory of Fatigue.' The Trend in Engineering, 13, 9±14. Parvatareddy H and Dillard D A (1999). `Effect of mode-mixity on the fracture toughness of Ti-6Al-4V/FM-5 adhesive joints.' International Journal of Fracture, 96(3), 215±228. Parvatareddy H, Dillard J G, McGrath J E and Dillard D A (1998). `Environmental aging of the Ti-6Al-4V/FM-5 polyimide adhesive bonded system: implications of physical

208

Adhesive bonding

and chemical aging on durability.' Journal of Adhesion Science and Technology, 12(6), 615±637. Reedy E D (2002). `Strength of Butt and Sharp-Cornered Joints.' Adhesion Science and Engineering ± I: The Mechanics of Adhesion, D A Dillard and A V Pocius, eds, Elsevier Science, Amsterdam, 145±192. Rice J R (1968). `A Path Independent Integral and the Approximate Analysis of Strain Concentrations by Notches and Cracks.' Journal of Applied Mechanics, 379±386. Seely F B and Smith J O (1952). Advanced Mechanics of Materials, John Wiley & Sons, Inc., New York. Sih G C (1973). Handbook of Stress-Intensity Factors: Stress-Intensity Factor Solutions and Formulas for Reference, Lehigh University, Bethlehem, PA. Suo Z G and Hutchinson J W (1990). `Interface Crack between Two Elastic Layers.' International Journal of Fracture, 43(1), 1±18. Thouless M D and Yang Q D (2002). `Measurement and Analysis of the Fracture Properties of Adhesive Joints.' The Mechanics of Adhesion, D A Dillard and A V Pocius, eds, Elsevier, Amsterdam, 235±272. Timoshenko S P (1953). History of Strength of Materials, McGraw-Hill, New York. Towse A, Potter K D, Wisnom M R and Adams R D (1999). `The sensitivity of a Weibull failure criterion to singularity strength and local geometry variations.' International Journal of Adhesion and Adhesives, 19(1), 71±82. Williams J G (1984). Fracture Mechanics of Polymers, Halsted Press, New York. Xu S Y and Dillard D A (2003). `Determining the impact resistance of electrically conductive adhesives using a falling wedge test.' IEEE Transactions on Components and Packaging Technologies, 26(3), 554±562.

10

Fatigue I A ASHCROFT

10.1 Introduction Fatigue in engineering structures is a loss of structural integrity over time due to the repeated or continuous application of stress. The response to a constant stress is sometimes called `static fatigue', however, the term `fatigue' is more generally associated with intermittent or cyclic stressing and only this aspect will be explicitly addressed here. However, the effects of creep and fatigue are closely related and a cyclic stress with a non-zero mean may be considered as a superposition of a fully reversed cyclic stress and a constant stress. The engineering importance of fatigue lies in the frequently observed occurrence of failure under repetitive loading at far smaller loads than those required to cause quasi-static failure. Failure may occur suddenly after many years in service, at which point many similar parts may be in service and an expensive remedy required. The fatigue phenomenon is common to most types of materials and it has been estimated that 80% of all engineering failures can be attributed to fatigue.1 The cost of these failures is high in economic terms and also in the injuries and loss of life incurred in major engineering failures. It is not surprising, therefore, that a determined effort has been aimed at understanding fatigue failures and developing fatigue lifetime prediction methods. These studies started in earnest in the 1850s with the widespread use of steel on the railways and received added impetus with the introduction of steel ships and aluminium alloy aircraft. Although the majority of the published material on fatigue concerns metals, there is a growing literature on the subject of fatigue in polymers and polymer composites. Although these materials share many of the characteristics of fatigue failure with metals, there are also a number of key differences. The mechanisms of fatigue in polymers differ from those in metals, as will susceptibility to environmental factors such as moisture and temperature. Also, the visco-elastic nature of many polymers at modest temperatures will affect their response to cyclic stresses. The study of fatigue in bonded joints is further complicated by the fact that we are dealing with a heterogeneous system in which the adhesive itself is usually a composite material. It is not surprising,

210

Adhesive bonding

therefore, that the level of understanding of fatigue in adhesive joints is not as advanced as that for many metals. However, adhesive joints are generally considered to have good fatigue resistance compared with alternative joining techniques, such as spot welding or bolting. This has been attributed to the reduction in stress concentrations, which deleteriously affect the fatigue life of metals. In addition, the adhesive layer prevents fretting fatigue, which may be a problem in mechanical joints. There is, therefore, a case for using adhesive joints in applications subjected to fatigue loading and hence the need to understand and predict fatigue in bonded joints. Before the response of adhesive joints to fatigue is discussed, it is worth considering the sources of fatigue loading. Stresses in an adhesive joint can be attributed to mechanical loads or to residual effects. The latter arise from a number of sources including the differential thermal expansion of adhesive and adherends (thermal stresses) and the expansion of the adhesive as moisture is absorbed (hygroscopic stresses). In the following discussion only fatigue stresses due to applied mechanical loads are discussed, however, fatigue due to fluctuating residual stresses may also arise from periodic changes in the service environment. Mechanical loads can be further divided into static loads, working loads, vibratory loads and accidental loads, which together comprise the fatigue spectrum. In an aircraft, for example, a static load will be associated with self weight and working loads will arise from standard manoeuvres such as taking off and landing. Vibratory loads are high-frequency loads that are superimposed on the working loads and an example of these would be the vibrations due to the interaction of an aeroplane's tyres with the runway. Accidental loads are caused by events outside normal operating practice. These events, though usually rare, may be important in initiating fatigue damage. These loads will be to some degree irregular but may conveniently be represented by a simplified variable amplitude fatigue spectrum of loads seen in a standard flight. A further distinction to be made is between high cycle fatigue (HCF) and low cycle fatigue (LCF). HCF considers events that may occur millions of times in the life of a structure, with a predominantly elastic response. LCF may involve only thousands of events before failure, but is associated with more widespread plasticity. In the aircraft example, high cycle fatigue will be concerned with the working loads and vibratory loads, of which there may be hundreds or thousands per flight. LCF may be associated with pressurisation of the cabin, which only occurs once per flight. The latter, together with stress concentrations in the fuselage, was the source of failure in the famous Comet aircraft disasters.2 LCF may operate over a similar time-span as HCF but involve lower frequency. Although the fatigue spectra seen by many structures will be rather complex it is often convenient in laboratory investigations to consider fatigue in terms of a sinusoidal waveform, as shown in Fig. 10.1. In this case a sinusoidal wave with a constant stress amplitude is shown, however, constant load, strain or

Fatigue

211

Figure 10.1 Constant amplitude sinusoidal waveform.

displacement amplitude waves are also used. A number of key parameters used to describe fatigue spectra are defined below. Maximum stress ˆ max Minimum stress ˆ min Mean stress ˆ m ˆ

max ‡ min 2

Stress range ˆ
 ˆ max ÿ min
 2 min Stress ratio ˆ R ˆ max

Stress amplitude ˆ a ˆ

Frequency ˆ f ˆ

1 Hz cycle…sec s†

Only two of the stress parameters, together with frequency, are required to characterise a constant amplitude spectrum. This means that it is impossible to investigate each of these parameters independently, e.g., with a constant stress amplitude it is impossible to investigate the effect of the mean stress without also affecting the maximum stress. In metal testing, fully reversed fatigue testing

212

Adhesive bonding

is common, in which the mean stress is zero and R ˆ ÿ1. Most adhesively bonded joints are designed for tension loading, hence the mean stress is usually positive and R is greater than zero (typically 0.1 and 0.5). However, the fact that these samples are always under a positive tensile load means that they are susceptible to creep, which may complicate the fatigue response. Other waveforms such as trapezoidal, square or saw-tooth may be used to explore such issues as the effect of loading rate, the relative importance of the loading, unloading and hold periods and creep-fatigue interactions. Three main approaches have been used to analyse fatigue in metal structures. These are the stress-life approach, the strain-life approach and the fatigue crack growth (FCG) approach. The stress-life and strain-life approaches are associated with the safe-life design philosophy, in which a component is considered flaw free and is designed for a fixed service life after which the part should be replaced. The stress-life approach is associated with HCF and will be discussed in more detail in section 10.2. The strain-life approach considers the effects of localised plastic deformation during cyclic stressing and is generally associated with LCF of ductile metals. This method has seen little application to adhesively bonded joints and texts on metal fatigue should be referenced for further information.1±3 In the FCG approach, an initial flaw distribution is assumed and knowledge of the conditions and rates at which these cracks will grow, together with a comprehensive inspection programme is used in a fail-safe design approach. This allows for less conservative designs and is more commonly used in the aircraft industry. The study of fatigue in bonded joints can be considered in terms of the mechanisms of fatigue damage, i.e., the changes in the physical and chemical structure of the adhesive during fatigue stressing, or in terms of the mechanical response of the joint. For reasons of space this chapter mainly concerns the latter approach and only passing reference will be made to failure mechanisms. However, it is worth considering that the total fatigue life of a component is often considered as being composed of an initiation and a propagation phase. Although there is no clear distinction between the two, it is useful to think of the propagation phase as starting when a macro crack has formed and the conditions of fracture mechanics are fulfilled. After this point the FCG method can be used to predict the propagation fatigue life. The initiation phase is less easily characterised because the highest stresses in a bonded joint are often internal and hence damage initiation is difficult to observe directly. The mechanisms occurring during the initiation phase include micro-plasticity, micro-crack nucleation and growth, localised creep and damage coalescence. The small scale of these mechanisms also hinders direct observation. The propagation phase is concerned with the propagation of a dominant crack, however, similar damage mechanisms to those discussed above may still be occurring in the stress field ahead of the dominant crack front and this will influence crack growth kinetics.

Fatigue

213

10.2 The stress-life approach 10.2.1 Introduction In this method the sample or component is subjected to a cyclic load with constant stress or load amplitude. In most cases this is a sinusoidal waveform with a constant frequency, as shown in Fig. 10.1. Load cycling continues until complete failure of the part and this is repeated at different levels of stress or load amplitude. The results are usually presented as a plot of stress (S) against number of cycles to failure (NF) and this is referred to as an S-N curve. S is often a nominal or average stress and in the standard specimens used in metal testing can be related to the maximum stress by a stress concentration factor, kt. In the case of bonded joints, the applied load divided by the total bonded area is often used as the nominal stress. However, in this case the relationship between the nominal stress and the maximum stress is less easily defined which makes the results from an S-N curve for one sample geometry difficult to apply to a different sample geometry. A number of features are worth noting in the schematic S-N curve shown in Fig. 10.2. Firstly, the fatigue life increases as the stress amplitude decreases and, considering the log scale, small decreases in stress amplitude can result in large increases in the life. It can also be seen that at high cycles the curve is asymptotic to an infinite life at a stress amplitude of
Se . This is the fatigue (or

Figure 10.2 S-N curve.

214

Adhesive bonding

endurance) limit and is seen in many materials, including many steels and plastics. Where this is not seen, such as in aluminium alloys,
Se is defined as the stress amplitude at some arbitrary, large number of cycles (typically 107) or can be used to indicate a change in slope of the S-N curve. In adhesive joints,
Se is typically quoted at between 20 and 50% of the quasi-static failure stress. A change in slope may also be seen in the high-stress/low-cycle region, however, as this is the LCF region, where the strain-life approach is more valid, this is often ignored and the S-N curve can be represented by the straight line S ˆ C ‡ D log NF and
Se (or a straight line of reduced gradient). In some cases both S and NF are plotted on log scales in which the data can be fitted to the curve S ˆ ANFB. The S-N curve is useful in predicting the life to failure of a component, however, it tells us nothing of the evolution of damage during fatigue stressing. Therefore, there is no means of determining the residual life of a component by using a visual inspection with the stress-life approach. A particular deficiency in the standard stress-life approach is that no differentiation is made between the crack initiation and growth phases. As mentioned above, the crack initiation phase is difficult to monitor in bonded joints, however, some progress in investigating the initiation phase has been made by use of the back-face strain method.4,5 In this method strain gauges are bonded to the back face of the adherend near a site of anticipated failure. The change in measured strain can then be used to indicate the fatigue damage. The relation between the strain response and the damage incurred depends upon such factors as the position of the gauge, the size of the gauge and the geometry and materials used in the joint. These factors have been explored using finite element analysis (FEA),4,5 which has also been used to determine the correlation between the strain readings and the damage incurred. Figure 10.3 shows the strain gauge placement and typical response for fatigue crack growth in a single lap joint. In this case there appears to be an initiation stage of approximately 15,000 cycles, after which we get an increase in strain in one of the gauges and a decrease in the other. This indicates crack growth from the S2 end of the overlap. It was noted, however, that when the adhesive fillets were removed that the initiation stage disappeared.5 Other methods that have been used to examine the evolution of damage in bonded joints include optical microscopy, laser interferometry, x-radiography and ultrasonics.6±8 The use of these techniques has seen the emergence of champions for both propagation and initiation dominated fatigue, however, in all likelihood the relative importance of these phases will be sensitive to many factors including materials, sample geometry, test environment, load range and the definition of the transition point from initiation to propagation. In all events, it appears that at present the initiation phase (or early damage phase) is poorly understood in bonded joints and is worthy of further investigation.

Fatigue

215

Figure 10.3 Back-face strain detection of fatigue crack growth (data from ref. 5).

10.2.2 Test methods Samples used in the stress-life testing of bonded joints are similar to those used in quasi-static testing, e.g., single lap joints, double lap joints, lap-strap joints, etc. The dimensions of these samples may vary from standard samples if issues such as crack initiation period, crack propagation rate or creep-fatigue are to be investigated. Attention must be paid to sample control and test conditions as the response of bonded joints to fatigue loading can be sensitive to small differences in the sample geometry and test environment. As a fatigue-testing programme is likely to take a considerable period of time to complete, the stability and storage of test samples should also be controlled to ensure that samples towards the end of the test programme are in the same state as those tested earlier. This can be verified by periodic testing of control samples. In fatigue testing of metals, bars are often tested in bending. This is easily achieved by rotation of the beam under a static load. For bonded joints, reciprocating axial loading is more common and this can be achieved by various mechanical systems, including machines based on resonant vibration and servohydraulic systems. The latter tend to be the most costly option but also allow greater control over waveform, amplitude and frequency of testing. Modern systems tend to have sophisticated control systems capable of ensuring close adherence to the demand waveform as sample compliance changes and are also capable of applying complex and semi-random waveforms and of high frequency monitoring of output data such as load and displacement. In a

216

Adhesive bonding

standard stress-life curve it is important to ensure that the minimum and maximum load (or stress) remain constant through the test and that the number of cycles to failure is recorded. Displacement, stress or crack growth may also be monitored to provide additional information about the evolution of damage during the test. The fatigue testing of adhesive lap joints is covered by the standards BS EN ISO 9664:1995 and ASTM D3166-99. In the former it recommends that at least four samples should be tested at three different stress amplitude values for a given stress mean, such that failure occurs between 104 and 106 cycles. This standard also advises on statistical analysis of the data. In general, fatigue data exhibits greater scatter than quasi-static data and this need to be taken into account when using safety factors with fatigue data. Further advice on the application of statistics to fatigue data is found in BS 3518-5:1966.

10.2.3 Fatigue loading effects Mean stress effects can be investigated by testing samples at several values of mean stress and at various stress amplitudes to generate a family of S-N diagrams. An example of this is shown in Fig. 10.4 for a steel-epoxy lap joint (data from reference 9). It should be noted that a load range rather than a stress range is used in this case. This is more useful as the relation between the average shear stress and the maximum stress is not well defined and once crack propagation occurs the bonded area decreases, resulting in an increase in the average stress and also a change in the relationship between average stress and

Figure 10.4 Effect of R-ratio on the S-N curves for steel-epoxy lap joints (data from ref. 9).

Fatigue

217

Figure 10.5 Constant life diagram for steel-epoxy lap joints (data from ref. 9).

maximum stress. It should also be noted that in this figure the mean load is indicated by means of the R ratio. It can be seen from the definitions in section 10.1 that R and the mean load are related and that if the load amplitude is held constant, a lower value of R indicates a lower mean. In Fig. 10.4 it can be seen that as R decreases, the load for a given fatigue life increases, indicating an increase in fatigue resistance. This can probably be attributed to the reduction in the maximum load as R decreases. An alternative way of presenting the effect of mean stress is in a constant life diagram, in which mean stress is plotted against stress amplitude for a given fatigue life. A constant life diagram using the data in Fig. 10.4 is shown in Fig. 10.5. It can be seen that as the stress amplitude increases the mean stress must be reduced to maintain a constant life. The stress amplitude in the constant-life diagram can be normalised with respect to the stress amplitude for that particular life at R ˆ ÿ1…ar †, to produce a normalised amplitude-mean diagram, which tends to consolidate the data at different lives to a single curve. This curve can be fitted to a straight line (Goodman equation) or a curved line, such as a parabola (Gerber equation): a m ‡ ˆ1 10:1 Goodman equation: ar u  2 a m Gerber equation: ‡ ˆ1 10:2 ar u

218

Adhesive bonding

These curves are often not accurate for compressive mean stresses, in which case a conservative estimate may be made by assuming that compressive mean stresses provide no benefit, i.e, a ˆ 1: for m  0; ar Many metals are relatively insensitive to fatigue frequency (unless creep or corrosion is present) and this can usefully be used to accelerate fatigue life tests by the application of high frequency testing. The visco-elastic nature of polymeric materials means that the effect of frequency has to be treated more cautiously in bonded joints. In some polymers, cyclic loading can result in significant hysteretic heating that can lead to thermal fatigue failure. Hysteretic heat generation is proportional to the frequency and the square of the stress amplitude. The localised increase in temperature depends on the comparative rates of heat generation due to the cyclic stressing and heat dissipation to the surrounding material. In bonded joints, high stress regions tend to be highly localised and attempts to measure the temperature at the crack tip of bonded joints subjected to cyclic loading have demonstrated only very modest temperature rises,10,11 confirming the high rate of heat dissipation compared to heat generation in these systems. This may explain why high frequency thermal softening has not been reported in the literature for bonded joints. However, there has been little reported work on the effect of high frequency on the fatigue strength of bonded joints and hysteretic heating may still be a factor under certain conditions of loading, environment and geometry. At low frequencies, creep may be important in the fatigue testing of bonded joints. Hart-Smith12 observed that lap joints with small overlaps that were capable of surviving 107 cycles at high frequency, failed in only a few hundred cycles at low frequency. This effect was not seen in similar joints with longer overlaps as creep was restricted to the edges of the joint and a large region of the adhesive at the centre of the joint remained elastic. Romanko and Knauss13 similarly found that the total time to failure could be more important than the number of cycles in predicting the fatigue failure of short overlap joints. It is worth noting that in BS EN ISO 9664 it states that the test frequency should be 30 Hz unless indicated otherwise and that frequencies greater than 60 Hz should be avoided to prevent excessive heating of the adhesive.

10.2.4 Effect of test environment It is well known that adhesives and adhesion can be adversely affected by environmental exposure and that this in itself is a topic of considerable complexity. When environmental effects are combined with fatigue testing we have a further complication owing to the introduction of coupled time dependent effects. The main effects of environmental exposure can be classified as those

Fatigue

219

affecting the adhesive, those affecting the adherend and those affecting the interphase between the two. In terms of the adhesive, an increase in temperature or the absorption of moisture generally sees increased plasticization with accompanying reduction in modulus and failure load. However, strain to failure and fracture toughness may increase. This may affect fatigue initiation and propagation in a number of ways. The plasticization will tend to reduce stress concentrations, although stresses may now be significant over a larger area, hence the resistance to brittle fatigue failure may increase but the resistance to creep-fatigue decrease. These effects will become more significant close to the glass transition point (Tg) and similarity can be seen between the effects of moisture, temperature and test rate (or frequency). Some of these issues are illustrated by the results shown in Table 10.1 (data from references 14 and 15). This table shows the fatigue limits for the bonded CFRP lap-strap and double lap joints shown in Fig. 10.6. Looking at the lap-strap results first, it can be seen that temperature has little effect on the fatigue limits of those samples stored and tested in nominally dry conditions. However, samples tested in hot-wet conditions experience a significant reduction in the fatigue limit. Samples were also conditioned under high humidity conditions until saturation. Samples subsequently tested wet at 22 ëC saw no change in fatigue limit compared to those tested dry whereas samples tested at 90 ëC, whether wet or dry, experienced a large reduction in fatigue resistance. Interpretation of these results was complicated by the fact that complex mixed mode failure paths were observed. In order to explain these results the effect of temperature and moisture on the mechanical behaviour of the adhesive needs to be considered. It can be seen in Fig. 10.7(a) that the stress to failure and modulus of the adhesive decreases as the temperature increases but that strain to failure and total strain energy density increase. The competing Table 10.1 Effect of environment on the fatigue limit for bonded CFRP-epoxy lapstrap and double lap joints (data from references 14 and 15) Sample

Pre-conditioning

Test conditions

Fatigue limit, kN

Lap-strap joint

Vacuum desiccator

ÿ50 ëC/ambient 22 ëC/ambient 90 ëC/ambient 90 ëC/97% rh

14 15 14 7

22 ëC/95% rh 90 ëC/ambient 90 ëC/97% rh

15 5 5

45 ëC/85% rh

Double-lap joint Vacuum desiccator

ÿ50 ëC/ambient 22 ëC/ambient 90 ëC/ambient

10 10 3.3

220

Adhesive bonding

Figure 10.6 Schematic drawings of (a) lap-strap and (b) double-lap joints.

effects of these different trends conspire to maintain the fatigue limit at a relatively constant value between 22 and 90 ëC when stored and tested dry. DMTA was carried out on samples saturated to different moisture levels and it was seen that for every 1% of moisture absorbed, the glass transition point of the adhesive decreased by approximately 15 ëC.14 Hence the saturated adhesive tested at 22 ëC would not be expected to behave markedly differently from those tested dry at 22 and 90 ëC. However, the saturated sample tested at 90 ëC is in the glass transition range and the effect of this on the mechanical behaviour of the adhesive is clearly shown in Fig. 10.7(b). The adhesive is clearly less capable of resisting stress under these conditions and hence the fatigue limit is greatly reduced. The transient effects of moisture were investigated to explain the results of dry stored samples tested at 90 ëC/wet and wet stored samples tested at 90 ëC/dry.16 The moisture concentration at the failure initiation site for these samples as a function of number of fatigue cycles is shown in Fig. 10.8, and Fig. 10.9 shows the correlation between the maximum water concentration at the initiation site and the fatigue limit. If the lap-strap results are now compared with the double-lap joint results in Table 10.1 it can be seen that whereas the lap-strap joints are relatively temperature insensitive over the test range when dry, the double-lap joints experience a large decrease in fatigue resistance as temperature is increased from 22 to 90 ëC. This was attributed to creep enhanced failure at the higher temperature15 and this can be seen in the plots of displacement against cycles at constant load amplitude testing in Fig. 10.10.

Fatigue

221

Figure 10.7 Effect of (a) temperature in ambient humidity and (b) moisture at 90 ëC on the mechanical properties of an epoxy adhesive (data from ref. 14).

Little17 carried out fatigue tests on aluminium single lap joints bonded with the same adhesive as that in the experiments discussed above. He found that with chromic acid etched (CAE) adherends, there was no difference in the fatigue threshold of those samples tested dry and those tested immersed in distilled water at 28 ëC. However, those samples with grit blasted and degreased

222

Adhesive bonding

Figure 10.8 Change in moisture concentration at the failure initiation site with cycles for CFRP-epoxy lap-strap joints stored and tested in different environments (data from ref. 16).

Figure 10.9 Correlation between maximum water concentration at the failure initiation site and fatigue limit (data from ref. 16).

Fatigue

223

Figure 10.10 Creep in CFRP-epoxy double-lap joints during fatigue testing at 90 ëC (data from ref. 15).

(GBD) adherends exhibited a significantly lower fatigue threshold when tested wet and the locus of failure changed from cohesive failure in the adhesive to failure in the interfacial region.

10.2.5 Effect of test geometry and lifetime prediction Once an S-N curve has been generated for a particular joint geometry, the question arises as to the validity of this data to other joint geometries constructed from the same materials. Clearly failure criteria are needed and the most obvious ones are those based on strength of materials or fracture mechanics concepts. If the fatigue life is dominated by the crack propagation phase then the fatigue crack growth (FCG) method can be used to predict the fatigue life and this is discussed in section 10.3. However, if the fatigue life is initiation dominated or if only fatigue limits are to be predicted then analysis of the failure initiation site in the uncracked joint may be sufficient. A number of strength of materials and fracture mechanics criteria have been investigated for the prediction of fatigue thresholds in lap-strap joints.18 Results from tests with unidirectional CFRP adherends were used to predict the fatigue limits in similar joints with multidirectional adherends. Some of the results are shown in Table 10.2. With the strength-based failure criteria, the stress singularity was handled by using the `stress at a distance' and the `stress over an area' methods. In the case of elastic and elasto-plastic fracture mechanics an initial crack of similar dimension to flaws seen in sectioned samples of the adhesive was chosen. The method above is reasonable for the prediction of fatigue limits but a damage mechanics

224

Adhesive bonding

Table 10.2 Prediction of fatigue limits in lap-strap joints with multidirectional CFRP adherends (data from reference 18) Failure Criterion

Test temperature

Experimental fatigue limit, kN Maximum principal strain at a distance Von Mises strain at a distance Maximum principal stress at a distance Von Mises stress at a distance Shear stress at a distance Peel stress at a distance Maximum principal stress over an area Plastic zone size Strain energy release rate J-integral

ÿ50 ëC

22 ëC

90 ëC

11.0 10.9 11.2 10.4 11.1 11.1 7.8 10.4 ö 8.2 11.1

11.0 11.8 12.2 11.0 12.0 12.0 8.3 11.0 13.2 8.8 11.8

9.0 11.2 11.3 14.3 11.4 11.4 >15 14.3 12.0 8.2 10.9

approach would be more useful to characterise the initiation phase. A continuum damage mechanics (CDM) method was used to predict S-N curves for the double-lap joints shown in Fig. 10.6 and this method compared favourably with a fracture mechanics approach.19

10.2.6 Variable amplitude fatigue As discussed in section 10.1, the actual fatigue spectra experienced by many structures and components in service are irregular and not well presented by a constant amplitude sinusoidal waveform. In some instances fatigue tests are performed using input waveforms directly related to the in-service loads, however, in most cases this is not practical and some sort of accelerated test that is still representative of the service loads is required. A popular method of reducing irregular load histories to a manageable variable amplitude fatigue spectra is `rainflow cycle counting' and this, together with other methods, is discussed in other texts.1±3 A number of methods can be used to predict the variable amplitude fatigue life from constant amplitude S-N data. The most common is probably the Palmgren-Miner (P-M) rule,20,21 which for block loading can be defined as: NB

nB X ni iˆ1

Ni

ˆC

10:3

where NB is the number of loading blocks to failure; nB is the number of constant amplitude stages in a block; ni is the number of cycles in a stage with a stress level corresponding to a fatigue life of Ni and C is called the Miner's (or damage) sum. C is equal to 1.0 for 100% damage with no load interaction effects.

Fatigue

225

One of the limitations of the P-M rule is that cycles below the endurance limit do not contribute to the fatigue life. To account for this an extended Miner's rule in which the S-N curve is extended below the endurance limit has been proposed.22 The P-M rule and extended P-M rule have been applied to bonded double-lap joints subjected to the variable amplitude block loading spectrum shown in Fig. 10.11(a).11 The results, in Fig. 10.11(b), show that the Miner's

Figure 10.11 (a) Variable amplitude fatigue spectrum and (b) Miner's sum against maximum load for CFRP-epoxy double-lap joints subjected to this load spectrum (data from ref. 11).

226

Adhesive bonding

sum using both these rules is consistently below 1, which indicates damage acceleration effects. To account for these effects, a strength wearout model has been proposed, which accounts for damage acceleration due to changes in the mean load in the fatigue spectrum.11 This method results in a much improved prediction of fatigue life in bonded joints subjected to variable amplitude fatigue and could also be extended to account for other types of load interaction effect. The weakness of this approach is that no account is taken of where in the life the interactions occur and the damage model parameters are, therefore, only directly applicable to a single sample geometry.

10.3 The fatigue crack growth (FCG) approach 10.3.1 Introduction In this method, fatigue crack growth rate (da/dN) is related to some fracture mechanics parameter. In the case of metals, the stress intensity factor range (
K) is the usual fracture mechanics parameter used. In the case of bonded joints, the strain energy release rate (G) is usually a more easily derived fracture parameter and has hence been used by many researchers.23±30 Both G and K are applicable only if the assumptions of linear elastic fracture mechanics are valid.31 If widespread plasticity is present the J-integral32 may be a more relevant parameter and this has also been applied to bonded joints.18,33,34 This also has its limits and if widespread creep is present then a time dependent fracture mechanics parameters such as C* or C(t) ave may be more applicable.35,36 In testing adhesives and composites, the maximum strain energy release rate, Gmax, is sometimes used in preference to the strain energy release rate amplitude (
G ˆ Gmax ÿ Gmin). This is because with polymeric materials, facial interference on unloading may artificially raise Gmin and therefore reduce the
G value. In the following sections
G will be used as a representative fracture mechanics parameter. A typical plot of log
G against log da/dN for a bonded joint is shown in Fig. 10.12. It can be seen that the fatigue crack growth (FCG) curve has a characteristic sigmoidal shape. Region I of this curve is associated with a fatigue threshold (
Gth), below which measurable crack growth does not occur.
Gth is an important parameter when designing with materials in which fatigue crack growth is to be avoided. In adhesives and composites, the crack propagation rate is often more sensitive to changes in load than in metals and considerable scatter is often seen in experimental FCG plots. This means that it is often desirable to base designs on fatigue thresholds with these materials. In region II, the FCG curve is essentially linear and in many cases the Paris relationship,37 given below, fits the data well. da ˆ C…
G†m dN

10:4

Fatigue

227

Figure 10.12 Fatigue crack growth curve.

where C and m can, within limits, be considered material constants. The value of the exponent, m, indicates the load sensitivity of the crack propagation rate. Region III of the FCG curve signifies unstable crack growth as Gmax approaches the critical strain energy release rate in quasi-static loading, Gc. The ratio
Gth/ Gc indicates fatigue sensitivity. The Paris curve together with
Gth and Gc can be used as a simple representation of the complete FCG curve. Alternatively, a more complex equation can be used to represent the full curve, an example of which is shown below.38   9 8 Gth m1 > > > > 1 ÿ = < da Gmax m   ˆ C…Gmax † m2 > > dN Gmax > > ; :1 ÿ Gc

10:5

where m1 and m2 are additional material constants.

10.3.2 Testing In FCG tests the sample is usually pre-cracked and only the propagation phase is studied. The double cantilever beam (DCB) and tapered double cantilever beam (TDCB) are probably the most popular types of joint for characterising FCG in bonded joints.23±25,30 This type of specimen consists of two cantilever arms, of uniform width, bonded together with a starter crack at one end. The load is

228

Adhesive bonding

applied perpendicularly to the loading arm, resulting in nominally pure mode I (opening mode) loading. It is a useful joint for studying crack propagation as the samples are relatively simple to manufacture and methods of calculating G for these joints are well established. The end notched flexure (ENF) sample has been used to generate mode II FCG data and the cracked lap shear (CLS) sample is a common mixed mode sample.8,14,39±41 Test machines similar to those described in section 10.2 are used in FCG testing but it is now necessary to monitor load, deflection and crack length as a function of cycles. This data must then be analysed to determine the fracture mechanics parameter and crack growth rate as a function of crack length. Testing is usually carried out in either load or displacement control. The latter is more commonly used as this favours a decreasing FCG rate with crack length, which is more useful for characterising the fatigue threshold region.

10.3.3 Fatigue loading effects Mean stress effects can be explored by testing at different R ratios in an analogous way to that described in section 10.2. Figure 10.13 shows the effect of R ratio for CFRP-epoxy DCB joints bonded with an epoxy paste adhesive.42 It can be seen that when Gmax is the chosen fracture parameter the crack growth rate for a given value of Gmax increases when R is reduced. However, when
G is used the curves at different R ratios come closer together. This is because whilst both curves shift to the right when converting from Gmax to
G, the effect increases as R increases. A similar effect has been seen by other investigators.43±45

Figure 10.13 Effect of R-ratio on fatigue crack growth in CFRP-epoxy DCB samples (data from ref. 42).

Fatigue

229

Various modifications to the Paris crack growth law have been proposed to take R-ratio effects into account.46,47 A modified version of the relationship proposed by Forman is given below.2 da C…
K†m …
K ÿ
Kth †0:5 ˆ …1 ÿ R†Kc ÿ
K dN

10:6

The effect of frequency on FCG in bonded joints has been investigated by a number of researchers44,48±50 and in most cases it is seen that either frequency has little effect or as frequency decreases the FCG rate for a given value of
G increases and the fatigue threshold decreases. This is illustrated in Fig. 10.14(a) for CFRP-epoxy DCB samples.51 This trend has been attributed to creep assisted fatigue and in Figure 10.14(b), where the data has been re-plotted in terms of creep crack growth (da/dt) rather than fatigue crack growth (da/dN), it can be seen that crack growth is predominantly time rather than cycle dependent. It should be pointed out that this is not always the case and crack growth may be predominantly cycle dependent under certain conditions and dependent on both cycles and time in others.51,52 A simple method of predicting fatigue crack growth where frequency varies was proposed by Al-Ghamdi et al.51 In this method, several fatigue tests need to be carried out over the range of frequencies seen in service. The FCG curves can then be represented by the Paris equation in region II and Gth and Gc in regions I and II respectively. Hence the complete crack growth behaviour over the range of frequencies is characterised by three material constants, which vary with frequency (the Paris constants m and D and Gth) and one which is assumed frequency independent (Gc). An example of how these parameters vary with frequency for mild steel-epoxy DCB samples is shown in Fig. 10.15. It can be seen

Figure 10.14 Effect of frequency on fatigue crack growth in mild steel-epoxy DCB samples (data from ref. 51).

230

Adhesive bonding

Figure 10.15 Effect of frequency on the Paris constants for steel-epoxy and CFRP-epoxy DCB samples (data from ref. 51).

that all these parameters vary approximately linearly with frequency over the range tested and hence a FCG curve at any frequency in the range can be constructed by interpolation from the known constants. This method was used to predict crack growth in samples tested at a constant 5 Hz frequency and in samples subjected to variable frequency loading. The results, shown in Fig. 10.16, demonstrate that the technique predicts a good approximation of the experimental behaviour.

Figure 10.16 Prediction of fatigue crack growth under variable frequency fatigue using the `empirical crack growth law' method (data from ref. 51).

Fatigue

231

Figure 10.17 Prediction of creep-fatigue crack growth using a damage partition method. Steel-epoxy DCB samples tested at (a) 90 ëC and variable frequency, (b) 90 ëC and 0.1 Hz (data from ref. 52).

In the case of creep-fatigue it has to be questioned whether
G is the most appropriate fracture mechanics parameter or whether a time dependent fracture mechanics parameter such as C*, Ct or (Ct)avg is more appropriate. The most suitable parameter will depend on the size of the creep affected zone at the crack tip (in a similar way that the size of the plastic zone at the crack tip determines

232

Adhesive bonding

Figure 10.18 Crack jump in CFRP-epoxy DCB samples subjected to intermittent overloads (data from ref. 42).

whether an elasto-plastic fracture parameter such as the J-integral is appropriate). This has been investigated for creep-fatigue in metals35 but little work has been published for adhesives. A damage partition method can be used to predict creepfatigue. Crack growth is partitioned into cyclic dependent (fatigue) and time dependent (creep) components and crack growth is predicted by simply adding the two components. In some cases this still underestimates crack growth, in which case an empirical creep-fatigue interaction term may be added. This approach has been used to predict creep-fatigue crack growth in steel-epoxy DCB samples and some of the results from this work are shown in Fig. 10.17.52 The effect of variable amplitude fatigue on crack propagation in bonded joints has received little attention in the published literature, but interesting results have been observed when periodic overloads have been applied to CFRP-epoxy DCB joints tested in fatigue.42 Figure 10.18 shows that if Gmax exceeds a certain value, that a crack jump is seen after a few thousand cycles, after which crack retardation is seen. If Gmax is below this critical level then the crack jump is not observed but a slight crack acceleration effect is observed. This phenomenon has been explained with reference to the formation of a damage zone ahead of the crack tip that affects the rate of crack growth and is dependent on the load history and a simple model has been proposed to predict this effect.7,53

10.3.4 Effect of environment FCG curves for CFRP-epoxy DCB samples tested at different temperatures are shown in Fig. 10.19.54 It can be seen that as temperature increases there is also

Fatigue

233

Figure 10.19 Effect of temperature on fatigue crack propagation in CFRPepoxy DCB samples (data from ref. 54).

an increase in Gth and Gc. This is consistent with the increased ductility of the adhesive as temperature increases in this range, as shown in Fig. 10.7(a). The results from fatigue testing steel-epoxy DCB samples at different temperatures are presented in Table 10.3.52 It can be seen that at 0.1 and 1 Hz, there is a slight increase in the fatigue threshold as the temperature increases from 22 to 90 ëC but this then decreases as the Tg of the adhesive is approached at 120 ëC. At 10 Hz there is a small decrease in Gth as the temperature increases from 22 to 90 ëC and a more significant decrease as the temperature increases further to 120 ëC. The effect of moisture on fatigue crack growth in epoxy-aluminium TDCB samples was investigated by Little and co-workers.17,30 It was seen that moisture had little effect on the FCG curve for samples using chromic acid etched Table 10.3 Effect of temperature and frequency on fatigue thresholds in steel-epoxy DCB samples (data from ref. 52) Fatigue threshold (Gth), J/m2

Test temperature

22 ëC 90 ëC 120 ëC

0.1 Hz

1 Hz

10 Hz

82 86 69

111 120 74

133 124 77

234

Adhesive bonding

aluminium but that resistance to FCG greatly decreased in the presence of moisture for aluminium that had only been grit blasted and degreased prior to bonding. This decrease in fatigue resistance was accompanied by a change in the failure locus from cohesive failure in the adhesive to interfacial failure.

10.3.5 Fatigue life prediction using FCG analysis The great advantage of the FCG approach compared to the S-N approach is that the extent of cracking can be calculated (and also checked experimentally) throughout the fatigue life and that the FCG curve can potentially be used to predict crack propagation and failure in samples of different geometry, providing the test materials, failure locus and failure mechanisms are the same. However, in changing the test geometry the mode mix may change and a mixed mode failure criterion needs to be adopted. The most popular failure criteria are the GI and GT failure criteria. In the former it is assumed that crack propagation is dominated by mode I failure and that crack growth can be calculated based on the mode I component alone in a mixed mode test. In the GT criterion it is assumed that crack growth is determined by the total strain energy release rate (GT). In adhesive joints the mode III component is often insignificant and hence GT ˆ GI ‡ GII. In order to predict fatigue crack growth under constant amplitude loading a fatigue crack growth law must be selected, which may take the following form: da ˆ f …
G; R† 10:7 dN where the effect of environment, frequency, etc., will be included in the material constants for the specific crack growth law used. The life in cycles in the crack growth phase of the fatigue life can then be determined by solving this equation for dN and integrating both sides: Z Nf Z af da 10:8 dN ˆ Nf ÿ Ni ˆ NF ˆ f …
G; R† Ni ai In some cases, closed form solutions to this equation exist but in many cases numerical integration is necessary. Abdel-Wahab et al.55 proposed a generalised numerical procedure using finite element (FE) analysis to solve this equation. This method can be used to predict S-N curves for adhesively bonded lap joints of any geometry if it is assumed that the fatigue life is dominated by the propagation phase. Figure 10.20 shows the results of using this method to predict S-N curves for single and double-lap joints using FCG data from DCB tests and both the GT and GI failure criteria.56 An alternative approach is to use Gth from the FCG curve to predict the fatigue limit for a lap joint, assuming an initial flaw size. This approach has been investigated by using the data from a mode I FCG test to predict the fatigue

Fatigue

235

Figure 10.20 Prediction of S-N curves for CFRP-epoxy single lap joints using FCG data from CFRP-epoxy DCB samples (data from ref. 56).

limits in double-lap joints and lap-strap joints tested at different temperatures.54 The results are shown in Table 10.4. It can be seen that reasonably good predictions are made, except when creep dominates failure in the double lap joints at high temperature.

10.4 Summary and future trends Two main approaches have been used to investigate the fatigue of adhesively bonded joints. The S-N curve can be used to predict the number of cycles to failure but tells us little about the evolution and progression of damage. This approach can be used in a safe-life design method. The other main approach is Table 10.4 Prediction of fatigue limits in lap joints using FCG data from DCB samples (data from ref. 54) Temp.

ÿ50 ëC 22 ëC 90 ëC

Lap-strap joint

Double-lap joint

Expt.

Predicted

Error

Expt.

14 kN 15 kN 14 kN

8 kN 10 kN 11 kN

43% 33% 21%

10 kN 10 kN 3.3 kN

Predicted

Error

10 kN 12.5 kN 13 kN

0% 25% 294%

236

Adhesive bonding

based on the assumption of flaws in the joints and analysis of the rate of flaw propagation. Both these methods have shown that the fatigue of bonded joints is dependent on many parameters including R ratio, frequency, test geometry and test environment. Methods of predicting the fatigue life of bonded joints based on both these approaches have been suggested. The FCG approach is potentially the most useful as this can be transferred to different sample geometries more easily and can be used to predict the evolution of fatigue damage. However, this method still has limitations and more work is needed to develop a generally applicable and physically realistic model of fatigue failure in bonded joints on which to base an improved predictive method. This will not be easy as the mechanisms of failure in a bonded joint depend strongly on such parameters as loading conditions, test environment, adhesive chemistry and the nature of the inter-phase. In addition, the characterisation of damage evolution in bonded joints is far from simple. It is the author's opinion that future work should be aimed at gaining a better understanding of the failure mechanisms in fatigue and more clearly differentiating between the initiation and propagation phases of fatigue damage accumulation. The effects of environment and creep assisted fatigue and the effects of complex fatigue loading spectra have been little investigated to date and are worthy of further work also. The limitations of the strength of materials and LEFM methods currently used in lifetime prediction should be more accurately defined and alternative approaches such as elasto-plastic fracture mechanics, time dependent fracture mechanics and continuum damage mechanics could be developed as supplements or replacements of the existing methods for certain definable conditions.

10.5 Further information As has been stated previously, the analysis of fatigue has a longer life and is more developed for metal structures than it is for polymers or adhesive joints, hence good sources of information on fatigue are textbooks and journals concerned primarily with fatigue in metals. The author has found the following books useful on the general subject of fatigue.1±3,35,36 Academic journals dedicated to fatigue research are International Journal of Fatigue (Elsevier) and Fatigue and Fracture of Engineering Materials and Structures (Blackwell). Hertzberg and Manson's Fatigue of Engineering Plastics57 is a useful book to those investigating fatigue in polymeric adhesives and serves as an excellent introduction to the subject. Useful fatigue articles can also be found in journals dedicated to adhesives, polymers or polymer composite research such as International Journal of Adhesion and Adhesives (Elsevier), Composites Part A (Elsevier) and Polymer Testing (Elsevier). Useful information on fatigue testing and data analysis can be found in ASTM, BSI and ISO standards.

Fatigue

237

A number of professional organisations are involved in the area of fatigue and numerous international conferences deal with the subject. The web sites of the Engineering Integrity Society (www.e-i-s.org.uk) and European Structural Integrity Society (www.esis.org) are useful sources of further information.

10.6 References 1. Dowling N E, Mechanical behaviour of materials: engineering methods for deformation, fracture and fatigue, Upper Saddle River, NJ, Prentice Hall, 1998. 2. Suresh S, Fatigue of Materials, Cambridge, Cambridge University Press, 1998. 3. Schijve J, Fatigue of Structures and Materials, London, Kluwer Academic, 2001. 4. Zhang Z, Shang J K and Lawrence F V, `A backface strain technique for detecting fatigue crack initiation in adhesive joints', J Adhesion, 1995 49 23±36. 5. Crocombe A D, Ong A D, Chan C Y, Abdel-Wahab M M and Ashcroft I A, `Investigating fatigue damage evolution in adhesively bonded structures using backface strain measurement', J Adhesion, 2002 78 745±778. 6. Ashcroft I A, Erpolat S and Tyrer J, `Damage assessment in bonded joints', Key Engineering Materials, 2003 245 501±508. 7. Ashcroft I A, `A simple model to predict crack growth in bonded joints and laminates under variable amplitude fatigue', J Strain Anal, 2004, 39 707±716. 8. Ashcroft I A, Gilmore R B and Shaw S J, `Cyclic fatigue and environmental effects with adhesively bonded joints', AGARD Conference Proceedings 590, Bolted/ Bonded Joints in Polymeric Composites, NATO, 1997, 14.1±14.9. 9. Crocombe A D and Richardson G, `Assessing stress state and mean load effects on the fatigue response of adhesively bonded joints', Int J Adhesion and Adhesives, 1999 19 19±27. 10. Jethwa J K, The Fatigue Performance of Adhesively-Bonded Metal Joints, PhD Thesis, London, Imperial College of Science, Technology and Medicine, 1995. 11. Erpolat S, Ashcroft I A, Crocombe A D and Abdel-Wahab M M, `A study of adhesively bonded joints subjected to constant and variable amplitude fatigue', Int J Fatigue, 2004 26 1189±1196. 12. Hart-Smith L J in Developments in Adhesives ± 2, London, Applied Science Publishers, 1981, 1±44. 13. Romanko J and Knauss W G in Developments in Adhesives ± 2, London, Applied Science Publishers, 1981, 173±205. 14. Ashcroft I A, Abdel Wahab M M, Crocombe A D, Hughes D J and Shaw S J, `The effect of environment on the fatigue of bonded composite joints. Part 1: Testing and fractography', Composites: Part A, 2001 32 45±58. 15. Ashcroft I A, Abdel-Wahab M M, Crocombe A D, Hughes D J and Shaw S J, `Effect of temperature on the quasi-static strength and fatigue resistance of bonded composite double lap joints', J Adhesion 2001 75 61±88. 16. Abdel-Wahab M M, Ashcroft I A, Crocombe A D and Shaw S J, `Diffusion of moisture in adhesively bonded joints', J Adhesion, 2001 77 43±80. 17. Little M S G, Durability of Structural Adhesive Joints, PhD Thesis, London, Imperial College of Science, Technology and Medicine, 1999. 18. Abdel Wahab M M, Ashcroft I A, Crocombe A D, Hughes D J and Shaw S J, `The effect of environment on the fatigue of bonded composite joints. Part 2: fatigue

238

Adhesive bonding

threshold prediction', Composites: Part A, 2001 32 59±69. 19. Abdel-Wahab M M, Ashcroft I A, Crocombe A D, Hughes D J, Shaw S J, `Prediction of fatigue threshold in adhesively bonded joints using damage mechanics and fracture mechanics', J Adhesion Sci and Tech, 2001 15 763±782. 20. Palmgren A, `Durability of ball bearings', Zeitschrift des Vereins Deutscher Ingenieure, 1924 68 339±41. 21. Miner M A, `Cumulative damage in fatigue', J Appl Mech, 1945 12 159±64. 22. Schutz W and Heuler P in Advances in Fatigue Science and Technology, London, Kluwer Academic, 1989, 177±219. 23. Mostovoy S, Crosley P B and Ripling E J, `Use of crack-line-loaded specimens for measuring plane strain fracture toughness', J Materials, 1967 2 661±681. 24. Kinloch A J and Shaw S J, `The fracture resistance of a toughened epoxy adhesive', J Adhesion, 1981 12 59±77. 25. Mall S and Johnson W S, Characterization of Mode I and Mixed-Mode Failure of Adhesive Bonds Between Composite Adherends, NASA Technical Memorandum 86355, Hampton, NASA, 1985. 26. Fernlund G and Spelt J K, `Failure load prediction of structural adhesive joints. Part 1: analytical method', Int Adhesion and Adhesives, 1991 11 213±220. 27. Lin C and Liechti K M, `Similarity concepts in the fatigue fracture of adhesively bonded joints', J Adhesion, 1987 21 1±24. 28. Ashcroft I A, Hughes D J and Shaw S J, `Mode I Fracture of Epoxy Bonded Composite Joints, Part 1: Quasi-static loading', Int J Adhesion and Adhesives, 2001 21 87±99. 29. Erpolat S, Ashcroft I A, Crocombe A and Abdel-Wahab M, `On the analytical determination of strain energy release rate in bonded DCB joints', Eng Fract Mech, 2004 71 1393±1401. 30. Hadavinia H, Kinloch A J, Little M S G and Taylor A C, `The prediction of crack growth in bonded joints under cyclic-fatigue loading II. Analytical and finite element studies', Int J Adhesion and Adhesives, 2003 23 463±471. 31. Miannay D P, Fracture Mechanics, New York, Springer, 1998. 32. Rice, J R, `A path-independent integral and the approximate analysis of strain concentration by notches and cracks', J Appl Mech, 1968 35 379±386. 33. Fernlund G and Spelt J K, `Analytical method for calculating adhesive joint fracture parameters', Eng Fract Mech, 1991 40 119±132. 34. Ashcroft I A, Abdel-Wahab M M and Crocombe A D, `Predicting degradation in bonded composite joints using a semi-coupled FEA method', Mech Adv Matls & Structs, 2003 10 227±248. 35. Saxena A, Nonlinear Fracture Mechanics for Engineers, London, CRC Press, 1998. 36. Miannay D P, Time-Dependent Fracture Mechanics, New York, Springer, 2001. 37. Paris P C and Erdogan F, `A critical analysis of crack propagation laws', Trans ASME D, 1963 85 528±535. 38. Ewalds H L, Fracture Mechanics, London, Edward Arnold, 1984. 39. Brussat T R, Chiu S T and Mostovoy S, Fracture mechanics for structural adhesive bonds, Air Force Materials Laboratory Technical Report 77-163, Ohio, 1977. 40. Romanko J, Liechti K M and Knauss W G, Integrated Methodology for Adhesive Bond Joint Life Predictions, AFWAL-TR-82-4139, Air Force Wright Aeronautical Laboratories, Ohio, 1982. 41. Johnson W S, `Stress analysis of the cracked-lap-shear specimen: an ASTM round-

Fatigue

239

robin', J Test Eval, 1987 15 303±324. 42. Erpolat S, Ashcroft I A, Crocombe A D and Abdel-Wahab M M, Fatigue crack growth acceleration due to intermittent overstressing in adhesively bonded CFRP joints, Comp. A. 2003 35 1175±1183. 43. Mall S, Ramamurthy G and Rezaizdeh M A, `Stress ratio effect on cyclic debonding in adhesively bonded composite joints', Composite Structures, 1987 8 31±45. 44. Pirondi A and Nicoletto G, `Fatigue crack growth in bonded DCB specimens', Eng Fract Mech, 2004 71 859±871. 45. Knox E M, Tan K T, Cowling M J and Hashim S A, `The fatigue performance of adhesively bonded thick adherend steel joints', European Adhesion Conference (EURADH 96), Cambridge, UK. , 1996, vol. 1, 319±324. 46. Forman R G, Kearney V E and Engle R M, `Numerical analysis of crack propagation in cyclic-loaded structures', J Bas Engng, 1967 89 459±64. 47. Walker K, `The effect of stress ratio during crack propagation and fatigue for 2024T3 and 7075-T6 aluminium', STP 462: Effects of Environment and Complex Load History for Fatigue Life, ASTM, Philadelphia, 1970, 1±14. 48. Luckyram J and Vardy A E, `Fatigue performance of two structural adhesives', J. Adhesion, 1988 26 273±291. 49. Xu X X, Crocombe A D and Smith P A, `Fatigue crack growth rates in adhesive joints tested at different frequencies', J Adhesion, 1996 58 191±204. 50. Joseph R, Bell J P, MvEvily A J and Liang J L, `Fatigue crack growth in epoxy/ aluminium and epoxy/steel joints', J Adhesion, 1993 41 169±187. 51. Al-Ghamdi A H, Ashcroft I A, Crocombe A D and Abdel-Wahab M M, `Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading', J. Adhesion 2003 79 1161±1182. 52. Al-Ghamdi A H, Fatigue and creep of adhesively bonded joints, PhD Thesis, Loughborough, Loughborough University, 2004. 53. Ashcroft I A, `Fatigue of adhesively bonded joints', Proc. 27th Annual Meeting of the Adhesion Society, Wilmington, N.C., The Adhesion Society, 2004, 416±418. 54. Ashcroft I A and Shaw S J, `Mode I Fracture of Epoxy Bonded Composite Joints, Part 2: Fatigue Loading', Int J Adhesion and Adhesives, 2002 22 151±167. 55. Abdel-Wahab M M, Ashcroft I A, Crocombe A D and Smith PA, `Numerical prediction of fatigue crack propagation lifetime in adhesively bonded structures', Int J Fatigue, 2001 24 705±709. 56. Abdel-Wahab M M, Ashcroft I A, Crocombe A D and Smith P A, `Finite element prediction of fatigue crack propagation lifetime in composite bonded joints', Composites Part A 2004 35 213±222. 57. Hertzberg R W and Manson J A, Fatigue of Engineering Plastics, New York, Academic Press, 1980.

11

Vibration damping M HILDEBRAND

11.1 Introduction Vibration damping or structural damping has become a more and more important property in many applications. For example, passenger comfort in many vehicles can be enhanced by increasing vibration damping. Also, fatigue life of machinery can be extended by increasing the vibration damping of certain critical components. Vibration damping can be a key issue in high-precision high-speed machinery where vibrations have to be kept at a minimum level. A suitable level of vibration damping is of high importance in many sporting goods, such as tennis rackets, skis or golf clubs. Vibration damping is an essential part of the dynamic behaviour of structures. This chapter includes some aspects about damping in general and how adhesively bonded joints can specifically be utilised to increase vibration damping of the structure. The term `damping' as used here refers to the energy dissipation of materials or structures under cyclic stress but excludes energytransfer devices such as dynamic absorbers. With this definition, energy must be dissipated within the vibrating system. The energy of the vibration is dissipated into a non-recoverable form of energy, in most cases into heat. As in many other structures, the damping of adhesively bonded products is composed of material damping and damping due to mechanical construction, which includes the adhesive joint. A possible domination of the damping due to mechanical construction depends on the complexity of the structure and on the type, number and the stress-state of the joints. There are a large number of mechanisms by which vibrational energy can be dissipated within the volume of a material element. These mechanisms are usually associated with internal reconstructions of the micro and macro structure, ranging from crystal lattice to molecular scale effects. The majority of published information on material damping is of empirical nature and hence, the underlying physical effects are not always fully understood. The mechanisms of energy dissipation at structural joints and discontinuities are complex. Friction is normally involved in most mechanical joints (such as in

Vibration damping

241

bolted and riveted joints), but is usually less important in adhesively bonded joints or combined joints (for instance bolted-bonded joints), unless in the case of substantial voids in the adhesive layer. The stress concentrations typically near the joint ends and at the joint interfaces lead also to an increase in damping in many cases. For analysis and testing purposes, several mathematical models are used to represent damping. It should be noted that these models do not necessarily imply a particular mechanism for energy dissipation: · viscous damping (damping force is proportional to the velocity) · hysteretic damping (damping force is proportional to the displacement) · Coulomb damping (damping force is constant). Various tests are used to quantify the damping of materials and structures and several measures are frequently used for damping: 
 Q dW/W tan

specific damping capacity loss factor logarithmic decrement damping ratio Q-factor ratio of dissipated and stored energy tangent of the phase angle.

Additional information about the corresponding test methods and definitions of these measures can be found in the literature (Nashif et al., 1985). The relationship between these measures is expressed in Equation 11.1, which is valid for harmonic vibration at resonance (Rawal et al., 1986).  ˆ =2 ˆ dW=2W ˆ 1=Q ˆ 2 ˆ
= ˆ tan

11:1

Damping is the most weight efficient solution for resonance issues related to noise, vibration, or service life concerns. This is particularly true when the intrinsic structural damping is low and vibratory excitation energy is high. When comparing the different damping material values or test results it is important to take into account the possible differences in stress level and stress distribution of the tested specimens as these have an influence on the quantity of damping for most of the materials. The damping properties of various structural materials, such as metals, polymers, ceramics and their composites have been reviewed recently by Chung (2001). However, adhesives have not been treated separately in this review.

11.2 Damping in joints The damping due to mechanical construction (e.g. joints and discontinuities) in many complex metal structures is dominant compared to the usually very low

242

Adhesive bonding

damping of the metallic material itself. Local and discontinuous joints (spot welds, short intermittent welds, rivets, bolts, screws, adhesively bonded joints) can effectively contribute to the damping behaviour of the whole structure. Friction, which dissipates energy during the vibration of a structure, is always present in mechanical joints. The vibrational damping occurs when small relative movements take place between the joint interfaces. Using additionally an adhesive in a bolted joint can increase the stiffness of the joint but can lessen vibration damping compared with the pure bolted joint. The information available on the damping behaviour of structural joints is rather limited and usually not sufficient for optimising damping at the design stage of the product. There are relatively few studies that compare the damping behaviour of alternative joining methods. However, adhesive bonding is known to provide attractive solutions for achieving joints with high damping (Prucz 1987; Srivatsan et al., 1988, 1989). Tough structural adhesives (e.g. modified epoxies and polyurethanes) have excellent strength and good damping properties due to their visco-elastic behaviour. Due to the shear and peeling stress peaks normally present at the edges of the adhesive bond line, local stresses are high, thus emphasising the damping properties of the adhesive joint, because the damping of many adhesive materials increases with increasing stress level. This naturally also depends on the geometry and loading components of the joint. Srivatsan et al. (1989) measured the damping of adhesively bonded steel-steel double-lap joints in flexural vibration at frequencies of around 250 Hz. A plasticised epoxy resin was used as adhesive. For joints without defects they achieved specific damping capacities ( ) of 16%. Introducing defects (partly debonded joint area) the specific damping capacity increased significantly but, naturally, the strength of the joints decreased considerably. Scott and Orabi (2000) investigated strain-effects on damping and other elastic properties of co-axial tube to rod joints. Two different epoxy adhesives were compared and both show a clear increase in damping ratio with increasing strain. There is hardly any literature about the damping behaviour of adhesively bonded joints between dissimilar materials. Nevertheless, the damping effects arising in adhesively bonded joints are fairly similar regardless of the adherends being of similar or dissimilar materials. In the case of adherends with high damping properties (such as certain fibre-reinforced composites materials), adhesive joints can be used to broaden the frequency range at which the damping is high. The presence of joints is essential in multi-material products. Consequently, adhesively bonded joints offer a major potential for passive vibration control in multi-material products.

11.3 Prediction methods of vibration damping Although vibration damping has been the subject of many recent studies, it remains a relatively poorly understood phenomenon. The confidence of any

Vibration damping

243

analytical prediction method lies far below comparable structural analyses (Spence and Kenchington, 1993). Theoretically the finite element method (FEM) allows us to analyse the manifold damping features of structures as it is possible to include damping in the analysis in several ways. Damping effects can be introduced by using discrete damping elements, by introducing modal damping or through the material model (for instance, using a visco-elastic material model). One method commonly used to assess the damping of structures is the modal strain energy (MSE) approach, which utilises a finite element analysis representation of a structure as the basis for modelling the damping effect. This method has been shown to be an accurate predictor of damping levels in structures comprising layers of elastic and visco-elastic elements (Johnson et al., 1981; Nashif, 1983). The MSE principle states that the ratio of system loss factor to the material loss factor for a given mode of vibration can be estimated from the ratio of elastic strain energy in the visco-elastic elements to the total strain energy in the model for a given mode (Nashif et al., 1985). Typically, the MSE approach is used in conjunction with an undamped, normal-mode analysis to compute the strain-energy ratio. The strain energies are determined from the relative mode shapes. It is assumed that the visco-elastic properties are linear in terms of the dynamic strain rate. This, however, is not necessarily the case with many structural adhesives. Explicit finite element codes also offer possibilities to assess damping directly in the time domain. In terms of analysis cost it is, however, still a rather costly and time-consuming way compared with the type of conventional structural dynamic analysis, which is today common practice in the industry. In practice, although some finite element codes allow the inclusion of damping in several ways into the model, there are many obstacles to a successful, efficient and reliable structural damping analysis, specially if the analysis is required to be not only specially dedicated to vibration damping, but also to dynamic behaviour in general. The main key to such vibration damping analysis efforts is the proper understanding of visco-elastic material behaviour and an accurate characterisation of the dynamic properties of the adhesive materials used in these joints. In general, there is a substantial shortage of material input data that would be accurate enough for the damping models used in these analysis methods. At present, damping materials developed specifically for damping layers are the only group of materials for which sufficient input values have been generated experimentally, i.e., damping has been measured usually as a function of both frequency and temperature. Some of these damping materials are even used like an adhesive, for instance, bonded between two sheets of metal to form a sandwich sheet with high vibration damping capacity and high flexural stiffness. However, equivalent data of the same accuracy is rarely available for common structural adhesives. Typically, the damping characteristics of adhesive

244

Adhesive bonding

materials are both frequency and temperature dependent and therefore a thorough material characterisation is needed to achieve analysis input data broad enough for various joint types and applications. Much of the damping in a larger structure is often due to joints and structural discontinuities. Thus, to achieve confidence in the analysis, these regions should be modelled very accurately. In case of finite-element analyses, this means in practice that a considerably finer mesh than normally used in a structural analysis should be used in the joint areas. Provided that the damping behaviour of the joints and discontinuities is well understood, it is also possible in certain cases to use discrete spring and damper elements to model the damping behaviour of these regions. In brief, a reliable analysis of structural damping requires not only a highly evolved software but also a better understanding of the damping properties of adhesive materials and joints, a very accurately meshed structural model and in most cases a non-linear analysis method. Experimental validation will be necessary in most of the cases where damping plays an essential part and might involve testing efforts on material, joint and structural levels.

11.4 Experimental data on vibration damping of adhesively bonded joints A rather extended experimental series on vibration damping of adhesively bonded single-lap joints with various adhesives has been performed by Hildebrand and Vessonen (1998). The experimental series included vibrationdamping experiments both on joints and adhesives. Additionally, some structural properties of both joints and adhesive materials were determined. The data illustrate typical levels of vibration damping, which can be achieved by adhesively bonded structural joints. It also shows the large differences in damping due to different adhesives. Finally, the data can be used to compare, verify and further develop methods for assessing structural vibration damping in adhesively bonded joints. Adhesively bonded single lap joints have been manufactured with various adhesives. For comparison some bolted, bolted/bonded and welded alternatives with similar geometries are also included. The adherends are of steel (AISI304) and before bonding, the surfaces of the adherends were cleaned with acetone, grit-blasted (aluminium oxide) and cleaned again. Then, the specimens were adhesively bonded. The overlap length was 50 mm, resulting in a total length of the joint specimen of 400 mm. The thickness of the adhesive layer was controlled during the manufacturing of the joints, resulting in bond line thickness of 0.2, 0.5 or 2.5 mm. Several adhesive materials were used, as shown in Table 11.1. They represent a broad range of structural adhesives used in various industries and applications. The following experiments were performed.

Vibration damping

245

Table 11.1 Adhesive materials used in the different specimens (Hildebrand and Vessonen, 1998). The adhesives A±I are representative for a broad range of typical structural adhesives, adhesive K is a less structural hot-melt adhesive which has been included for comparison Specimen

Adhesive type

A B C D E F G H I K

2K polyurethane 2K epoxy 2K epoxy 2K epoxy 2K epoxy 2K polyurethane 2K polyurethane 1K polyurethane 1K polyurethane Hot melt

Henkel Makroplast 8202 + 5430 Eurepox 710 + 140 3M DP 110 Ciba AV 138 3M DP 460 Teopur 4012 Kiilto Kestopur PL 240 Sikaflex 360 HC (0.5 mm thickness) Sikaflex 360 HC (2.5 mm thickness) Hot Melt Bostik 9951

Lap joint modal test The experiments were performed as follows: two additional steel masses (each 0.98 kg) were attached to both ends of the specimens in order to decrease the eigenfrequencies of the specimens. The size of the steel masses was 80  40  40 mm. During the modal testing the specimens were allowed to hang freely supported by one flexible rope attached to the steel mass on the other end of the specimen. The test was carried out by applying the impact hammer method, in which the structure is excited into vibration by using a proper instrument hammer. Frequency response functions (FRF) in the form of acceleration/excitation force (m/s2/N) for the same seven response locations on each test specimen were measured using an 8-channel spectrum analyser. Seven accelerometers (each weighing about 2.5 g) were used to measure vibration responses. A force transducer was used to measure the impact force. Figure 11.1 illustrates the test arrangement. Measured frequency response functions were analysed using a modal analysis program to get the natural frequency and damping data for the first bending and torsional mode of each test specimen. Lap joint tensile test The lap joint tensile test was performed on a universal testing-machine at a speed of 2 mm/min at room temperature. As a result, the mean shear strength of the joint was obtained. The mean shear strength is a rough indication of the joint strength level. Adhesive tensile strength The tests were performed according to the standards ISO/DIS 527 at a loading rate of 2 mm/min. As a result, tangent modulus, maximum strength and

246

Adhesive bonding

Figure 11.1 Arrangement of the modal test (Hildebrand and Vessonen, 1998).

elongation at break were obtained. Additionally, the stress-strain curves are also shown. Adhesive DMTA The dynamic mechanical thermal analysis (DMTA) has been made in the torsional mode. Modulus and loss factor are obtained as a result over a temperature range between ÿ20 and 100 ëC (between ÿ80 and 100 ëC for adhesive K). The testing frequency was 10 Hz. This data can be used to evaluate the effect of temperature on vibration damping of the joint. Additionally, the DMTA test results can be used to determine the temperature resistance of the adhesive. The experimental results of the tests are summarised in Table 11.2. Additionally, the DMTA and adhesive tensile test results are shown in Figs 11.2, 11.3 and

Table 11.2 Summarised test results. All values at 20 ëC. The joint modal, joint tensile test and adhesive tensile test results are mean values of three specimens. Damping values are given in percentage of critical damping (c/ccr) (Hildebrand and Vessonen, 1998) Joint

Modal test

Adhesive Damping layer (torsion) thickness (mm) (%) A B C D E F G H I K

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.5 2.5 0.2

0.112 0.139 0.144 0.088 0.114 0.311 0.345 0.149 0.119 0.669

Frequency Damping (torsion) (flexure)

Joint tensile test Frequency (flexure)

(Hz)

(%)

(Hz)

Mean shear strength (MPa)

53.9 53.8 54.5 54.0 54.2 53.9 53.7 50.6 51.0 52.0

0.262 0.179 0.173 0.151 0.133 0.516 0.466 0.209 0.215 0.73

33.7 33.6 34.7 34.1 34.1 34.0 33.5 30.8 31.7 32.2

13.1 11.7 14.3 12.2 15.0 10.3 9.94 6.20 8.11 3.44

Adhesive tensile test

DMTA

Tensile strength

Tensile modulus

Elongation Adhesive at break damping (tan ) (%) (ÿ)

(MPa)

(MPa)

13.9 36.4 26.8 24.5 38.1 8.6

341 2 570 1 470 3 940 2 790 197

50.9 1.5 11.5 0.8 3.9 21.7

7.6 7.6

16 16

296 296

0.085 0.013 0.060 0.014 0.013 0.066 0.096 0.202 0.202

Adhesive torsional modulus (MPa) 1 079 1 092 921 1 654 941 1 202 1 264 5.01 5.01

248

Adhesive bonding

Figure 11.2 Shear moduli of the adhesives as a function of temperature as measured in the DMTA in torsion. Testing frequency is 10 Hz (Hildebrand and Vessonen, 1998).

Figure 11.3 Damping of the adhesives as a function of temperature as measured in the DMTA in torsion. Testing frequency is 10 Hz (Hildebrand and Vessonen, 1998).

Vibration damping

249

Figure 11.4 Tensile stress-strain curves of the adhesives. The speed of testing is 2 mm/min (Hildebrand and Vessonen, 1998).

11.4. The damping of the steel base material, which was tested for reference, with a 400 mm long specimen without a joint is 0.048% and 0.095% in torsion and flexure, respectively. Hence, the vibration-damping level of the specimens with adhesive joints is up to ten times higher than that of the corresponding structure without joint. The experimental data provided in this chapter is intended to serve analytical or numerical efforts for predicting structural vibration damping in structures with adhesively bonded joints. It is interesting to note that the modal damping of the joint does not clearly correlate with the adhesive material damping capacity. This is shown in Fig. 11.5. There are also other factors which determine the damping capacity of the joints, such as joint geometry and mechanical properties of the joint. It is important to note that there is usually a trade-off between damping and structural characteristics of adhesives. The adhesives with the highest damping properties usually have limited structural properties. This makes structural design and optimisation a challenge, since limited joint strength is not usually allowable with adhesively bonded joints. An attempt to create a structural joint between metal sheet and composite sandwich panels dedicated to high vibration damping has been made by (Hentinen et al., 1997). Compared to corresponding purely structural joint types, a higher degree of flexibility was introduced to achieve higher damping, in combination with an adhesive material with suitable visco-elastic properties

Figure 11.5 Comparison between torsional damping of the joints and the corresponding adhesive material (Hildebrand and Vessonen, 1998).

Vibration damping

251

Figure 11.6 Three types of structural joints between metal and composite sandwich panels. The joints to the left and to the right are purely structural, the joint type in the middle is dedicated to high vibration damping in combination with relatively high structural properties (Hentinen et al., 1997).

(Fig. 11.6). In addition to the increased flexibility, one potential drawback of the concept is that creep has to be considered if high long-term static loading is expected, particularly if it occurs in combination with elevated temperatures. The same visco-elastic properties that are responsible for the high vibration damping, on the other hand lead to an increasing risk of creep under high longterm static loads.

11.5 Future trends Adhesively bonded joints offer an interesting potential to considerably increase the level of structural vibration damping without increasing structural weight. In many applications today, vibration damping is taken as an additional benefit of adhesively bonded joints. The importance of vibration damping will further increase in many future applications. The quest for faster machines, lighter structures, more comfortable vehicles, higher precision, etc., leads to the need for even greater control of dynamic properties. Whereas nowadays, vibration damping is still in many cases a property classified as `nice to have', it will be an exactly specified property in many future applications. This implies that vibration damping will play an increasing role in the analysis and design phase of many applications. Not only analysis and design tools have to be further developed, but also much new material data has to be established in order to fully exploit the potential of adhesively bonded joints.

252

Adhesive bonding

`Design for damping' and in this context, `adhesively bonded joint design for damping' will become a new aspect in the structural design process. It is not enough that the adhesive joints be designed to have high vibration damping, it is even more important to locate the joints in suitable areas where they can contribute in an optimal way to energy dissipation without compromising other structural properties, such as strength or stiffness. To find a suitable balance between strength, flexibility and vibration damping by utilising adhesively bonded joints will be a new challenge to many engineers. Adhesively bonded joints are a good alternative in multi-material structures compared to other joining methods. Multi-material structures also offer other benefits in respect to vibration damping. For instance, with fibre-reinforced composite materials, which have substantially higher damping properties than most metals, damping properties can be quite easily adjusted to requirements by changing fibre orientations and laminate lay-up of the structures. In combination with adhesively bonded joints the possibilities to achieve control of vibration damping behaviour are further increased. In addition to the above-mentioned means of passive vibration control, it is highly probable that active vibration control will find its way into adhesive joints for industrial applications. Active methods make use of sensors and actuators to achieve vibration sensing and activation to suppress vibration in real time. Active, more-or-less discrete structural members are already in use today to control vibrations and act as dampers in some industrial applications. For instance, shape memory alloys have been incorporated into joints between pipes and their couplings in laboratory scale (Li and Dunne, 2000) with some promising results. Adhesively bonded joints, being usually limited in size compared to the whole structure, offer a good alternative for integrating such active elements. Taking into account the fast ongoing development of new active materials, it is probable that active properties will be incorporated into adhesive joints in the near future.

11.6 References Chung D D L (2001), `Review ± Materials for vibration damping'. Journal of Material Science, 36, 5733±5737. Hentinen M, Hildebrand M and Visuri M (1997), `Adhesively bonded joints between FRP sandwich and metal. Different concepts and their strength behaviour'. VTT Research Notes 1862, Espoo, Technical Research Centre of Finland. Hildebrand M and Vessonen I (1998), `Experimental data on damping of adhesively bonded single-lap joints', in Workshop on modelling of sandwich structures and adhesively bonded joints. Porto. IDMEC. Johnson C D, Kienholtz D A and Rogers L C (1981), `Finite Element Prediction of Damping in Beams with Constrained Viscoelastic Layers', Shock Vibr. Bul., 51(1), 71±81.

Vibration damping

253

Li H and Dunne D P (2000), Final Report on Shape Memory Alloy Couplings for Joining Pipe and Tube, CRC Australia. Nashif A D (1983), `Control of Noise and Vibration with Damping Materials', Sound and Vibration Magazine, July, 28±36. Nashif A D, Jones D I G and Henderson J P (1985), Vibration Damping. New York, John Wiley & Sons. Prucz J C (1987), `Advanced joining concepts for passive vibration control', in 58th Shock and Vibration Symposium, Oct. 13±15, 1987, Huntsville, AL. Washington, DC. NASA Conf. Publication 2488, pp. 459±471. Rawal S P, Misra M S and Rath B B (1986), `Introduction: Material damping ± how to define it', Proc. Roles of Interfaces on Material Damping, ed. B B Rath and M S Misra, Toronto, 13±17. Metals Park, Ohio. ASM 1986, pp. 1±3. Scott J E and Orabi I I (2000), `Prediction and measurement of joint damping in scaled model space structures', in Fourteenth Engineering Materials Conference, Austin, Texas. Spence P W and Kenchington C J (1993), `The role of damping in finite element analysis', NAFEMS Finite Element Methods & Standards Report R0021. Srivatsan T S, Place T A, Mantena R, Gibson R F and Sudarshan T S (1988), `The influence of processing variables and defects on the performance of adhesively bonded joints', in Proceedings of the 2nd Int. SAMPE Metals Conference, Aug. 2±4, Dayton, Ohio, Vol. 2, pp. 368±380. Srivatsan T S, Mantena R, Gibson R F, Place T A and Sudarshan T S (1989), `Electromagnetic measurement of damping capacity to detect damage in adhesively bonded material', Materials Evaluation, 47, 564±570.

Part III Applications

12

Joining similar and dissimilar materials E J C KELLAR

12.1 Introduction The versatility of adhesives to join almost any material combination is one of the largest benefits of this technology. The advantages that adhesives bestow upon a joint are many and include: · joints with a uniform stress distribution in contrast to the localised approach of mechanical fastening and the inherent thermal stresses from welding processes · the ability to form an aesthetically pleasing structure with the absence of holes or additional forming of the substrates · the ability to form a complete seal over the entire joint area · the ability to form a joint with controllable properties as a function of the adherends, the adhesive and the geometry of the joint · the ability to join almost any material irrespective of type, shape or form. Although there are often significant advantages in using fusion techniques, i.e., welding, where the two substrates in question are similar or compatible, the flexibility of the adhesive option brings other benefits. These include the ability to avoid `hot' processes and associated distortion effects, the incorporation of novel or complex joint designs and the ability to control physical properties of the joint. When one then considers joining of dissimilar materials, adhesives become the primary joining option alongside mechanical fastening. However, there are distinct differences between joining similar and dissimilar materials using adhesives that one must be aware of in order to maximise the chances of success. This chapter endeavours to clarify those similarities and differences and to enable users to benefit from such a versatile joining process. Areas of specific relevance include surface quality and the effects of thermal expansion where either the size of the joints is substantial, e.g., external panels for buildings, vehicles or aircraft or there is a specific effect on the function of the structure through the build up of expansion stresses and subsequent creep effects. There is a need to design the joints to benefit from adhesive properties and to take into consideration the vast

258

Adhesive bonding

array of adhesives to choose from and the types of surface pre-treatment required to obtain the desired bond. In addition an overview of tooling and assembly aids are discussed in the context of similar and mixed material systems.

12.2 Joint design 12.2.1 Overview The fundamental requirement of any joint between two or more components is to carry load effectively and in many cases this function should be invisible, i.e., the joint is not seen by the overall structure. Only when the joint is designed to have additional function should it have any measurable effect. The reality is, however, that in the majority of situations the adhesive will consist of a material that is not the same as the adherends it is joining, the chemical and mechanical properties will differ and the resulting joint properties will be influenced by that combination. The situation is further complicated if the adherends being joined differ significantly from each other so that the adhesive joint has to function as a transition point and ideally not provide a locus for failure (unless designed specifically to do so). Careful joint design is therefore required to take these factors into account to maximise the benefits offered through the use of adhesive technology and to avoid premature failure. When considering the bonding of metallic, ceramic or continuous fibre adherends, the material properties such as modulus and strength of almost all adhesives, which are organic polymers, will be at least one order of magnitude less. Such a difference results in their primary application to bond relatively thin sheet materials or panels where yield in the adherend under loading is common due to the large bond areas employed. In addition, the toughness and strain capability of adhesives is usually higher when measured in compression and shear as opposed to cleavage, tension or peel. These facts have a major contribution to make when choosing an appropriate joint design. In contrast to most types of metals and ceramics some polymeric materials have material properties similar to or even less than the adhesives employed. This aspect will influence the types of adhesive chosen and bring into play other material-specific factors such as surface preparation, stress cracking and joint stiffness. It can be seen therefore that the route to a successful bonded structure, especially where dissimilar materials are used, requires careful consideration of the types of materials to be joined, their physical properties and the type/design of joint employed.

12.2.2 Material effects ± coefficient of thermal expansion (CTE) When joining different materials, another significant factor to consider after modulus and strength, is the difference in CTE between all the materials within

Joining similar and dissimilar materials

259

the joint, i.e., the adherends and the adhesive. The combination of two adherends with a large CTE mismatch, for example, stainless steel bonded to aluminium, can result in high thermally induced stresses if the joint is very large and no allowance made for expansion. Selection of a high strength, high modulus adhesive with a low strain to failure value could result in failure. An anecdotal example of this was when some large aluminium-skinned sandwich panels were faced with stainless steel for decorative purposes in an external environment where temperatures could range from 10 ëC at night to over 40 ëC during the day. Within months the steel panels were seen to buckle and detach from the underlying structure despite stringent surface preparation and careful assembly. This outcome was not surprising when one looked at the change in strain across the joint, especially at the ends of the panel. For a structure three metres in length, assuming zero stresses at the lowest temperature, the dimensional difference between the two skins is ~0.6 mm over 30 ëC. Assuming the stainless steel sheet is not constrained in any way there will be ~0.3 mm over-expansion of the aluminium panel which for a bold-line thickness (BLT) of 0.1 mm equates to ~1.5% strain at the end of the joint, potentially too high for some epoxy systems. In general, polymers have high CTE values in comparison with metals and ceramics (Table 12.1) with a modulus that decreases in value as the temperature Table 12.1 Coefficient of thermal expansion () for common materials Material ABS Aluminium Brick Epoxy Cement and concrete Copper Polyamide 6,6 Polycarbonate PET LDPE Polypropylene Polystyrene Polyurethane (thermoset) PVC (rigid) Silica (fused) Silicon Soda glass Steel Titanium Woods, typical (along grain) Woods, typical (across grain) Zinc

 (10ÿ6 Kÿ1) 65±69 22 3±10 20±60 10±14 16.7 80 68 65 100±220 81±100 50±83 100±200 50±100 0.4 2.6 8.5 11 9 3±5 35±60 31

260

Adhesive bonding

is increased. This combination usually means that adhesives have the capacity to partially decouple some of the thermal stresses seen between less compliant materials such as ceramics and metals. Due to this property, adhesive bonding is often seen as one of the few routes to bond such materials, especially when the relative dimensions are large. However, as has been shown in the earlier example, selection of the correct adhesive in conjunction with an appropriate BLT is important. A very thin bond-line is much more strain sensitive than a thick one and so the adhesive will take on the role of being an inter-layer where the bulk material properties play an significant part within the behaviour of the structure. The compromising aspect of this situation is a challenge that has to be considered carefully by the design engineer, i.e., reduced modulus and associated low strain values versus higher modulus and high localised thermal induced stresses. Consideration needs to be taken of the type of adhesive used in terms of the curing temperature, as this will set the level of residual stresses within the structure at the normal operating temperature. An adhesive cured at ambient room temperature conditions will impart significantly different properties into the joint, over one that has been cured at 180 ëC. When joining dissimilar materials where a CTE mismatch is significant, adhesive selection becomes critical in that the adhesive function provides a structural load-carrying capacity and acts as an interlayer to smooth or dissipate thermal stresses. Another area where such selection is critical is within the electronics sector because although the components are often perceived as small, the local stresses brought about by thermal change can become significant due to either the increasingly high strain sensitivity of the actual devices themselves or the risk of bond/component failure due to fatigue effects arising from thermal cycling. A good example of this is the use of adhesives as an underfill component for flip-chip bonding. Flip chips are silicon semiconductor devices which are attached to a glass/resin substrate via melted bumps of solder. The bumps are added to the base of the chip which is then `flipped' over into position on the circuit board and soldered in place. The CTE mismatch between the silicon (2.6), the solder (21) and the circuit board (14±17) is sufficient to result in fatigue failure at the the silicon/solder interface when subjected to thermal test cycling between ÿ40 ëC and 125 ëC as would be used for `under the hood' automotive applications. This problem is addressed through the careful selection of an underfill adhesive (usually epoxy) with a typical CTE of  18  10ÿ6 Kÿ1 . The adhesive is injected under the soldered device and allowed to cure (Fig. 12.1). The adhesive provides key structural support and resists movement between the components thereby lowering stresses in the solder/silicon area. However, incorrect selection of adhesive in terms of CTE (greater or lower value) can have the opposite effect providing results that are worse that having no underfill adhesive at all.

Joining similar and dissimilar materials

261

Figure 12.1 Underfilling a flip chip device with an adhesive.

Finally, it should not be forgotten that adhesives are complex mixtures of various monomers/oligimers, activators, catalysts, adhesion promoters, rheology modifiers and various fillers. Considerable effort has been expended over the years to create adhesives with a range of material properties and in many cases to match those of selected substrates. With respect to bonding metals, fillers consisting of matching metallic particles are added to lower the adhesive's CTE and bring it closer to that of the metal. A common example would be aluminium where aluminium filled adhesives are routinely used in the bonding and sealing of aerospace alloy structures. The effect of different filler compositions on the CTE for a typical epoxy system is shown in Fig. 12.2.

Figure 12.2 Effect of various fillers on adhesive CTE.

262

Adhesive bonding

12.2.3 Factors affecting adhesive properties Polymers are organic materials which are affected by: · · · · · ·

chemical composition physical structure glass transition temperature (Tg) cure type (two part, moisture cure, UV, heat, etc.) susceptibility to solvent uptake (especially water) unreacted material.

Each of these factors can have a significant effect on the long-term properties and performance of the adhesive which in turn could affect the properties of the joint and associated structure. If the adhesive becomes progressively more brittle or starts to soften, the initial design parameters may be compromised. Taking each factor in turn, their effect will be considered. Chemical composition This is a broad area which includes actual adhesive component chemistry which dictates material properties (modulus, strength, Tg, etc.) and the type of cure (condensation, radical, etc.), presence of volatiles (solvents, uncured monomer/ oligomers). Physical structure Adhesives, in common with other polymers, adopt thress primary physical forms: · thermoset (considerable cross-linking) · thermoplastic (no cross-linking) · elastomeric (flexible with or without cross-linking). The type and level of form produced affects the primary material properties of adhesive. Glass transition temperature (Tg) It has been indicated earlier in this book that Tg is an extremely important polymer property. Below Tg, the modulus decreases gradually as the temperature increases whereas around Tg, the adhesive becomes very soft with limited load-bearing capacity but it may act as a buffer between two dissimilar materials whose CTEs are significantly mismatched (Fig. 12.3). At temperatures well above Tg the polymer chains become mobile with the possible consequence of irreversible plastic flow under load for systems with little or no cross-linking (creep). In summary, all thermoplastic adhesives will exhibit creep under load if they are heated to above their Tg and to a lesser extent at temperatures just below Tg. For thermoset systems, creep is less of a problem and secondary cross-linking

Joining similar and dissimilar materials

263

Figure 12.3 Effects of CTE mismatch and adhesive compliance with temperature increase: (a) cured structure at ambient conditions using room temperature cure adhesive (upper adherend has larger CTE than lower adherend); (b) at higher temperature but below Tg of adhesive; (c) at higher temperature above Tg of adhesive or at higher temperature with a high compliance adhesive.

can occur if post heating is applied. This will result in an increase in Tg, potential increase in modulus and so affect properties irreversibly. Cure type Different adhesives cure in different ways depending upon the chemistry selected and mixture composition of the product. Examples include: · · · · ·

moisture heat

radiation UV microwave, etc.

This in turn will affect factors such as cure shrinkage, degree of cross-linking and thermal stresses should heat be required for curing. For some products, subsequent heating cycles in service can significantly alter the original adhesive properties in an irreversible manner. Solvent uptake Adhesives will absorb solvents (via chemisorption and physisorption) including water, over time. Whilst the effect for water is low in dry ambient conditions, submerged and wet environments can cause some adhesives, e.g., epoxies and acrylic based systems, to show a significant drop in modulus due to the plasticising effect of the water. Such a property may be seen as an advantage in that it effectively toughens the system. Unreacted materials Some adhesives can contain a proportion of unreacted materials which may take the form of low molecular weight material which can be driven off during additional heating cycles. Typical examples include polyester and acrylic adhesives. Loss of such species will affect properties such as modulus and toughness.

264

Adhesive bonding

12.2.4 Corrosion Metallic corrosion (especially that which is aqueous based) is an issue that cannot be ignored especially in the case of aluminium and steel structures. In view of this, many processes have been developed to control and even eliminate the problem. Such processes are predominantly pre-treatments based upon chemical modification such as conversion coatings, addition of silane chemicals and anodisation or physical coatings such as primers, paints and metallic coatings (Zn, Cr, etc.) or a combination thereof. All result in a surface for which the corrosion-resisting properties may be compromised should it be significantly breached. This needs consideration during joining operations in that this additional layer may influence the type of joining processes and significantly affect the joint properties. For joining similar materials such as sheet steel or aluminium, welding operations may now be compromised if the weld process is intolerant of the materials within the coating. Welding may require local removal of the coating, which is an additional process step or the heat from the welding process may cause local heat damage of the coating especially in the case of paint systems. All of this will have to be `made good' after welding, costing time and money. Mechanical attachment offers an alternative approach but holes still have to be formed and the act of fastening may cause local coating damage. Adhesive bonding offers an attactive joining solution in that the coating in the joint area does not need to be damaged and an adhesive can be selected on the basis of compatibility with the coating rather than the underlying adherend. In addition, the adhesive provides a barrier between the adherends which prevents the spread of corrosion from one side of the joint to the other. The barrier effect also addresses the issue of galvanic corrosion where different metals such as steel and aluminium are to be joined. It should also be noted that carbon fibres can cause galvanic corrosion when in direct contact with metals, highlighting the need for an effective polymeric seal between the components. As adhesives are complex mixtures, additional corrosion-resisting functionality can be added, such as the incorporation of coupling agents (silanes), hydrophobic molecules or other ionic species which migrate to the adherend/ adhesive interface during the curing stage. Once in position such species can form an additional layer of defence to corrosive attack when in service.

12.2.5 Anisotropy Although a majority of engineering materials are isotropic and as such, present joining challenges that have no specific directional quality, there are increasing numbers of anisotropic materials which require careful consideration. Key examples include ordered fibre reinforced composite materials, crystalline materials and drawn materials such as metals and polymers. The anisotropic

Joining similar and dissimilar materials

265

qualities that these materials possess mean that often the only way to join them to themselves or other materials is by adhesive bonding, where an adhesive is selected on the basis of a number of properties including compliance, chemical compatability, cohesive strength, cure shrinkage, CTE, etc. In addition consideration must also be made of an appropriate joint design, tolerances and BLT value. Anisotropic materials present a number of joining challenges, in that their internal alignment is based upon the direction, type and magnitude of anticipated loading conditions. Effective transfer of these loads from one side of the joint to the other is key to producing an optimum structure. However, the anisotropic nature of adhesives means that the primary load path will be through the adherend surfaces, especially in the case of unbalanced joints such as the single lap shear or flange joint. A typical example would be with the bonding of continuous fibre composites either to themselves or to metals. The most direct load path results in peak stresses being seen within the first two layers of the composite ply structure. The bulk composite material is isolated from the load, which compromises many of the benefits that such materials have to offer such as high strength, high stiffness and low mass.

12.3 Adhesive selection 12.3.1 Overview It has already been shown that adhesives offer many challenges over other processes when joining materials and in particular when the materials are dissimilar. The need to select the correct adhesive to address issues such as joint strength, flexibility, CTE mismatch, etc., is extremely important to the design engineer. Additionally, the adhesive must also be selected with the surface properties of the adherend in mind. Some adherends can have adverse effects on the bond strength of the adhesive either chemically via the presence of certain ions or molecular species, or physically via low energy surfaces or through surface structure, i.e., friable, smooth, soft/hard, discrete domains, laminar, etc. These effects will now be considered in more detail.

12.3.2 Chemical effects The chemical interaction between the adhesive and the adherend surface can take a number of forms but the main ones include: · Promotion or inhibition of cure. Some adhesives such as cyanoacrylates require a surface containing free moisture and a high pH to initiate cure, hence they are good at bonding skin tissue. However, the presence of copper ions will inhibit cure. Conversely, anaerobic adhesives require an absence of oxygen and will not cure unless certain metal ions such as copper are present.

266

Adhesive bonding

· Solvents. Adhesives containing solvents or low molecular weight monomers used to be used with care when bonding some polymers otherwise stress cracking/crazing around the joint area can result. · Reactive agents. Adhesives often contain systems called adhesion promoters or coupling agents, which chemically react with the adherend surface and enable a chemical bond to be formed with the adhesive. · Chemical compatibility. In general the best adhesives for bonding a particular adherend are those with similar chemistries or compositions, this being most relevant to polymeric materials. Epoxy adhesives will bond much more successfully to epoxy-based composites than to acrylic materials and the converse is also true although acceptable bonds can still be achieved. This effect is seen more acutely where low energy surfaces are involved such as with polyolefins and silicones. Generally, only adhesives with chemistries very close to these materials will have any chance of achieving a successful bond with minimal surface pre-treatment. In the case of inorganic and metallic adherends, compatibility can be enhanced through the use of appropriate fillers although the benefits more usually relate to the modification of the adhesive physical properties such as CTE than to the chemistry. · Cure shrinkage. Adhesives primarily consist of polymer resins or short chain oligomers which chemically react with either themselves or some other curing species to form a cross-linked network. For most systems, the curing process results in a net volume decrease and resultant shrinkage. However, the extent of shrinkage can vary considerably. For example, toughened acrylic adhesives will commonly shrink by 1±3% whereas epoxy adhesives may have shrinkage values as low as 0.05%. The final modulus of the adhesive in conjuction with the type and proportion of filler will determine the extent of shrinkage that the joint will experience. A further factor to consider is if heat is required to cause the curing reaction to happen. In this case thermally induced stresses can also be generated around the joint when it cools to room temperature especially when the adherends are mixed and have differing CTE values.

12.3.3 Physical effects Physical effects take many forms including: · Surface energy. The surface energy of a material (commonly termed surface tension) is a function of the number of active polar groups at that surface and for good adhesion to be achieved, the surface energy of the adhesive must be lower than that of the adherend material. Examples of surface energy values of a range of common materials are provided in Table 12.2. While this is not

Joining similar and dissimilar materials

267

Table 12.2 Surface free energy values ( S) for some common materials Solid surface PTFE Polypropylene Polyethylene Polystyrene PMMA Nylon 6,6 PVC PET Epoxy (averaged) Carbon fibre reinforced plastic (abraded) Silica Alumina Fe2O3 Copper

Surface free energy s (mJ/m2) 19.1 30.2 32.4 40.6 40.2 41.4 41.5 45.1 46 58.0 287 620 1357 1360

usually difficult to achieve for metals, which all possess high energy surfaces, bonding polymers is more challenging. In particular, the surfaces of polyolefins and polyfluoro-carbons have extremely low surface energies making them a real challenge to bond to without specialist surface pretreatments. · Surface profile or surface roughness. In general, a roughened surface will enhance bonding for most materials and in particular metals, ceramics and glasses. The reasons for this relate to the increased surface area that a rough surface has over a smooth one and the opportunity for mechanical keying of the adhesive to the surface. In addition, roughening serves to remove any friable loose material or oxides and expose a fresh, more reactive, surface which will be more receptive to bonding. The effect of roughening on polymer materials is variable. The removal of a passive top surface and the increase in surface area will enhance bonding but localised damage of the surface due to bond breaking or in the case of fibre reinforced composites, damage to the underlying fibres can be detrimental to achieving an acceptable bond. When bonding mixed material systems appropriate consideration of the needs of each substrate is required.

12.3.4 Design needs When considering adhesives for a particular application there is often the mistaken perception that the `strongest' adhesive is the best. In reality the most appropriate adhesive needs to be carefully selected on the basis of a range of factors, the majority of which are considered within this chapter. In particular,

268

Adhesive bonding

the design of the joint in conjunction with the adherend properties and dimensions can play a critical role. In instances where the adherends are thin and both of low modulus, it would be inappropriate to select a high modulus adhesive which would stiffen the joint area and result in areas of high stress and possible failure. A lower modulus, toughened system would be a better choice, the ultimate failure load may be lower but the energy carrying capacity of the joint would be significantly higher and failure may be seen outwith the joint, in the parent material. This phenomenon is often seen when bonding sheet metal with a thickness of 1 mm or less. Thin adherends will also have a tendency to bend under load making a flexible adhesive a more effective choice. Conversely, if both adherends are of high modulus and/or their thickness is such that bending around the joint area is not envisaged, a high modulus, high strength adhesive will be more effective for some applications. Should the materials to be bonded have significantly different stiffnesses, e.g., rubber to steel or cloth to thick sheet polymer then the selection process may not be so straightforward. All joints should be designed to minimise peel and cleavage and where this is not possible, the adhesive should be sufficiently flexible to tolerate large strains at the ends of the joint. In addition to the static loading conditions, consideration is also needed as to how the joint will perform under more complex loading scenarios such as fatigue and impact and what sort of operating environment the structure will see.

12.4 Surface pre-treatments 12.4.1 Overview There is a very close relationship between the adhesive selection issues discussed in sections 12.3.2 and 12.3.3 (chemical and physical compatibility) and the effect of surface pre-treatment which in many respects can be regarded as an extension of these areas. Pre-treatment of all surfaces to be bonded is crucial in establishing control over the quality and the effectiveness of the bond produced. Specific details relating to the benefits and actual treatments are covered elsewhere in this book. However, an overview is provided in the following two sections on the two main classes of adherends i.e. metals and plastics. In many applications a common pre-treatment or coating can be applied to both substrates thereby addressing specific compatibility issues. This can be especially useful for plastics materials.

12.4.2 Metals A whole range of pre-treatments are possible ranging from simple degreasing through roughening, to the generation and chemical modification of the oxide

Joining similar and dissimilar materials

269

layer. The pre-treatment is usually selected on the basis of the performance requirements of the structure being produced with an emphasis on strength and durability. Most treatments are compatible with a wide range of adhesives enabling considerable choice of the most suitable material combination. However, in some instances it may be necessary to modify the surface to enhance compatibility or impart additional properties such as corrosion resistance. Examples include the use of: · coupling agents such as silanes which are formulated to chemically react both with active chemical species in the oxide layer and with the adhesive · primers which are often dilute solutions of adhesive formulations designed to completely wet out the adherend surface and provide a more active bonding surface · metal coatings such as galvanising present another set of problems in that the zinc layer can be less well adhered to the underlying steel than to the adhesive resulting in coating failure under certain load conditions · conformal polymer coatings such as ParaleneÕ used for fine electrical insulation and to provide a barrier to moisture. The first two examples promote adhesion whereas the latter two may require additional processes to enable bonding to take place or limit the types of adhesives that can be used. The ability to bond metallic adherends together without any pre-treatment is highly desirable but for obvious reasons is not normally recommended due to the lack of quality control that such an approach will result in. However, an exception to this approach has been developed in conjunction with the automotive industry where there was a need to adhesively bond/seal oily coated steel structures. The industry was resistant to implementing an additional cleaning stage on the grounds of cost and time and placed the onus back on the adhesive producers to develop a solution. The result was a family of single part heat curing adhesives which absorb the oil on the metal surface during the curing cycle allowing the adhesive to fully wet out and bond to the metal.

12.4.3 Polymers Polymeric materials differ from metals in that they can be more sensitive toward the types of adhesive that can be used. A key point to note is that simple mechanical abrasion has been shown to produce very variable results depending upon the extent of abrasion and the type of polymer, in some instances it merely produces a friable surface which results in a poor adhesive bond. A broad range of alternative processes exist, similar to those for metals, including chemical etching and priming in addition to other energetic systems such as laser, corona, flame and plasma methods.

270

Adhesive bonding

12.5 Assembly issues and hybrid joining 12.5.1 Introduction In general, for all material combinations some type of tooling or assembly equipment, in the form of jigs and fixtures is required to achieve a high quality, reproducible adhesively bonded joint. The function of the tooling is to locate and retain components together throughout the bonding process. As the majority of adhesive systems exist as liquids or pastes before they cure or harden, jigs and fixtures are there to control the following key factors within an adhesively bonded joint: · bond-line thickness (BLT) · joint alignment · fillet profile. All of these key factors have a direct impact on · mechanical performance · cosmetic appearance · assembly time ± and hence cost. A broad range of jigging and assembly aids for adhesively bonded systems exist. They can be loosely placed in three groups: · internal agents, e.g., glass beads, wires or shims · external agents, e.g., clamps, presses and plates · combination or hybrid systems, e.g., riv-bonding (adhesive plus rivets). It is important to understand the role of pressure when assembling an adhesively bonded joint, i.e., how much pressure to apply to a joint during the adhesive curing process. In some instances, guidelines supplied by manufacturers indicate a certain minimum pressure; in others cases, no magnitude is indicated and this is often interpreted as `as much pressure as possible'. As adhesives exist as liquids or partial liquids for a period of time prior to curing/hardening, care must be taken to control the application of pressure, to avoid squeezing all adhesive from the joint. To address this issue, jigging or tooling is required to control the spacing between the adherends and therefore the BLT. This can be either internal, i.e., filler particles, glass beads, wires, carrier films, joint details, etc., or external, i.e., tooling, external shims, joint details, etc. With virtually all adhesive systems there exists an optimum BLT range for achieving the desired mechanical properties of high bond strength and resistance to creep. A joint starved of adhesive will be very weak and highly susceptible to defects such as voids and dry/poorly wetted areas. If there is too much adhesive present, the properties of the bulk adhesive will dominate the joint which is often undesirable. When the BLT is within the optimum range the adhesive will not fail prematurely as load transfer is maximised and creep is minimised.

Joining similar and dissimilar materials

271

The ideal BLT range varies for different types of adhesive, for example: epoxy, 50±350 m; acrylic, 100±500 m; polyurethane, 500±5000 m. It is, therefore, very important that this is taken into account during the design phase and appropriate assembly aids are employed during fabrication.

12.5.2 Internal agents Fillers Many filler materials are inorganic in origin and often consist of finely ground particles. The size and distribution of these particles can be used to control BLT. Particle sizes of up to 500 m can be used, although less than 100 m is more common because large particles affect the handling and dispensing of the adhesive. Glass beads Instead of relying on the filler's maximum particle size to define BLT, ballotini (glass beads) of controlled diameter can be used (Fig. 12.4). Typical diameters for ballotini range from 100±300 m. Ballotini can be added during the adhesive manufacturing process, or it can be mixed with the adhesive at the point of use. The advantages of ballotini are: · controlled size ± gives required BLT consistently · controlled quantity and distribution ± can be mixed evenly within the adhesive or added in very small quantities at specific locations within the adhesive joint. Ballotini also has some disadvantages: · may compromise adhesive performance and reduce strength · are easily crushed if too few are used or too much pressure is applied · human error can result in too much or too little being used or placed in the wrong locations ± results in the formation of artificially starved of adhesive areas or insufficient compensation for surfaces with variable tolerances.

Figure 12.4 Lap joint using ballotini-loaded adhesive for BLT control.

272

Adhesive bonding

Wires or shims Placing wires or metal shims within the joint will also control BLT and is a common approach for test specimens as it is extremely localised and in many instances the wire can be cut out of the final test-piece. Care must be taken with selecting the type of material, however. `Non-stick' materials create defects which can cause fatigue loading failures or environmental attack. Carrier materials and tapes The physical form of the adhesive product can also be used to control BLT. In the case of film adhesives, the presence of the carrier material defines the minimum adhesive thickness. For pressure sensitive systems, the dimensions of the supporting film provide a similar function. Adhesive tapes can also be used for adhesive containment purposes, i.e., they can be used as a `stop-off' by placement between adherends at one or more extremes of a bonded joint. Joint detail (threads, ridges, pips, troughs, etc.) A series of individual peaks or ribs strategically placed within the bonding area ensures that adherends are accurately spaced. The forming of these features, if incorporated into the design of the component, usually adds little or no cost to the manufacture of the component.

Figure 12.5 Extruded component with ribs for BLT control.

Joining similar and dissimilar materials

273

Figure 12.6 Detail showing BLT control ridges on Lotus Elise chassis.

Components produced by plastics moulding, pressure die-casting, extrusion, etc., can all be manufactured with designed-in bonding aids. With extruded parts, the extrusion process dictates that ribs and recesses are unidirectional (Fig. 12.5). One particular example of note would be the chassis for the Lotus Elise car which is a fully bonded structure produced from a combination of extruded and sheet aluminium alloy components. All BLT control is achieved through the use of ridge features as can be seen in Fig. 12.6. For machined parts, such bonding aids can take any design form but will have an associated cost. Similarly, features for containment of adhesive, or adherend alignment purposes, can often be economically provided as part of the manufacturing process.

12.5.3 External agents Clamps and shims The simplest method of securing bonded structures during adhesive cure is by clamping. The clamping can range from a simple spring clip, manual screw clamps (`G' or toolmakers' clamps, etc.) to fully automated hydraulic or pneumatic `systems'. All clamping methods will require some form of BLT control. This can be `internal' to the adhesive joint (as described earlier), `external' to the joint, or by pressure or distance `limits' acting within the clamping system. Tooling In a production environment, tooling is employed to align, apply pressure and support components during adhesive cure. Tooling may provide other benefits to a bonded joint, such as the formation of radiussed fillets. The presence of fillets at the perimeter of a bonded joint enhances joint performance by reducing stresses at the adhesive's edges.

274

Adhesive bonding

Presses and plates On flat components, heat curing of the adhesive is often achieved using a press with heated platens. Using either a liquid or film adhesive, components can be aligned between the platens prior to applying the pressing load. The heated platens are then set to carry out the adhesive cure cycle. When using a film adhesive, platen pressure can be pre-set so that the carrier material within the film adhesive controls the BLT. In the case of liquid adhesives, the heated platens are usually set to `close' against a hard stop, the distance between platens being the thickness of adherends plus the BLT. Alternatively, the work-piece may be clamped between metallic plates, and then the entire assembly is placed in an oven for the curing operation. Fasteners (nails, bolts, rivets) A simple and usually cost-effective way of controlling an adhesive joint is by the addition of standard fasteners (nails, screws, bolts, rivets, etc.). `Screwed and glued' is a well-known term in the woodworking industry; the main purpose of the screws is to align the components until the glue sets. It is a fact that once the glue has set the screws add little to the joint's strength. With woodworking joints, the BLT is not an issue. Since both adherends are porous, fully tightening the screws will not completely expel the adhesive. In the case of metallic or nonporous bonded joints, the screw and glue approach is equally simple and attractive but some form of BLT control is necessary. Internal BLT control is usually the most appropriate and whilst any of the methods previously described can be used, the simple expedient of placing a standard washer between the adherends, at the bolting point or points, can be an economic option.

12.5.4 Combination or hybrid joining The terms `hybrid joint' or `combination joint' are used to describe a joint formed using two or more distinct joining technologies. The use of jigging aids or tooling which become part of the final structure can therefore offer a further function as a supplementary joining technique, the most common ones being: · · · ·

weldbonding ± resistance spot welding riv-bonding ± rivets (including self-piercing) clinch-bonding ± clinching glue and screw ± threaded fasteners.

All of the above enable the joint to be held together during the adhesive curing process and are left in place, thereby providing a second level of joint retention should the adhesive bond fail. This approach is commonly adopted within the automotive, woodworking and aerospace industries. It should be noted however,

Joining similar and dissimilar materials

275

that weldbonding is unsuitable for most mixed material systems and clinchbonding relies upon plastically deformable adherends such as metals. The visible presence of fasteners may be regarded as an additional level of security or, alternatively, as compromising the aesthetic appeal of a bonded structure. On a more practical level, a hybrid joint is more resistant to catastrophic failure such as impact or fire where the fastening point acts as a crack arrest mechanism or as a thermally resistant system respectively.

12.5.5 Conclusions The need to hold components together during an adhesive bonding operation is in most cases self evident. However, the way in which those components are retained is much less clear. This is primarily due to the wide range of options available, some of which are highly complex, bespoke solutions, while others are very simple and inexpensive. With such a variety of choices available it is important to make the correct selection and this will usually be based on a range of factors, among which cost, speed, performance and precision are probably the most important. These factors cannot be effectively evaluated without an appreciation of the mechanism behind each assembly aid, together with its associated technical benefits. This text has provides a framework within which an informed selection can be made.

12.6 Future trends Due to the ever increasing use of adhesives and associated joining methods in all areas of industry, many new opportunities and challenges exist to be explored and exploited. Of particular interest are those areas relating to new materials (e.g., composites, nanomaterials, etc.), green technologies (e.g., disassembly on demand, natural materials, environmentally friendly pretreatments, etc.), medicine (e.g., devices, bionics, orthotics), nanomaterials, electronics (e.g., microtechnology, optoelectronics, etc.) and more demanding structural applications within such sectors as aerospace, automotive, building and construction. Much work is being done to improve the level and type of adhesion that can be achieved between adherends. Various approaches are being taken and include: · The development of micro-hairs on the adherend suface to mimic the `adhesive' properties of gecko feet to produce a dry re-useable system that bonds using van der Waals forces by virtue of intimate contact that can be achieved between the hairs and the surface. Such an approach could reduce the level and extent of pre-treatments currently needed for many materials.

276

Adhesive bonding

Figure 12.7 ComeldTM joint between stainless steel and a glass polyester composite.

· Addition of nanomaterials to the adhesive to improve adhesion, toughness and strength. It has been demonstrated that the addition of nano-sized clay particles to epoxy adhesive systems significantly improves the adhesive performance for structural applications. · Work is being done to produce a graded joint to smooth the transition between dissimilar materials such as composites to metals. TWI has developed one such process called ComeldTM which utilises power beam technology to pre-treat the metal at the surface and in the bulk to form macro features which can interact more extensively within the layup of a composite system thereby enabling greater and more directed load transfer between the materials (Fig. 12.7). A similar approach is used by nature to connect bone to soft tissue. · In the fields of microtechnology and optoelectronics, adhesive bonding often offers the only way to join components of different materials where thermal effects can critically affect performance. Functional adhesives are being developed to have matched properties (e.g., CTE, thermal conductivity, etc.) to a particular application. In consumer electronics, low viscosity UV curing adhesives are being exploited to fabricate double layered DVD disks. · Impending EU legislation will result in the elimination of chromium containing compounds from pre-treatment systems. Adhesive bonding makes extensive use of such systems, especially in the areospace sector. Alternatives are being sought in the form of both alternative pre-treatments and in developing adhesives that will function without the need of a pre-treatment. · The ever growing need to recycle components requires adhesives that will debond upon command, not an insignificant challenge, but adhesive systems are being developed that will do just that, through the incorporation of a blowing agent and the application of heat or by causing a secondary weakening reaction to occur under the action of UV radiation.

Joining similar and dissimilar materials

277

The ability to understand and control both the surfaces of the materials to be bonded and the structure/properties of the adhesive at all levels of scale, will be the key to fully exploiting adhesive technology.

12.7 Bibliography A wide range of sources and publications were used in the preparation of this chapter and the main ones are listed below.

12.7.1 Books and publications Engineered Materials Handbook, Volume 3 ± Adhesives and Sealants (1990), prepared under the direction of the ASM International Handbook Committee, ISBN 0-87170279 (v.1). Handbook of Aluminium Bonding Technology and Data (1993) J. Dean Minford, Marcel Dekker, Inc., ISBN 0-8247-8817-6. Joining of Advanced Materials (1993) Robert W. Messler, Jr., Butterworth-Heinemann, ISBN 0-7506-9008-9. Joining Processes ± An Introduction (1997), David Brandon and Wayne D. Kaplan, John Wiley and Sons, ISBN 0 471 96488 3. Analysis and Design of Structural Bonded Joints (1999), Liyong Tong and Grant P. Steven, Kluwer Academic Publishers. Designing Plastic Parts for Assembly (1994), Paul A. Tres, Hanser Publishers, ISBN 3446-17594-6. Principles of Polymer Engineering (1988), N. G. McCrum, C. P. Buckley and C. B. Bucknall, Oxford University Press, ISBN 0-19-856152-0. Development of Design Rules for Structural Adhesive Bonded Joints ± A Systematic Approach (2001), Ijsband Jan van Straalen, privately printed in the Netherlands, ISBN 90-9014507-9. Structural Adhesive Joints in Engineering, 2nd edn (1997), R. D. Adams, J. Comyn and W. C. Wake; Chapman and Hall; ISBN 0 412 70920 1 Adhesion and Adhesives ± Science and Technology (1987), A. J. Kinloch, Chapman and Hall, ISBN 0-412-27440-X. Textbook of Polymer Science, 3rd edn (1984), Fred W. Billmeyer Jr., John Wiley and Sons, ISBN 0-471-03196-8. Joining Fibre-Reinforced Plastics (1987), F. L. Matthews (ed.), Elsevier Applied Science, ISBN 1-85166-019-4. Joining Technologies for the 1990s ± Welding, Brazing, Soldering, Mechanical, Explosive, Solid-State, Adhesive (1986), John D. Buckley and Bland A. Stein, Noyes Data Corp., ISBN 0-8155-1095-0.

12.7.2 Internet resources www.adhesivestoolkit.com ± a useful resource bringing together a number of interactive modules and data sources including DTI funded research reports, stress analysis, adhesive selection, case histories, supplier links and design guidance.

278

Adhesive bonding

www.lclark.edu/cgi-bin/shownews.cgi?1030395600.1 ± the original announcement relating to developing a dry adhesive surface based on the science of gecko feet adhesion. www.psb-services.demon.co.uk/debond/ ± website for Debonding Ltd, a company with a patented approach for adhesive debonding where blowing agents are incorporated into the adhesive and are activated upon heating.

13

Bonding of composites

P DAVIES

13.1 Introduction This chapter will present an overview of the use of adhesive bonding to assemble composite components. The composite materials considered here are based on polymer matrix materials reinforced with glass or carbon fibres. Such composites are often employed to save weight, and adhesive bonding can provide additional weight savings compared to metallic fasteners. However, other advantages such as more efficient assembly procedures and improved corrosion resistance also influence the choice of this technology and can result in significant cost savings. In the following sections the influence of the specific nature of composite materials will be presented first, in section 13.2, then four particular aspects will be discussed in more detail in sections 13.3 to 13.6. A case of composite bonding of great industrial importance is that involving the skin/core bond in sandwich materials. This will be described in section 13.7, in which three examples of applications are presented. Long-term behaviour and durability are discussed in section 13.8. The chapter concludes with a look at future trends and recent developments in section 13.9. It should be emphasised that this chapter is not intended to be an exhaustive survey, but rather an indication of some of the particular requirements associated with composite assemblies and a pointer to some areas of current interest.

13.2 The specific nature of composite materials Many details of adhesion and adhesives are given in other chapters so this contribution will concentrate on the specific nature of composite materials and the aspects to be considered when assembling them, either to themselves or, as is more often the case, to metallic materials. There are several points which distinguish composites from other materials, Fig. 13.1. First these materials are laminated structures. They are themselves produced by bonding processes at two levels, between fibres and matrix and between

280

Adhesive bonding

Figure 13.1 Specific nature of composites with respect to assembly.

layers of reinforcement or prepreg, and their properties thus depend on the success of these bonding operations. The addition of a third bondline to produce a structure may not be a region of weakness for the structure as the throughthickness properties of the laminated composite alone are often quite poor. Second, in order to get the best performance out of composites they are generally more or less anisotropic. This introduces complexity in design and may result in unexpected coupling phenomena. Internal stresses resulting from this anisotropy may affect bond performance significantly. As a consequence of these factors we are not currently able to predict composite bond strength with confidence. This will be discussed further below but the practical result is that testing remains an essential part of composite assembly design. Thirdly, composite structures are tailored to respond to the expected loading, so surface layers can also be orientated. This may allow a degree of optimisation in assembly. On the other hand the surface to be bonded is composed of a thin veil of resin over load-bearing fibres so surface preparation must be carefully controlled. A further point of considerable practical importance is bond-line thickness. In non-aerospace applications this is not always controlled and it may have a strong influence on joint strength. These points will be explored in more detail in the following sections.

13.3 Design of bonded composite assemblies The main design principle in the development of bonded assemblies is to ensure that the joint works in shear and to minimise peel loading. The traditional approach to the design of metallic joints is to run a stress analysis, using more or less complicated analytical methods or finite element packages, and to compare the calculated peak stresses or strains with a failure criterion for the adhesive (typically a von Mises or a maximum strain criterion) and with a plasticity

Bonding of composites

281

criterion for the metallic substrates. There are several difficulties with this approach when the substrates are composites. However, first, whether it is composites or metals which are being assembled it should be noted that the properties of the structural adhesives being used today tend to be very nonlinear, and they may develop extensive damage zones before failure. For example, Fig. 13.2 shows a tensile stress-strain plot for an epoxy-based adhesive and the adhesive can support very large strains before failure. Estimating the failure of such a material using a linear stress-based analysis is clearly going to be conservative. On the other hand part of the non-linearity is generated by irreversible damage mechanisms, shear micro-cracks in the case shown here. A damage mechanics approach to design such as that proposed by Allix and LadeveÁze and colleagues may therefore prove more appropriate in the future (Allix et al., 1992, 1998). This approach requires more extensive testing to determine the damage model parameters and considerable modelling capabilities. To date such applications have been limited to aerospace projects but several groups are currently working to develop these in other areas. A second point concerning bonded composite or metal joints is the presence of singularities at the ends of the joint, which must be accounted for in the analysis, either by an averaging procedure or by a fracture mechanics approach. A third point specific to composite bonding is that the first damage in the assembly often occurs in the composite substrates due to high through-thickness stresses. Prediction of bond assembly strength therefore becomes prediction of composite through-thickness strength near stress concentrations. There is still no generally accepted failure criterion for through-thickness mechanisms in composites. An extensive exercise to compare composite failure criteria for well-defined case studies was reported recently, and included nineteen different approaches to predict deformation and failure response of laminates. That exercise was limited to in-plane loading and the authors concluded that, contrary to widely held misconceptions, much still remains to be done to ensure that reliable and accurate predictive tools are readily available for general use in design (Soden et al., 1998; Hinton et al., 2002). For out-of-plane loads there are several criteria available. The simplest are non-interactive of the type: 33 13 ‡ ˆ1 Z S where Z is the through-thickness tensile strength and S is the out-of-plane shear strength. In a recent study of bonded pultruded stiffeners this criterion was applied quite successfully to predict delamination onset in the stiffener (Davies et al., 2005). This linear criterion only requires two strength values but even these are not easy to measure. The out-of-plane failure envelope may not be linear in reality but insufficient data are available today to justify another form.

Figure 13.2 (a) tensile stress-strain plots for brittle and ductile epoxy adhesives, (b) illustration of damage in a double lap shear specimen bonded with the ductile adhesive, test stopped before failure.

Bonding of composites

283

More complex failure criteria do exist, including those with interaction terms such as that proposed by Hill, but these introduce many additional parameters which must be estimated as they are not easily measured. This underlines the fact that the choice of failure criteria is closely related to the limitations of current test methods. Recent overviews of through-thickness testing of composites (e.g. Lodeiro et al., 1998) have emphasised the need for further test development, as uncertainties concerning input data severely limit the usefulness of even the simplest failure criteria. Work is needed to improve stress uniformity in specimens, reduce stress concentrations and confirm failure modes. This will be discussed further in section 13.5 below. An exercise to evaluate the prediction of the strength of composite assemblies was performed within the DOGMA (Design Optimization and Guidelines for Multimaterials Applications) European thematic network. This involved the definition of various single and double lap shear joint geometries. Several academic and industrial engineers then ran blind failure load predictions using the same material and geometry input data. The results were compared between themselves and tests were also performed to enable predictions to be compared to test results. Seven finite element codes and three analytical solutions were used to predict failure load. The results have been presented elsewhere so only a brief summary will be presented here (Davies et al., 2001). That reference also gives the names of those who ran the analyses. Table 13.1 shows the predictive methods applied, and Figs 13.3 and 13.4 show two examples of results for single lap shear specimens with 3 mm thick adherends and either a brittle (Fig. 13.3) or a ductile (Fig. 13.4) epoxy adhesive. The adhesives correspond to those of Fig. 13.2. The specimen width was 20 mm and the overlap was 20 mm.

Figure 13.3 Example of correlation between prediction and test results, brittle adhesive.

284

Adhesive bonding

Table 13.1 Models used to predict failure loads in DOGMA project round robin exercise No. Model type, version 1 2 3 4 5 6 7 8 9 10

Type

FE ANSYS 5.3 FE NISA 7.0 FE ABAQUS 5.4

2D linear elastic adherends, NL adhesive, NL geometry 3D, linear elastic adherends, NL adhesive, NL geometry 2D linear elastic adherends, NL elastic/plastic adhesive, NL geometry FE In-house 2D Special joint element SAMCEF 2D linear elastic adherends 7.1.3 and adhesive. Linear geometry FE COSMOS/M 2D linear elastic adherends, 2.0 NL plastic adhesive, NL geometry FE ADINA 2D orthotropic linear 7.2 adherend, plastic bilinear adhesive Analytical Stress and strain (CETIM CADIAC) Analytical Fracture mechanics Analytical

Stress and strain

Failure criteria Von Mises stress, adhesive max. strain Max. stress in adhesive or adherend Composite ILT, ILSS, adhesive max. strain Composite max. stress adhesive max. strain Elasto-plastic, von Mises Max. stress adherend, max. strain adherend Composite Tsai-Hill, adhesive max. strain Composite elastic adhesive elastic-plastic Mixed mode fracture envelope Five failure criteria

Figure 13.4 Example of correlation between prediction and test results, ductile adhesive.

Bonding of composites

285

In general the finite element (FE) codes and associated criteria tended to underestimate measured failure loads. However certain FE codes gave good indications of failure for the brittle adhesive. For the more ductile adhesive all the FE predictions underestimated failure even when bilinear elastic-plastic adhesive material models were employed. Most of the predictions were based on stress or strain criteria but one analytical fracture mechanics solution was also evaluated. It involves the approach developed by Fernlund and Spelt and colleagues (Papini et al., 1994) who applied it quite successfully to the prediction of bonded aluminium joint strength. Here the mixed mode fracture envelope for the ductile adhesive on the same glass/epoxy adherends had been determined in a previous study (Ducept et al., 2000). The fracture mechanics predictions were quite good for the brittle adhesive but tended to overestimate the ductile assembly failure load. The other analytical methods employed were also reasonably accurate for the brittle assemblies, but tended to underestimate the ductile adhesive cases. The results from the thematic network studies suggest that composites assembled with brittle adhesives can be modelled reasonably well. When more ductile adhesives are used the FE predictions were conservative in all cases so while the full potential of such adhesives may not be fully exploited, there is an intrinsic safety factor in the models. In a more detailed study for an aerospace application (EleÂgoet, 2000) carbon/ epoxy assemblies were examined. Numerical analyses were performed but in this case the failure criterion was identified using simple lap shear specimens. The criterion which appeared to give the best results for the simple specimens was based on a critical value of the normal stress in the adhesive at a certain distance from the interface. This criterion was then used in the analysis of a more complex industrial structure and a good prediction of failure load was obtained. Although we are not currently able to predict adhesive bond strength with confidence from material input data, many structures are being designed and built today with adhesively bonded composites. As demonstrated by EleÂgoet the design process generally involves both analysis and testing with iterations between the two, and the development of efficient test programmes has become an essential element of the design process. This will be illustrated in later chapters for different industrial applications, but it should be emphasised that design safety factors will reflect the experience of designers, the level of quality control and the consequences of structural failure. These differ widely between the different application sectors where composite materials are used.

13.4 Surface preparation Surface preparation is a critical part of adhesive bonding. It is covered in more detail in previous chapters so here only a few words are included to describe composite surface treatments. The aims of these treatments are to:

286 · · · ·

Adhesive bonding

remove contamination increase the polarity of the surface increase the surface energy increase the surface area.

Typical surface treatments for composites include solvent wipe, grit blasting, abrasion, peel ply removal, and grinding. These may be applied alone or in combination and the first two are the most common for composite surfaces. In certain cases, notably for thermoplastic matrix composites which have low energy surfaces, more sophisticated treatments such as corona discharge or plasma may be employed. The epoxies widely used as composite matrix resins are more polar than polyolefins and the main role of surface treatments is therefore to remove contaminants, particularly mould release agents. Several studies have presented results from mechanical tests on bonded composite joints prepared with different treatments. Kinloch summarised some of these (Kinloch, 1987). Problems with peel plies were discussed, in particular the influence of fluorinated contaminants originating from the peel ply. These are applied to make peel ply removal easier and the most effective approach is to abrade and apply a solvent wipe after peel ply removal, otherwise these release agents can result in low bond strength. More recently Hart-Smith signalled the adverse influence of pre-bond moisture, that is unable to escape during the bond cycle (Hart-Smith, 1999, 2002). He noted that this may prevent adhesion even more effectively than a layer of silicone. Chin and Wightman studied the influence of three surface treatments, peel ply, grit blast and oxygen plasma treatment on a carbon fibre reinforced epoxy composite (Chin and Wightman, 1994). They showed that all reduced contact angles compared to as-received composites. All resulted in significantly improved double lap shear strength when tests were performed under ambient conditions. However, when tests were run in a hot wet environment grit blasting resulted in lower strengths than the asreceived reference samples and the improvement caused by the other surface treatments was marginal. This underlines the importance of durability tests to ensure the long term effectiveness of surface treatments. In addition to the surface treatment the interfacial fibre orientation at the bonded surface also influences the bond strength. For example Johnson and Mall examined crack behaviour at 0ë, 45ë and 90ë interfaces in carbon/epoxy specimens under cyclic loading (Johnson and Mall, 1986). The damage initiation stresses were similar for 0ë and 45ë interfaces as debonding in the adhesive was the main failure mechanism. At 90ë interfaces, however, damage initiation occurred much earlier as matrix cracking was observed in the composite plies. This resulted in a much lower allowable stress level indicating that 90ë interface plies should be avoided. For glass/polyester composites chopped strand mat layers are often placed at the surface to be bonded as there is a widely held belief that this improves adhesion strength. However, tests have shown that the

Bonding of composites

287

assembly strength is significantly higher when woven layers rather than mat layers are placed in contact with the adhesive (Roy, 1994).

13.5 Testing Testing is essential to ensure the short- and long-term performance of adhesively bonded composite structures. It is covered in detail in another chapter of this book but tests to obtain the data needed for design of composite assemblies may be grouped in four types: · · · ·

adhesive properties substrate (composite) properties composite assembly tests sandwich interface adhesion tests.

The standard tests are grouped in annual reference books (e.g. ASTM, 2003), and that volume lists over 130 test methods. Adhesives are generally designed to work in shear, so details of their stress-strain behaviour under shear loading are the first requirement for design. There are many tests available to obtain these characteristics but the most reliable is the thick adherend shear test (TAST), ASTM D5656. This was developed by Krieger and is described elsewhere in this volume. The disadvantage of this type of test is that it provides adhesive behaviour under only one type of loading. For this reason much effort has been devoted to developing tests which allow a complete fracture envelope to be obtained. One example is based on the Arcan fixture (Arcan et al., 1987). An example of an Arcan fixture is shown in Fig. 13.5. The disadvantage of this type of fixture is that high stress concentrations occur at the specimen edges. The use of profiled substrates enables these to be minimised and recent work has enabled an adhesive fracture envelope to be obtained over a wide range of tension, shear and compression loading conditions (Cognard et al., 2005). In order to measure the properties of the composite substrates to be used in assembly modelling there is an extensive range of tests available, but as noted previously the tests for through-thickness tensile and shear strengths are not easy to run. Tensile strength may be determined by bonding metal blocks to both faces of a waisted composite specimen and pulling them apart (Mespoulet et al., 1996) but this requires careful machining. Shear strength may be measured by a double notched (Iosipescu) specimen in a special test fixture using the standard method ASTM D5379. The Arcan fixture (Fig. 13.5) can give more information on the full failure envelope of composites as well as adhesives but is limited by stress concentrations. An alternative is to apply fracture mechanics tests, which enable interlaminar composite fracture resistance to be quantified under simple loadings. Fracture mechanics based design methods have so far seen few applications and one of the reasons for this has been the lack of standard test methods. This

288

Adhesive bonding

Figure 13.5 Arcan fixture to measure failure strength envelope.

situation is evolving and ESIS (European Structural Integrity Society) has developed test methods which include a mode I DCB (double cantilever beam) test for adhesively bonded composite fracture toughness (Blackman and Kinloch, 2001). Using this test for mode I and the MMB (mixed mode bending) test fixture for mixed mode loading it is possible to obtain mixed mode fracture envelopes for adhesively bonded composites. Figure 13.6 shows an example for a glass/epoxy composite and its bonded assemblies (Ducept et al., 2000). Other workers have presented similar results for carbon/epoxy joints (e.g. Blackman et al., 2001; Ashcroft et al., 2001). Once such data are available their use in structural analysis may become more widespread. An alternative approach to determining input data for failure criteria is to run tests directly on composite assemblies and to analyse the specimen failure. The

Bonding of composites

289

Figure 13.6 Mixed mode failure envelopes, quasi-unidirectional glass/epoxy delamination and debonding of same composites in adhesively bonded assembly with ductile epoxy adhesive. Values correspond to onset of crack propagation at non-linearity (NL) on load-displacement plot.

most common are tests on lap shear specimens. These can provide information on failure modes and potential problems due to poor surface preparation or curing errors, and are therefore useful for quality control. They can also be used to check modelling assumptions, and to adjust analyses without the expense of full-scale testing. However, it should be emphasised that while the loading is usually simple, in these tests the stress state in the overlap is certainly not. Analysis of `simple' lap shear tests has been the subject of a vast number of research papers since the first analytical studies over 60 years ago. The development of computing power now enables full 3-D analyses to be run with geometrical and material non-linearity including adhesive viscoplasticity (Pandey and Narasimhan, 2001; Goncalves et al., 2002). The results obtained indicate significantly different stress distributions from those predicted by simpler plane strain analyses. For sandwich materials there are few standard test methods to characterise the adhesive bond between composite facings and core materials. Several test variants have been proposed which impose a largely mode I (peel) loading on the interface. Two examples are shown in Fig. 13.7. The first is the climbing drum peel test described in ASTM D1781. This may be useful for thin facings but in many cases it is not possible as the facings are too thick. Special tests have therefore been developed and again fracture mechanics has proved useful, particularly to quantify resistance to interfacial crack propagation. Figure 13.7(b) shows one such test developed by Cantwell et al. based on the single cantilever beam specimen (Cantwell et al., 1994, 1996). The loading of short sandwich beams in flexure introduces mainly shear loading (mode II), and a

290

Adhesive bonding

Figure 13.7 Tests to study sandwich facing/core interface bonding: (a) Climbing drum, (b) Single cantilever beam, (c) Mode II shear, (d) TSD mixed mode.

French standard (NFT54-606) for testing an uncracked beam has been proposed and is sometimes used as a quality control check for the core/facing interface. Cracked beams have been tested in flexure by Carlsson and colleagues, Fig. 13.7(c) (Carlsson et al., 1991). Finally, another configuration, designed to promote mixed mode loading of the sandwich interface, is based on the TSD (Tilted Sandwich Debond) specimen, Fig. 13.7(d). This was proposed by Grenestedt and then developed by Li and Carlsson (1999). It should be emphasised that the development of these sandwich interface fracture tests is quite recent and relatively few data are available. None of these tests has been standardised yet and several variants exist. Nevertheless, such tests offer the potential to obtain quantitative data under both quasi-static and higher rate loading conditions. Significant efforts are being made to characterise adhesives and adhesively bonded assemblies. The rapid progress made in video extensometry in recent years, allowing both damage detection and the measurements of full field displacements through image analysis techniques, is providing new tools and bonded assemblies are receiving considerable attention (Court and Sutcliffe, 2001; Davies and Sargent, 2003).

Bonding of composites

291

13.6 Influence of bondline thickness When composites are bonded in aerospace structures the adhesive is generally a film, often on a light woven textile base. This allows constant bondline thickness to be achieved, typically a few tenths of a millimetre, when structures are cocured in an autoclave. In many other applications the adhesive is applied in the form of a one- or two-component paste, applied manually, and the bondline thickness is not controlled. The final adhesive thickness then depends on the geometry of the parts and the local pressures applied and is often much thicker. In the large structures typical of marine applications there may also be significant variations of bondline thickness. This parameter can significantly affect the development of the plastic zone in more ductile adhesives. Several authors have examined the influence of bondline thickness on joint strength, and fracture mechanics tests have proved useful for this type of study (Bascom et al., 1975; Kinloch and Shaw, 1981). Typically, as bondline thickness increases, fracture resistance increases up to a maximum and this has been explained in terms of the development of the adhesive plastic zone. At higher thickness plateau values are obtained, but few results are available for composite substrates and adhesives thicker than 2 mm. In a recent study for marine applications, the EUCLID RTP 3.21 project (see section 13.10), joints consisting of two adhesives on infused glass reinforced composite substrates were characterised (Davies et al., 2003). One adhesive was a rigid epoxy (Araldite 2015, Young's modulus 1.8 GPa, strain to failure <5%), the second was a much more ductile polyurethane based adhesive (Axson 220, Young's modulus 50 MPa, strain to failure >50%). Figure 13.8 shows an example of the crack propagation in thin and thick bondlines, and the mode I fracture toughness results obtained. It is apparent that there is little influence for the more rigid epoxy and for this material the crack propagated within the adhesive layer. In the more ductile adhesive the crack oscillated between the adhesive/composite interfaces. Variability of measured values was higher but significantly higher fracture resistance was measured when the ductile adhesive was used in a thick bondline. More details of this study are given in (Davies et al., 2005).

13.7 Examples of bonded composite structures Three examples of adhesively bonded composite structures will be described briefly. The first is the bonding of composite pipework, one of the largest industrial composite assembly operations. The second is a composite/metal assembly used for large ship structures. The third is the bonding of stiffeners to sandwich panels. The assembly of 12-metre lengths of filament wound tubes into pipelines which extend tens of kilometres involves hundreds of adhesive joints which

292

Adhesive bonding

Figure 13.8 Influence of bondline thickness on mode I fracture toughness for propagation between glass/vinylester adherends, two adhesives. Mean values from over 500 data points, error bars show standard deviations. Insert photos show epoxy specimen edges. (a) 1 mm, (b) 4 mm bondline.

must be performed in the field often under difficult environmental conditions. Fluid transport, firewater and cooling systems are produced in this way. The main design requirement is water-tightness, and considerable work has been performed to ensure that reliable pipe-to-pipe joints can be produced economically. Pipe suppliers have developed bonding procedures and train and certify adhesive bonders (e.g. Ameron, 1997). Figure 13.9 shows an example of the bonding operations employed to produce a bonded cone and taper connection. The tube ends are tapered and the surfaces are carefully sanded and dried. Adhesive is applied and heating blankets are placed around the joint to cure it. The success of these operations shows that adhesive bonding has developed into a viable industrial assembly method for composites. The second example is that of the connection between a steel structure, the hull of a frigate, and the composite superstructure. The introduction of composites often involves the need to connect to existing metallic structures. A patented joining method was developed for the Lafayette frigates which allowed traditional welding to be used, Fig. 13.10. This application has been described in detail elsewhere (LeLan et al., 1992) and this type of joint has been the object of much study recently (Clifford et al., 2002; Cao and Grenestedt, 2003). This is a good example of combining the advantages of composite structures, in this case low weight and stealth, with the traditional advantages of steel such as weldability and low cost. A third example is that of stiffened sandwich panels. These are complex assemblies in which large-scale bonding between sandwich facings and cores is

Bonding of composites

293

Figure 13.9 Adhesive bonding to assemble composite pipelines: (a) machining pipe ends and drying, (b) application of adhesive, (c) assembly, (d) tightening and cure (heated blanket).

combined with local bonding between the other side of the facing and the stiffener. Such structures are common in many transport applications. Three examples are shown, in the first, Fig. 13.11(a), the stiffener is produced by hand laminating over a foam profile onto an existing glass reinforced composite balsa sandwich panel. An alternative approach is to produce the panel and stiffener

Figure 13.10 Metal/composite hull/superstructure connection.

294

Adhesive bonding

Figure 13.11 Sections through stiffened sandwich panel assemblies: (a) traditional overlaminated stiffener (hand layup) on glass fibre composite/balsa sandwich; (b) adhesively bonded pultrusion on glass fibre composite/balsa sandwich; (c) carbon reinforced composite stiffener of honeycomb sandwich.

Bonding of composites

295

separately, then to adhesively bond them together. An example of this design is shown in Fig. 13.11(b) and involved a pultruded stiffener. This is further described in Davies et al. (2005). In the third example, Fig. 13.11(c), the stiffened panel is produced by co-bonding under vacuum the stiffener and the carbon reinforced composite honeycomb sandwich. Considering first the sandwich assembly, there have been several published studies of interface behaviour. The difficulties encountered are strongly dependent on the type of core employed and the manufacturing parameters. Thus balsa wood is very sensitive to moisture and is generally primed to prevent excessive resin entry. The interface in a composite/honeycomb sandwich is more complex as the cells are much larger and the adhesive fillet, Fig. 13.12, appears to play an important role in interface crack propagation (Okada and Kortschot, 2002). This is not surprising as it has been shown in the past that adhesive spew fillets can strongly influence the strength of composite lap joints (Adams et al., 1997). Indeed, in Fig. 13.4 the failure load of a single lap shear specimen with a fillet is shown to exceed that of specimens with the same geometry for which the fillet was removed, by over 50%. The extent of the fillet will depend on the adhesive viscosity at the forming temperature but also on the wettability and the geometry of the honeycomb and the stiffness of the facings. With respect to the stiffener/facing interface several authors have worked on stiffened composite panels recently. For example, Shenoi and colleagues have worked on the analysis of top hat and T-stiffeners (Shenoi et al., 1995; Phillips et al., 1999) and used both stress-based and fracture mechanics approaches to examine damage mechanisms. In the cases studied it was mainly delamination in the curved area of the overlaminate rather than the stiffener/facing interface which was responsible for failure. For marine applications some guidelines on stiffener design can be found in the rules of the certification societies, much of which is based on developments for mine-hunter vessels. For example minimum flange overlaps and radii are given. Smith has summarised much of this early

Figure 13.12 Examples of adhesive at facing/honeycomb interfaces: (a) 1 mm thick facing, large resin meniscus; (b) 2.5 mm facing, little honeycomb wetting.

296

Adhesive bonding

work (Smith, 1990). A fracture mechanics approach has been used by Minguet and O'Brien to analyse stringer debonding for an aerospace application (Minguet and O'Brien, 1995).

13.8 Durability and long-term performance Concern about their long-term behaviour has been identified as one of the main factors limiting the adoption of adhesive bonding, particularly with respect to the influence of the environment. This subject has therefore received considerable attention. Special test methods have been developed to evaluate durability such as the Boeing Wedge test, but results tend to be qualitative indicating problems in surface preparation or adhesive cure, rather than quantitative (Hart-Smith, 1999). Kinloch has reviewed methods for the prediction of lifetime (Kinloch, 1994). More recently Crocombe has suggested how durability effects may be introduced into modelling tools and has predicted environmental degradation with some success for bonded aluminium (Crocombe, 1997). With respect to composite bonding it is moisture effects which can be particularly harmful. The analysis is complicated by the anisotropic nature of diffusion in composite materials and moisture ingress through the composite may degrade either the substrate or the composite/ adhesive interface. An example is shown in Fig. 13.13 for the case of a composite/steel assembly (Davies et al., 1999). If loads are applied during wet ageing the failure mechanisms may change. For the materials shown in Fig. 13.13 a high applied load during immersion results in failure in the composite while lower applied loads lead to failure at the steel/adhesive interface (Roy et al., 2000). This underlines the complexity of long-term predictions for such assemblies where the kinetics of multiple failure mechanisms must be evaluated. An additional concern has been expressed for assemblies involving carbon fibre reinforced composites with other materials as electrochemical coupling may result in degradation of the less noble material (Brown et al., 1995). Care is needed to avoid such effects, particularly in marine applications.

13.9 Future trends From a design point of view development is currently focused on interface models and cohesive zone descriptions of critical regions in joints, in order to enable more efficient numerical analyses to be performed. Several authors have described recent developments in this area (Mi et al., 1998; Blackman et al., 2003). With respect to material developments, the previous sections have shown that the strength of adhesively bonded composites is frequently limited by the through-thickness strength of the composite itself. Many developments have

Figure 13.13 Influence of wet aging on steel/glass fibre reinforced polyester composite bond strength, ductile epoxy adhesive, insert photo shows specimens after 12 months in tidal zone.

298

Adhesive bonding

been made to improve this strength. The use of tough thermoplastic matrix polymers such as PEEK is one approach, and this may also allow novel fusion bonding techniques to be applied (Davies et al., 1991; Ageorges et al., 2001). Local heating of the matrix polymer or the insertion of thermoplastic films with lower melting temperatures can provide excellent bond strengths and potential for repair. Considerable work has been published in this area but so far few applications have been described. An alternative and very promising approach that can be applied to traditional thermoset composites is to reinforce the composite locally in the thickness direction. Stitching is one option and it was shown many years ago that it could improve delamination resistance in composites (e.g. Guess and Reedy, 1985). More recent applications have included bonded composite joints (Tong and Jain, 1995). A different approach to through-thickness reinforcement is known as Zpinning (Partridge et al., 2003). This involves the insertion of stiff pins (ZfibersTM) into prepreg or wet laminate stacks using ultrasonic vibration. Figure 13.14 shows the application of such pins to the overlap region of a top hat stiffener. Pinning can be applied selectively to areas where out-of-plane loading is expected, such as around stiffeners or cut-outs, and can provide an order of magnitude improvement in joint strength. Figure 13.15 shows an example of the increase in force necessary to propagate a crack through the pinned area in a DCB specimen, the insert photo shows the corresponding fracture surfaces after propagation. The protruding pins are clearly in evidence, and energy is dissipated in overcoming friction during pullout. There are several parameters which can be tailored for a particular application, including pin diameter and density. When these are optimised the energy needed to cross the pinned region can be so high that the crack is completely blocked. Considerable development work has been performed at Cranfield University to understand the fracture mechanisms in these pinned materials (CartieÂ, 2000). The Z-pin preforms (Zpins plus foam) are commercially available from Aztex Inc. (see section 13.10).

Figure 13.14 Application of carbon Z-pins to an uncured stiffener overlap using ultrasonic gun.

Figure 13.15 Example of load-displacement plots from mode I tests on DCB carbon/epoxy, with and without pins. Insert photo shows fracture surfaces of pinned specimen.

300

Adhesive bonding

Through-thickness pinning appears to be one of the most promising techniques to improve adhesively bonded joint performance of existing materials provided they can be inserted in the uncured materials (i.e. assemblies produced by co-curing). Applications already include military aircraft and helicopters, and racing cars, and trials are underway for racing yachts. The local nature of this approach, which only adds reinforcement at `hot spots' in the structure, is expected to become more widespread in the near future. Another recently developed product is pinned sandwich material, X-cor, which may find applications where the sandwich interface must resist crack propagation for a minimum weight (Palazotto et al., 1999; Partridge et al., 2003; Cartie and Fleck, 2003). Again pinning may be applied locally in critical areas.

13.10 Sources of information There are many sources of information on adhesive bonding of composites. The internet is particularly useful to obtain data rapidly. Adhesive manufacturers provide guidelines on surface preparation, mechanical properties and design. Useful information on composite bonding is available at the following sites: http://www.loctite.com/int_henkel/loctite/entry.cfm http://www.adhesives.vantico.com/ourProducts/compositeBonding/ composite_bonding.rhtm http://www.3m.com/us/auto_marine_aero/aero/catalog/index.jhtml http://www.permabond.com http://www.hexcelcomposites.com/Markets/Products/Adhesives/Selector.htm amongst others. A large amount of background information on the performance of adhesive joints is available in the final reports of DTI project 1992±1995, Measurement Technology and Standards programme, from AEA Technology. The DOGMA project website provides information on many aspects of material assembly including composite design: http://www.dogma.org.uk/dogma/vtt/index.htm Two recently completed projects run by DNV in Norway on adhesive bonding of composites for marine applications are the EUCLID project RTP 3.21 (http:// research.dnv.com/euclid_rtp3.21/) and the Bondship project (http:// research.dnv.com/bondship/). Results from these two projects will be published in 2005 in a special edition of the Journal of Engineering for the Marine Environment, published by the Institute of Mechanical Engineers, London. Books devoted to this subject include Composites bonding, ASTM STP 1227, edited by Damico DJ, Wilkinson TL, Niks SFL, 1993. More general references which discuss the design of composite assemblies include:

Bonding of composites

301

Adams RD, Comyn J, Wake WC, Structural Adhesive Joints in Engineering, 2nd edn, Chapman & Hall, 1997. Tong L, Steven GP, Analysis and Design of Structural Bonded Joints, Kluwer Academic Press, 1999. Concerning through-thickness reinforcement (Z-fibres) more information can be found at: www.aztex-z-fiber.com

13.11 References Adams RD, Comyn J, Wake WC, Structural Adhesive Joints in Engineering, 2nd edn, Chapman & Hall, 1997. Ageorges C, Ye L, Hou M, `Advances in fusion bonding techniques for joining thermoplastic matrix composites: a review, Composites Part A', Applied Science and Manufacturing, 32, 6, June 2001, 839±857. Allix O, LadeveÁze P, `Interlaminar interface modelling for the prediction of delamination', Comp. Structures, 22, 1992, 235±242. Allix O, LeÂveÃque D, Perret L, `Identification and forecast of delamination in composite laminates by an interlaminar interface model', Comp. Sci. & Tech., 58, 1998, 671± 678. American Society for Testing and Materials ASTM, 2003, Annual book of standards, Volume 15.06 Adhesives. Ameron, Assembly instructions for Quick-Lock adhesive bonded joints, 1997. Arcan L, Arcan M, Daniel IM, SEM fractography of pure and mixed mode interlaminar fracture in graphite/epoxy composites, ASTM STP 948, 1987, 41±47. Ashcroft IA, Hughes DJ, Shaw SJ, `Mode I fracture of epoxy bonded composite joints, Part I Quasi-static loading', Int. J. Adhesion and Adhesives, 21, 2001, 87±99. Bascom WD, Cottington RL, Jones RL, Peyser P, J. Applied Polymer Sci., 19, 1975, 2545. Blackman BRK, Kinloch AJ, `Determination of the mode I adhesive fracture energy GIc of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens, ESIS TC4 protocol 2000, also BS 7991', in Fracture mechanics testing methods for polymers, adhesives and composites, eds Moore DR, Pavan A, Williams JG, ESIS Publication 28, Elsevier, 2001 pp 225±267. Blackman BRK, Kinloch AJ, Paraschi M, `The failure of adhesive joints under modes I and II loading', Proc. SAE6, Bristol July 2001, pp 103±106. Blackman BRK, Hadavinia H, Kinloch AJ, Williams JG, `The use of a cohesive zone model to study the fracture of fibre composites and adhesively-bonded joints', Int. J. Fracture, 119, 2003, 25±46. Brown R, Ghiorse S, Qin J, Shuford R, `The effect of carbon fiber type on the electrochemical degradation of carbon fiber polymer composite', Proc. Corrosion 95, NACE, 1995 paper 275. Cantwell WJ, Davies P, `A test technique for assessing skin/core adhesion on sandwich composite structures', J. Materials Science Letters, 13, 1994, 203. Cantwell WJ, Davies P, `A study of skin-core adhesion in glass fibre reinforced sandwich materials', Applied Composite Materials, 3, 1996, 407±420.

302

Adhesive bonding

Cao J, Grenestedt JL, `Test of a redesigned glass-fiber reinforced vinyl ester to steel joint for use between a naval GRP superstructure and a steel hull', Composite Structures, 60, 4, 2003, 439±445. Carlsson LA, Sendlein LS, Merry SL, `Characterization of face/core shear fracture of composite materials', J. Comp. Materials, 25, 1991, 101. Cartie DDR, PhD thesis, Effect of Z-FibresTM on the delamination behaviour of carbon fibre/epoxy laminates, Cranfield University, UK, 2000. Cartie DDR, Fleck N, `The effect of pin reinforcement upon the tthrough thickness compressive strength of foam-cored sandwich panels', Comp. Sci. & Tech., 63, 2003, 2401±2409. Chin JW, Wightman JP, `Surface pretreatment and adhesive bonding of carbon fiber reinforced epoxy composites', in ASTM STP 1227, 1994, pp 1±16. Clifford SM, Manger CIC, Clyne TW, `Characterisation of a glass-fibre reinforced vinylester to steel joint for use between a naval GRP superstructure and a steel hull', Composite Structures, 57, 2002, 59±66. Cognard J-Y, Davies P, Gineste B, Sohier, `Development of an improved adhesive test method for composite assembly design', Comp. Sci. & Tech, 65, 2005, 359±368. Court RS, Sutcliffe MPF, Tavakoli SM, `Ageing of adhesively bonded joints ± fracture and failure analysis using video imaging techniques', Int J. Adhesion and Adhesives, 21, 6, 2001, 455±463. Crocombe AD, `Durability modelling concepts and tools for the cohesive environmental degradation of bonded structures', Int J. Adhesion and Adhesives, 17, 1997, 229±238. Davies P, Sargent JP, `Fracture mechanics tests to characterize bonded glass/epoxy composites: application to strength prediction of structural assemblies', Proc. 3rd ESIS conference on Fracture of Polymers, Composites and Adhesives, Elsevier, 2003. Davies P, Cantwell WJ, Jar P-Y, Bourban P-E, Zysman V, Kausch HH, `Joining and repair of a carbon fibre-reinforced thermoplastic', Composites, 22, 6, November 1991, 425±431. Davies P, Roy A, Gontcharova E, Gacougnolle J-L, `Accelerated marine aging of composites and composite/metal joints', Proc. DURACOSYS 1999, Balkema, p 253. Davies P, Loaec H, Reynaud S, Ferreira A, Hentinen M, Hildebrand M, Mustakangas M, Gaarder R, Carli F, van Straalen IJ, Sargent JP, Adams RD, Broughton J, Beevers A, `Failure of bonded glass/epoxy composite joints: A benchmark study and correlation with test results', Proc. SAE6, Bristol July 2001, pp 233±237. Davies P, Choqueuse D, Bigourdan B, Gauthier C, Joannic R, Parneix P, L'hostis J, `Design, manufacture and testing of stiffened panels for marine structures using adhesively-bonded pultruded sections', Journal of Engineering for the Maritime Environment, 2004, 218(M4), pp 227±234. Ducept F, Davies P, Gamby D `Mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite/composite joint', Int J. Adhesion & Adhesives, 20, 3, 2000 233±244. EleÂgoet JY, PhD thesis, Approche numeÂrique et expeÂrimentale pour l'eÂtude du comportement et de la tenue de liaisons colleÂes de mateÂriaux composites, June 2000, CTA Paris. Goncalves JPM, de Moura MFSF, de Castro PMST, `A three-dimensional finite element model for stress analysis of adhesive joints', Int J. Adhesion & Adhesives, 22, 2002, 357±365.

Bonding of composites

303

Guess TR, Reedy ED, `Comparison of interlocked fabric and laminated fabric Kevlar 49/ epoxy composites', J. Comp. Tech & Research, 7, 4, 1985, 136. Hart-Smith LJ, `A peel-type test coupon to assess interfaces in bonded, co-bonded and cocured composite structures', Int. J. Adhesion & Adhesives, 19, 1999, 181±191. Hart-Smith LJ, `Adhesive bonding of composite structures ± Progress to date and some remaining challenges', J. Comp. Tech. and Research, 24, 3, July 2002, 133±153. Hinton MJ, Kaddour AS, Soden PD, `A comparison of the predictive capabilities of current failure theories for composite laminates, judged against experimental evidence', Comp. Sci. & Tech., 62, 2002, 1725±1797. Johnson WS, Mall S, `Influence of interface ply orientation on fatigue damage of adhesively bonded composite joints', Comp. Tech. Review, 8, Spring 1986, 3±7. Kinloch AJ, Shaw SJ, J. Adhesion, 12, 1981, 59±77. Kinloch AJ, Adhesion and Adhesives, Chapman & Hall, 1987. Kinloch AJ, Predicting the lifetime of adhesive joints in hostile environments, MTS Adhesives project, Report 5, DTI, 1994. LeLan JY, Parneix P, Gueguen PL, `Composite material superstructures, Proc. 3rd Ifremer conference on Nautical Construction with Composites, IFREMER publication 15, 1992, pp 399±411. Li X, Carlsson LA, `The tilted sandwich debond (TSD) specimen for face/core interface fracture characterization', J. Sandwich Struct. Mater., 1, 1999, 60±75. Lodeiro MJ, Broughton WR, Sims GD, `Understanding the limitations of throughthickness test methods', Proc. 4th European conf. On Composites Testing and Standardisation, Lisbon September 1998, Inst. of Materials, London, 80±90. Mespoulet S, Hodgkinson JM, Matthews FL, Hitchings D, Robinson P, `A novel test method to determine the through thickness tensile properties of long fibre reinforced composites', Proc ECCM7, Vol 2, Woodhead, 1996, pp 131±137. Mi Y, Crisfield MA, Davies GAO, Hellweg HB, `Progressive delamination using interface elements', J. Comp. Materials, 32, 14, 1998, 1246±1272. Minguet PJA, O'Brien TK, `Analysis of composite/stringer bond failure using a strain energy release rate approach', Proc. ICCM10, I, 1995, pp 245±252. Okada R, Kortschot MT, `The role of resin fillet in the delamination of honeycomb sandwich structures', Composites Science and Technology, 62, 14, 2002, 1811± 1819. Palazotto AN, Gummadi LNB, Vaidya UK, Herup EJ, `Low velocity impact damage characteristics of Z-fibre reinforced sandwich panels an experimental study', Comp. Struct., 43, 1999, 275±288. Pandey PC, Narasimhan S, `Three-dimensional nonlinear analysis of adhesively bonded lap joints considering viscoplasticity in adhesives', Computers & Structures, 79, 2001, 769±783. Papini M, Fernlund G, Spelt JK, `The effect of geometry on the fracture of adhesive joints', Int. J. Adhesion & Adhesives, 14, 1, 1994, 5±13. Partridge IK, Cartie DDR, Bonnington T, Manufacture and performance of Z-pinned composites in Advanced polymeric materials: structure-property relationships, ed. S. Advani, G. Shonaike, CRC Press, 2003, Chapter 3. Phillips HJ, Shenoi RA, Moss CE, `Damage mechanics of top hat stiffeners used in FRP ship construction', Marine Structures, 12, 1999, 1±19. Roy A, `Comportement meÂcanique en sollicitations monotone et cyclique d'assemblages colleÂs composite-composite et composite-acier', PhD thesis ENSMA/Universite de

304

Adhesive bonding

Poitiers, 1994. Roy A, Gontcharova E, Gacougnolle J-L, Davies P, `Hygrothermal effects on failure mechanisms in composite/steel bonded joints', ASTM STP 1357, 2000, 353±371. Shenoi RA, Read PJCL, Hawkins GL, `Fatigue failure mechanisms in fibre reinforced plastic laminated tee joints', Int. J. Fatigue, 17, 6, 1995, 415±426. Smith CS, Design of Marine Structures in Composite Materials, London, Elsevier Applied Science, 1990. Soden PD, Hinton MJ, Kaddour AS, `A comparison of predictive capabilities of current failure theories for composite laminates', Composites Science and Technology, 58, 7, 1998, 1225±1254. Tong L, Jain LK, `Analysis of adhesive bonded composite lap joints with transverse stitching', Applied Composite Materials, 6, 1995, 343±365.

Acknowledgements Thanks to J-Y. Cognard and L. Sohier for Fig. 13.5, J. Steen and R. Hofstede of Ameron for Fig. 13.9 and to D. Cartie of Cranfield University for the photo in Fig. 13.15.

14

Building and construction ± steel and aluminium IJ J VAN STRAALEN AND M J L VAN TOOREN

14.1 Basic needs Steel and aluminium are used for a wide variety of applications within the building and construction industry. The application of adhesive bonding as a joining technique on the other hand is less common. Sandwich wall and roof panels with thin gauge steel or aluminium faces bonded onto a thick core material made from foam or mineral wool material are very common for company complexes. House structures made of thin steel gauge members covered by wooden plate material, are promoted by the steel industry. Instead of using mechanical fasteners, adhesive bonding of the plate material on the steel joists seems to be a reliable alternative. Another example of a new alternative is the composition of aluminium extrusions which are bonded together. The advantages of adhesive bonding compared to welding for this alternative are the absence of a heat affected zone with lower strength properties, the larger tolerances that can be met and the possibility to reduce costs. Reduction of costs is also a motive in the design of offshore steel structures. Adhesive bonded joints with favourable fatigue properties might be a serious alternative for welded joints mostly used nowadays. The example of a building with a structure of steel and plywood as given in Fig. 14.1, illustrates the potential of adhesive bonded joints applied for steel and aluminium structures within building and construction. For a successful application it is necessary to incorporate the generally known adhesive bonding technology within the daily design practice of building and construction. Within the design process an initial, a conceptual, an optimisation and a validation phase can be distinguished. In the initial phase the problem statement and so-called objective functions are formulated. The aim of these objective functions is to quantify the required performances under given operating circumstances. The conceptual phase ends up with a number of potential outline solutions. The formulation of these is based on experience, insight and creativity of the engineer. The optimisation phase results in a final design and the validation phase shows formally that this final design meets the requirements.

306

Adhesive bonding

Figure 14.1 Example of a building with a structure of steel and plywood bonded together.

To support this design process for steel and aluminium in building and construction engineers make use of design codes. In Europe the Eurocodes for steel1 and aluminium2 are introduced, which contain a set of validation rules for a large variety of applications and load conditions. But the application of adhesive bonded joints is out of the scope of these two design codes. To apply adhesive bonded joints the following topics have to be covered within each phase of the design process: · the selection of an adhesive and surface preparation · the structural design of the joint and the validation of this design · the manufacturing and the quality assurance. Various issues related to these topics, which have to be considered by the engineers active within the building and construction industry, are discussed within the continuation of this chapter.

14.2 Adhesive characteristics required To select an adhesive for a given design, various issues have to be considered. Mechanical actions such as static loads, impact loads, long-term static loads and

Building and construction ± steel and aluminium

307

Figure 14.2 Mechanical characteristics of adhesives.

low or high cycle fatigue loads, should be identified. The result of this identification will give an idea of the stress and strain levels active within the adhesive. The matching required characteristics of an adhesive vary between high and low strength, and brittle and flexible as indicated in Fig. 14.2. Environmental actions which are active for the total lifetime of the joint should also be identified. Required adhesive characteristics for indoor applications differ, for example, significantly from those required for offshore applications. The performance of the joint under these mechanical and environmental actions also depends upon the geometry of the joint. A bondline loaded in shear is preferred and penetration of water can be avoided by selecting a proper shape for the adhesive bonded joint. Other issues to be considered during design are the steps to be followed during manufacturing. These steps are directly related to the required process properties of the adhesive such as viscosity, curing time, pot life, open time, need of pressure during curing stage, and allowable bondline thicknesses. All these issues should be taken into account during the optimisation of the design. Various kinds of adhesives are available for bonding steel and aluminium. For most common types a short description is given in this section, including an indication of the suitability for building and construction.

308

Adhesive bonding

Hot-melt A hot-melt can be applied by heating it until it becomes fluid. After cooling (normally within a few seconds) the adhesive becomes solid again and the substrates are connected. The properties of the hot-melt are determined by the composition of polymers, resins, waxes and other additions. So-called reactive hot-melts, e.g., based on polyurethane, have the advantage that after cooling, a curing process starts. An advantage of a hot-melt is the short time for fixing and a disadvantage is its poor creep properties. Within the building industry a hotmelt is more and more used for sandwich panels nowadays. Contact adhesive Another type of adhesive also used for panels like doors is a contact adhesive. The adhesive has to be applied to both substrates, which can be put together after approximately ten minutes. Due to its poor structural properties, a contact adhesive will mostly not be used for steel and aluminium structures. Anaerobic adhesive The curing process of an anaerobic adhesive starts as soon as it is in contact with a metal and is excluded from oxygen. At room temperature the connection can be handled after 30 minutes and is full cured within 3 to 24 hours. To get a proper strength, the bondline thickness has to be limited to 0.05 mm. The advantages of an anaerobic adhesive are its strength properties, also in fatigue, and its chemical resistance. Due to its characteristics, an anaerobic adhesive is used only for smaller metals parts and for locking bolts. Its use for building and construction is very rare. Acrylic adhesive Acrylic adhesives are two-part systems, of which part A is put on one substrate and part B on the other. After putting both substrates together the curing process is started. In contrast with the epoxy and polyurethane adhesives to be discussed later, the added initiator and accelerator will not react with the resin. The curing period is only a few seconds and after a few minutes final strength can be reached. An acrylic adhesive can be used for steel and aluminium, but it is known from experience that under long-term static loads and humidity the properties becomes less. Cyanoacrylate adhesive For a cyanoacrylate adhesive the curing time is also a few seconds and results in strong bonds. This process is initiated by humidity. Two significant

Building and construction ± steel and aluminium

309

disadvantages of a cyanoacrylate adhesive for steel and aluminium in building and construction are its brittle behaviour and its short processing time. Epoxy adhesive The curing process of an epoxy adhesive is a reaction between the resin and hardener. In the case of a one-component system, both parts are mixed and cure at higher temperatures above 120 ëC. For a two-component system both parts have to be mixed in the prescribed ratio and will cure at room temperature or elevated temperatures (e.g. 50 ëC). To avoid brittle behaviour most modern epoxy adhesives available are toughened by using additives. Due to their higher strength properties, epoxy adhesives can be used for steel and aluminium in building and construction. An example is the use of epoxy adhesives to bond steel plates onto existing concrete structures for strengthening. Polyurethane adhesive Polyurethane adhesives are also available in one- and two-component versions. The one-component version cures as a result of a reaction with moisture from the air. Strength properties are low. Within the Netherlands one-component versions are popular for bonding the face of a sandwich panel used for buildings. For the two-component version both parts have to be mixed in the prescribed ratio and will normally cure within one to eight hours. Final strength is reached within one week. The selection of additives influences the strength, adhesion, toughness, temperature resistance and curing time. Due to their favourable strength properties, two-component polyurethane adhesives can be used for steel and aluminium in building and construction. MS-polymer adhesive A MS-polymer adhesive (modified silane) behaves like a sealant; its strength properties are relatively low and large deformations are possible. Normally thicker bondlayers of a few millimetres have to be used. A MS-polymer adhesive is a onecomponent system, which cures as a result of a reaction with moisture from the air and it reaches its final strength after one week. Structural glazing is an example of a MS-polymer used for steel and aluminium in building and construction. It is noted here that these kinds of flexible adhesives can also be based on polyurethane (one- or two-component) instead of modified silane chemistry.

14.3 Surface preparation To influence the behaviour of the layer between the adhesive bond line and steel or aluminium adherends, different techniques have been developed to control

310

Adhesive bonding

and optimise the surface properties. The purpose of surface pretreatments is to remove contaminations and weak surfaces, to improve the adsorption of the adhesive onto the solid surface (good wettability) resulting in a good bonding, to make a conversion by modifying the texture of the surface or to add an additional layer. The available techniques are based on the principles of degreasing, chemical cleaning, mechanical cleaning and chemical conversion. Additional layers such as primers are used to change the adherend surface geometry or to introduce new chemical groups to provide a better bonding. The selection of the proper surface preparation is directly related to the alloy used and the selected adhesive. Essential within the selection process of a surface pretreatment is the estimation of the degradation of the properties due to ageing. These are not only influenced by the selected surface preparation, but also by the alloy and the adhesive used. It is difficult to quantify the degradation effects in general due to the large number of parameters influencing the degradation process, of which some are even unknown. The degradation properties can only be determined by tests for a given adhesive bonding system. For the selection of a pretreatment for steel or aluminium adherends, a number of issues and matching requirements and limiting conditions are relevant. As for the selection of an adhesive, mechanical and environmental actions should be identified. The result of this identification will give an idea of the stress and strain levels, and chemical attacks active within the interface. The matching required characteristics deal with the strength and durability properties. The actual surface condition is of prime importance. As well as plane steel and aluminium, it is also possible that the adherend to be bonded is coated. The type of adhesive used also influences selection of a matching pretreatment. For higher strength applications with, for example, an epoxy adhesive the required strength and durability properties of the interface between substrate and adhesive bondlayer are more rigorous than for lower strength applications with, for example, a MS-polymer. As a part of the manufacturing process, the stability of the pretreatment until bonding has to be guaranteed and the conditions during manufacturing should be controlled. All the issues mentioned should be taken into account during the optimisation of the design. For high strength applications under severe environmental conditions various pretreatments for steel and aluminium adherends have been developed. Techniques such as chemical etching and anodising developed and successfully applied for aluminium within the aerospace industry can also be used within building and construction. Within various handbooks (see, for example, ref. 3), these techniques are described in detail. Unlike aluminium, iron does not form coherent, adherent oxides, making it difficult to create a stable film with the fine microroughness needed for good adhesion. Grit blasting is commonly used as a pretreatment, but on its own it is not suitable in cases of severe environments. An effective etching or anodising process for low-carbon steels has not been developed.

Building and construction ± steel and aluminium

311

The best approach for preparing iron and low-carbon steels to obtain durable and bondable surfaces, consists of the following processing steps. First of all rust, dirt and oil have to be removed by scraping, brushing or hammering. In the next step the surface has to be cleaned by using solvent on a cotton cloth, followed by a final brushing with a clean cotton cloth. Then the surface has to be grit blasted. Aluminium oxide seems to give the best results. After grit blasting, the surface has to be cleaned with clean compressed air. Finally a primer is preferred to get durable bonds. Within aerospace and shipbuilding it is found that one-component corrosion-inhibiting epoxy primers give good results. These primers have to be cured at a temperature of 120 ëC. Two-component primers might be used as an alternative. It should be noted that the layer thickness of the primer (3±8 m) has to be controlled, to avoid failure within the primer layer. For lower strength applications the designer might look at more simple pretreatments.

14.4 Strength and durability 14.4.1 General By choosing an adhesive bonded joint to connect two adherends, a variety of alternative geometry solutions can be applied. In structural terms a distinction can be made between adhesive bonded joints globally loaded in shear, tension or peel. A design solution loaded in shear is preferred, while solutions loaded in tension or peel should be avoided as much as possible. This is because for tension and peel the stress state in the vicinity of the bondline is dominated by high tensile stresses, which are difficult to sustain. The magnitude of these stresses is much lower for a joint primarily loaded in shear. To optimise a potential solution the configuration and dimensions of the joint can be changed. Methods to evaluate the mechanical performance of adhesive bonded joints can be based on either theory or tests. Various analytical solutions have been proposed over the years and with finite element methods it is possible to make detailed calculations including physical and geometrical non-linear behaviour. Issues directly related to these theoretical analyses are the determination of material properties and the selection of a proper failure criterion. A test programme on the other hand focuses on particular applications. Theoretical analyses are mostly used for optimisation purposes, while tests are mostly used to validate a final design. Methods to evaluate degradation effects after ageing of adhesive bonded joints are based on tests. The ageing process is accelerated by intensifying the environmental actions in a climate chamber. Issues related to these tests are the definition of the environmental actions, how to accelerate the tests and the translation of the results for in-use conditions. These tests are mostly used to select an appropriate adhesive bonding system.

312

Adhesive bonding

Within the building and construction industry most designs are unique and have to be developed with a restricted budget and within a limited period of time. For this reason the number of tests has to be limited and the validation of the design is mainly based on design rules. To fulfil the safety requirements as defined for steel and aluminium structures, these design rules have to be based on the generally accepted philosophy of reliability.

14.4.2 Philosophy of reliability Modern design rules as given in the Eurocodes1,2 make use of structural reliability methods to reach the required targets of reliability, and are based on limit states. The limit state is defined as the condition in which the structural component is no longer able to fulfil its functions under given conditions. A mathematical presentation is given by the limit state function defined as the difference between the resistance (R) and the action effect (S): Z ˆRÿS As long as Z > 0 no failure will occur, while for Z < 0 the structure fails; the limit state is reached for Z ˆ 0. Both the resistance and the action effect are regarded as stochastic variables, which can be represented by their probability functions fR(r) and fS(s) respectively. If the resistance and the action effect are independent, their combined probability function is defined as fR(r)
fS(s). This function can be graphically presented by contours in its R-S plane, as indicated in Fig. 14.3. The probability of failure P() is equal to the capacity of the

Figure 14.3 Statistical presentation of the limit state concept.

Building and construction ± steel and aluminium

313

combined probability function for which Z < 0. To solve it, several probabilistic methods have been developed. With the use of these methods it is now possible to quantify the reliability of a structural component. Instead of presenting the results of the probabilistic methods in terms of probability of failure, the reliability index is commonly used. The relation between the probability of failure P…Z < 0† and the reliability index is given by: P…Z < 0† ˆ …ÿ † where  is the standard normal distribution function. For the static load case and an intended lifetime of 50 years a target value ˆ 3:8 is defined, which corresponds with a failure probability of 0.00007. The partial safety factor approach is mostly used for daily design. The reliability of a component has to be validated by comparing the characteristic values of the action effect Sk and the resistance Rk:

S
Sk 


Rk

R

where S and R are partial safety factors for the action effect and the resistance respectively. These partial safety factors take the stochastic nature of the action effect and the resistance into account. The conversion factor  corrects the resistance for durability effects. The characteristic values are mostly based on statistical means. To validate a design of an adhesive bonded joint the characteristic value of the action effect Sk and matching partial safety factor S can be taken from existing standards, as for example described in the Eurocode.4 The characteristic value of the resistance Rk has to be based on a prediction model, but the matching partial safety factor R and conversion factor  are still not generally available. The values of the partial safety factor for the resistance R and conversion factor  have to be determined by calibration. In the past this was mostly done by engineering judgement, but probabilistic analyses enable the use of additional quantitative information. Calibration methods for structural adhesive bonded joints are described by Van Straalen.5

14.4.3 Strength During all stages of the design process the nature of the load acting on the adhesive bonded joint has to be considered. The following distinction can be made between the various mechanical actions: · Short-term static load: during its lifetime the joint is incidentally loaded by a non-varying load for relatively short periods of time. · Long-term static load: the joint is loaded by a non-varying load for a longer period. This type of load might have a significant effect on the behaviour of

314

Adhesive bonding

the joint due to creep. Besides the fact that irreversible deformations occur, the joint might fail after a period of time. · Low cycle fatigue load: in fatigue the value of the load acting on the joint varies over time. The fatigue load is characterised by minimum and maximum load levels reached and the number of cycles during its lifetime. For low cycle loads fatigue cracks can be formed in the bondline, causing a degradation of the performance of the joint. · High cycle fatigue load: for high cycle fatigue load a continuing process of crack growth is active, until the joint fails. To validate the joint performances the significance of the above-mentioned mechanical actions and their values have to be known. Within the partial safety factor approach, the calculation of the characteristic value of the resistance is normally based on a mechanical prediction model. To guarantee the target of the required reliability level and to avoid high values for the partial safety factors as a result of the calibration of the design rule, it is necessary to make use of consistent prediction models. This means that the model has to be based on a failure criterion, on test methods and a theory to calculate the mechanical action effects. It is important to consider a failure criterion that is representative for the dominant failure mode. Its value has to be determined by testing, but test methods are also necessary to determine material properties that are used as input for the theory that calculates the effects of the applied mechanical actions. The prediction model has to give a relation between these mechanical actions and the failure criterion used. An issue that also influences the development of a prediction model, is the nature of the active mechanical action. The behaviour of the adhesive bonded joint strongly depends on it. For short-term static load conditions mostly models based on continuum mechanics are used. Under long-term static load conditions an adhesive bonded joint creeps, which might cause a reduction of strength in time, followed by final failure. To describe this type of failure special creep models for the adhesive material should be used. Up till now experience with these models is limited. Under a low cycle fatigue load cracks might appear within the bondline, which causes reduction of the ultimate strength of the joint in time. This behaviour can be described with an (empirical) degradation model. For high cycle fatigue load conditions a crack might initiate at a location within the bondline with high stresses. After initiation stable crack growth occurs. Available fatigue models are the experimentally based SN-curves and Miners rule. On the basis of this general overview it is concluded that the choice of a prediction model is directly related to the active load condition. To illustrate the development of a prediction model, Van Straalen6 has proposed and calibrated a design rule for a statically loaded overlap joint with steel adherends and an epoxy adhesive. The value of the partial factor R is

Building and construction ± steel and aluminium

315

Figure 14.4 Comparison of prediction model and test results.

determined by a probabilistic technique that compares experimental strength values with matching values according to a proposed prediction model. The proposed prediction model is based on tensile and compression tests to get the input of the Young's modulus, the Poisson's ratio, the stress-strain curve, the ratio between compressive and tensile stresses, and the ultimate tensile strain. Since adhesive failure occurred in the test series, a pressure dependent yield criterion as used for polymers seems to be fairly straightforward. The selected approach to calculate the stress and strain state in the adhesive bondline models the adherends as plates and the bondline as springs. With this model also the non-linear material behaviour and geometrical non-linearities are taken into account. Comparison with test results validated the proposed prediction model. In Fig. 14.4 the load deflection curve is given for the case of 4 mm thick steel adherends and a 20 mm overlap length. It is concluded that the physical non-linear model gives rather good predictions. Essential in the calibration procedure is the comparison of the set of data with the matching resistance predicted by the proposed model. By quantifying the differences between the set of test data and the matching values predicted by the model, the value of the partial safety factor R can be determined.5 In the example it is assumed that the characteristic value of the design rule is equal to the value predicted by the proposed model and for the static load case and an intended lifetime of 50 years a target value for the reliability index ˆ 3:8 is defined. With the probabilistic techniques it is found that the calibrated value of the partial factor for the proposed prediction model is equal to R ˆ 1.1. It is noted here that effects of, for example, load rate and differences in temperature are not yet incorporated within the value of this partial factor.

316

Adhesive bonding

14.4.4 Durability Environmental actions, which depend upon the application and the geographical site, can have a significant influence on the performance of adhesive bonded joints. Due to ageing the mechanical properties of a joint can degrade over time. To illustrate this phenomenon an example of the degradation of the strength of a single lap joint is given in Fig. 14.5. The performed ageing tests were accelerated by a higher temperature. It is generally concluded by research that water in a liquid or vapour state, temperature, time of exposure, long-term loads and cyclic behaviour of temperature have a major effect on durability.7 Water seems to be the most important factor in practice. A full understanding of degradation mechanisms in relation to these actions has still not been achieved. An issue not mentioned in the literature is how the magnitude of degradation develops during time. It is possible that due to ageing the mechanical properties of an adhesive bonded joint degrade and after a period of time stabilise. But it is also possible that the joint suddenly loses its strength completely. In Fig. 14.6 a schematic illustration is presented. For a successful application of adhesive bonded joints with aluminium or steel adherends a stable degradation of the strength has to be guaranteed. Current methods of analysing the durability of adhesive bonded joints mainly focus on the experimental comparison of combinations of adherends, adhesives and pretreatments. Specimens are aged under short-term laboratory-based environments and their strength is compared with the strength of specimens not aged. Alternative methods more frequently used for predicting the service life of all kind of products, compare the ageing effects of specimens exposed under laboratory-based environments with those exposed under in-use environments, but none of these methods is suitable for a quantitative prediction of durability.

Figure 14.5 Example of the degradation of the strength of a single lap joint with coated steel adherends and a two-component PU adhesive, aged at 60 ëC and 95% rh.

Building and construction ± steel and aluminium

317

Figure 14.6 Schematic presentation of the development of degradation of strength during lifetime.

Alternative methods not affected by the inadequacies of current methods are based on the reliability theory. Since the 1960s reliability methods have been successfully applied to many materials, components and systems.8 Most of these methods deal with the prediction of the service life, but the theory used can also be applied to the prediction of the degradation of the resistance. Reliability methods try to quantify the relationship between in-use environments and degradation mechanisms. Environmental actions are identified and quantified. For each of these separate actions degradation mechanisms are formulated. These mechanisms can be analyses using data from tests under longterm in-use exposures, using data from tests under short-term laboratory-based exposures, on the basis of fundamental physico-chemical studies and with statistical evaluation techniques. Data from mechanical tests under long-term inuse exposures provide valuable insight into dominant degradation mechanisms during lifetime. Data from tests under short-term laboratory-based exposure and physico-chemical studies provide information about the cause of degradation and the rate of degradation. Combining these three sources with statistical techniques and relating the environmental actions with the weathering circumstances, results in a quantitative description of the degradation of the resistance during lifetime. To analyse the reliability of the durability of a given adhesive bonded joint with aluminium or steel adherends, the following steps are necessary.5 · Identification of environmental conditions. A higher temperature normally reduces the stiffness and strength of the adhesive, while in an accidental fire most adhesives disintegrate rapidly. Other environmental actions like water, high humidity and salt spray normally have a long-term effect on the joint by reducing the strength. Cyclic behaviour of these actions might cause additional effects. · Determination of degradation mechanism. Where physico-chemical knowledge about the behaviour of the adhesive and the interface is available,

318

Adhesive bonding

this can be used to get an overview of the relevant degradation mechanism. Pre-testing under severe environmental conditions might be used as an alternative to show the dominant mechanism. · Accelerated ageing testing. Based on the dominant mechanism of the degradation effect the method to accelerate the ageing behaviour has to be defined. For adhesive bonded joints mostly the temperature is increased. To determine the degradation of the strength properties of the adhesive bonded joint, overlap joints will be tested after three or four periods of time. To get valuable data as a function of time, at least five specimens have to be tested for one period of time. · Statistical analysis of test results. The test results have to be extrapolated to the in-use environmental conditions, e.g., by using time-transformation functions. This interpretation has to be done with use of statistical techniques, that incorporate the required reliability level. Now the reduction in strength is equal to the extrapolated strength at a time equal to the reference period, divided by the strength at time zero. To illustrate this procedure, Van Straalen6 has presented the calibration of the value of the conversion factor for an overlap joint made of coated steel plate and a polyurethane adhesive. This calibration is based on accelarated ageing tests and to interpret the results with the use of statistical techniques, a relation that gives a proper description of the degradation process is defined. The results of this statistical evaluation are presented in Fig. 14.7. The predicted curve for

Figure 14.7 Results of the statistical evaluation of the tests of aged specimens.

Building and construction ± steel and aluminium

319

outside exposures is plotted assuming an average temperature of 10 ëC for the whole year. In Fig. 14.7 the results of additional specimens exposed outside under Dutch weathering conditions are also plotted. It is observed that the test results of these specimens are on the safe side of the predicted curve. Based on this interpretation the value of the conversion factor to be used in design rules is found to be  ˆ 0:76 in this example.

14.5 Common failures An engineer who wants to apply adhesive bonded joints in designs should take account of failures which might occur during the lifetime of steel and aluminium structures. For building and construction various kinds of failures might occur. The following are considered as common: · Adhesive failure of interface between adhesive layer and adherend. This is probably one of the most critical types of failure. Due to environmental conditions the properties of the interface might degrade. To avoid this type of failure within steel and aluminium structures, a proper pretreatment matching with actual load conditions is required. · Failure due to a fatigue load. In general less is known about the fatigue behaviour of adhesive bonded joints. For example offshore steel structures are dominantly loaded in fatigue, which means that for a reliable design an experimental programme is needed. The test set-up should match as closely as possible the actual situation of the geometry, adhesive and method of production. To guarantee the required reliability, probabilistic analyses based on statistical evaluations of results are needed. · Failure due to production errors. Many of the failures that occur in practice can be traced back to errors within the production process. Inadequate bonding of the faces with the core material of a sandwich wall panel appears as a bumpy surface within a period of one year and the strength properties of such a sandwich panel are no longer guaranteed. For this situation the sandwich panel has to be replaced. · Failure due to design errors. If a load condition and/or a failure mode is not validated during design, while it is critical, failure of the adhesive bonded joint might occur. For mechanically loaded sandwich panels failure modes such as yielding of the steel or aluminium faces, shear failure of the core material, wrinkling (local buckling) of the faces, delamination of the faces from the core material and overall buckling have to be considered. An example is given in Fig. 14.8. The sandwich roof panels of a cold-storage warehouse failed two weeks after occupation, because the difference in temperature in the cooled building and the outside surface of the panels heated by the sun caused bending in the panels. Since this effect was underestimated in the design, the outside surface failed by wrinkling of the

320

Adhesive bonding

Figure 14.8 Wrinkling of the face of a sandwich roof panel due to a design error.

faces. This example indicates that various types of failures have to be considered during design. · Failure due to a difference in expansion of adherends. Internal stresses within the bondline will be caused by differences in thermal expansion of adherends made from difference materials. For example, the bondline of steel beams strengthened with adhesively bonded fibre reinforced plastics will fail within a given temperature range. But differences in expansion will also occur in house structures made of thin steel gauge members covered by wooden plate material, since the length of wooden plate material is strongly influenced by the relative humidity of the air. · Failure due to brittle behaviour of adhesive bonded joint. Within building and construction an engineer should take the possibility of applied deformations of the structure into account. If the deformation capacity of an adhesive bonded joint is small, which might be the case for brittle adhesives, failure might occur. To avoid this problem, more flexible adhesives with thicker bondlines can be used but it is also possible to design the joint in such a way that the deformation capacity is generated by yielding of the steel or aluminium adherends.

14.6 Inspection, testing and quality control To make adhesive bonded joints in steel and aluminium structures competitive compared to more traditional joining methods such as welding and mechanical fastening, the production tolerances should be minimised. This means that a well balanced system of quality control is needed. The proper way to provide quality

Building and construction ± steel and aluminium

321

assurance of production is to systematically manage and control the whole operation from design of the joint through to final assembly. In this way, the possibility of poor quality adhesive bonded joints being produced is reduced to the minimum, because proven procedures are being followed at all times. Every adhesive bonding situation is unique but all adhesive bonding situations have a number of common features. Consequently, a generic model is preferred to cover all situations. Within the Eureka project Quasiat,9 such a generic model has been developed. This model acts as a guide in defining the specific quality requirements for a joint and the specific quality management actions needed to ensure the quality requirements are met. All the major stages from selection and sourcing of materials through to pre-use storage are listed which define the requirements and methods of quality verification at each stage. Each stage can be accessed individually, progressively in more detail leading ultimately to the identification of the specific measures needed for an individual application. The generic model for quality assurance of production is summarised in Table 14.1, with a focus on the building and construction industry. The model divides the whole production into eight activities from selection and sourcing of steel or aluminium up to pre-usage storage. For each activity the quality requirements to be considered are mentioned and matching methods to validate these requirements are summarised. In case one requirement is not met, possible corrective actions are indicated. Using this method it is possible to incorporate a system of quality assurance of production as a part of the design process. The use of such a type of quality assurance system is one way to guarantee reliability. During the design stage it can also be decided to use an inspection technique to control the reliability of the structure during its lifetime. The use of such inspection techniques can be incorporated in the philosophy of reliability as described in section 14.4. The probability of failure of a structure during its reference period shall fulfil the required reliability level indicated by its reliability index . For an adhesive bonded joint it might be expected that this probability varies over the years. Normally its value is higher than average during the first year and will increase near the end of the reference period as indicated in Fig. 14.9. This behaviour is also known as bath curve. The higher probability of failure during the first year is due to design errors, while the increase of its value near the end of the reference period is caused by degradation effects such as fatigue, chemical ageing of the adhesive or the interface between the adhesive and the steel or aluminium surface. The parameters that influence the probability might vary, but all in all the required reliability level, has to be reached. With the use of inspection techniques this reliability can be controlled. Where inspections are done during the lifetime of the structure, additional information about the properties of the adhesive bonded joints becomes available. An example of an inspection technique is the use of additional test

Table 14.1 Generic model for quality assurance of production Activity

Quality requirement

Method of validation/control

Corrective action

Selection and sourcing of steel or aluminium

· Joint design · Specified component

· Test records · Supplier certifications

· Return to supplier · Design change

Selection and sourcing of adhesives

· Joint design · Production requirements

· Test records · Supplier certifications · Published data · Experience of previous use

· Return to supplier · Re-select · Design change

Storage of adhesives

Requirements specified by adhesive supplier · Shelf life · Packaging · Temperature · Humidity

· Inspection of packages · Batch/data no. · Control of storage facility

· Reject · Re-test and re-life

Pre-treatment of surfaces

Requirements specified by material properties · Cleaning · Surface removal · Chemical treatment

· Use tested procedure (mistake proofed) · Trained staff

Re-treat

Assembly

· Component fit-up: correct components location · Application: type, mix, quantity, temperature, humidity

· Inspection · Use of jigs · Metering by calibrated dispenser · Use tested procedure (mistake proofed) · Trained staff

· Re-jig · Select correct components · Reject · Re-apply

Cure

Requirements supplied by adhesive supplier · Time · Temperature · Pressure · Heating/cooling rate

· Use tested procedure (mistake proofed) · Trained staff · Time/temperature records

· Reject · Re-cure

Final inspection

Joint meets design requirements · Strength · Environment · Appearance · Reliability · Durability

· Test programme · Review of process documentation and records

· Reject · Concession · Design change

Pre-usage storage

Joint meets design requirements · Strength · Environment · Appearance · Reliability · Durability

Correct storage review of test reports and supplier information

· Reject · Design change

324

Adhesive bonding

Figure 14.9 Schematic presentation of the development of the probability of failure during the reference period of an adhesive bonded joint.

Figure 14.10 Schematic presentation of the effect of inspection on the development of the probability of failure during the reference period.

samples placed under identical circumstances as the structures. These specimens should be tested over the years and using probabilistic methods the results can be used to update the predicted reliability level. Sensor techniques are also under development to monitor the degradation of the adhesive bondline and interface. This means that the uncertainty of the probability of failure decreases as indicated in Fig. 14.10.

14.7 Repair and strengthening There are various reasons for deciding to repair or strengthen a structure during its lifetime. The growth of fatigue cracks might be stopped and a structure that

Building and construction ± steel and aluminium

325

failed locally due to an accidental overload can be repaired. Instead of replacing an existing structure during its lifetime due to higher load, which is the case for various bridges nowadays, strengthening the actual structure is an alternative. Not only are the costs lower, but also traffic congestion during a longer period of time can be avoided. Another reason to strengthen an existing structure is to repair sections weakened by corrosion. If a fatigue crack is detected in a steel or aluminium structure, the user mostly wants to repair the crack to stop the crack growth. This can be done by welding or using a bolded cover plate, but a plate adhesively bonded upon the surface with the crack might be considered as a favourable alternative. To avoid delamination near the cover plate endings, it is advised to profile the plate near the edges. The actual knowledge about the fatigue behaviour of such a repaired structure is limited to some specific cases. Fatigue performance will depend upon the actual geometry and fatigue load, the cover plate material and dimensions, and the adhesive bonding system used. No general guidelines are available and for this reason fatigue tests are recommended to validate the required reliability of the structure during its sequential lifetime. After an accidental overload of a structure, the remaining reliability has to be determined and methods to repair local failures have to be considered to reach the required reliability level. The advantages and disadvantages of using adhesive bonding instead of welding or bolting, should be considered when deciding which method is preferred. Strengthening of a steel or aluminium structure can be done by bonding an additional element onto locations with the highest stresses. Mostly the same material is used for this element, but also the use of fibre reinforced plastics like pultrusions are considered as an alternative. This element is mostly of the same material. The validation of the chosen solution can be done by calculation, but for fatigue loads mostly additional fatigue tests are necessary. Repair and strengthening can also be explained within the scope of the philosophy of reliability. During its lifetime the reliability of the structure decreases due to, for example, increasing traffic loads or growing fatigue cracks. Due to repair or strengthening the probability of failure can decrease to an acceptable level as indicated in Fig. 14.11.

14.8 Other industry-specific factors As explained earlier, the validation of the design of a steel or aluminium structure is mainly based on analytical methods generally accepted within the building and construction industry. Probably one of the main disadvantages of applying adhesive bonded joints is the lack of guidelines and design codes for this field of applications. For this reason it is necessary that engineers are familiar with adhesive bonding technology and methods to validate the required reliability level. An exception are the design rules for sandwich roof and wall

326

Adhesive bonding

Figure 14.11 Schematic presentation of the effect of repair or strengthening on the development of the probability of failure during the reference period.

panels. The development of these codes started in the 1970s and nowadays a CEN-code10 is under development. The code covers a wide range of topics such as testing procedures, durability, fire resistance, design procedures and tolerances. This code can be seen as a state-of-the-art statement of all available knowledge. One load condition and failure mode not mentioned in this chapter up till now is fire. For various applications within building and construction, requirements dealing with fire safety are prescribed and are mostly an important performance criterion. For those situations the fire resistance of the chosen design has to be validated. To do this, fire tests are needed. In these tests a single structural element is placed in or on a furnace in which the gas temperature follows a standard specified curve. The fire resistance is usually expressed in classes, ranging from 30 to 120 minutes (and beyond) with 30 minute intervals. Where an adhesive bonded joint cannot withstand extremely high temperatures, the joint has to be covered by fire-resistant material. To facilitate the knowledge transfer needed to train people from industry, the European Federation for Welding, Joining and Cutting has defined the minimum requirements for education, examination and qualification.11 Three grades of staff with appropriate qualifications and experience are identified. A European Adhesive Bonder has industrial experience and can carry out bonding without supervision. A specialist is able to write and explain working instructions for the bonder in theory and practice. Finally an Adhesive Engineer is responsible for the integration of adhesive bonding into the design and manufacture of products, including design, evaluation of process parameters, problem solving and failure analysis. Courses have been developed by various training centres in the UK, Germany and France.

Building and construction ± steel and aluminium

327

14.9 References 1. ENV 1993-1-1, 'Eurocode 3: Design of steel structures; Part 1-1: General rules and rules for buildings', 1995. 2. ENV 1999-1-1, 'Eurocode 9: Design of aluminium structures; Part 1-1: General rules and rules for buildings', 1998. 3. Engineered Materials Handbook, Volume 3 ± Adhesives and Sealants, ASM International, 1990. 4. EN 1990, Eurocode: Basis of structural design, 2001. 5. Straalen IJ J van, Development of design rules for structural adhesive bonded joints ± A systematic approach, PhD thesis, Delft University of Technology, 2001. 6. Straalen IJ J. van, Tooren M J L van, Development of design rules for adhesive bonded joints, Heron, Volume 47, No. 4, 2002. 7. Kinloch A J, ed, Adhesion and adhesives ± Science and Technology, Chapman and Hall, United Kingdom, 1990. 8. Nelson W, Accelerated testing ± Statistical model, test plans, and data analysis, John Wiley & Sons, USA, 1990. 9. Quality Assurance in Adhesive Technology, EUREKA Project EU716, ISBN 1-85573259-9, Abington Publications, 1998. 10. prEN 14509, Self-supporting double skin metal faced insulating sandwich panels ± Factory made products ± Specification, CEN, European Committee for Standardization, 2002. 11. EWF, Minimum requirements for the education, examination and qualification of European Adhesive Bonder/Specialist/Engineer, Doc. EWF 515-01/516-01/517-01, 1998.

15

Building and construction ± timber È LLANDER E SERRANO AND B KA

15.1 Introduction and overview This chapter deals with the application of adhesives in timber engineering including wood-based products. First, an overview of the basic needs and prerequisites is given, followed by a brief overview of the characteristics of the wood material. An introduction to the general characteristics needed for wood adhesives, and an overview of the most commonly used wood adhesives is given in section 15.4. Surface preparation issues are discussed in section 15.5 together with the complex process of bond formation in wood adherends. Here emphasis is put on adhesive and wood properties affecting bond formation, including surface preparation techniques. The strength and durability characteristics of wood adhesive bonds are covered briefly in section 15.6. Here, some emphasis is put on the mechanical description of wood adhesive bonds, including experimental and numerical results from previous research. The most common reasons for failure and the most commonly used procedures for inspection, testing and quality control are given in sections 15.7 and 15.8, respectively. Repair of wooden structures is discussed in section 15.9, followed by some examples of the use of adhesive technology in timber engineering. Finally, some possible future trends and suggestions for further reading and information are given in section 15.11.

15.2 Basic needs and applications One of the main advantages of wood related to its use in load bearing structures is the favourable weight±load capacity ratio. In spite of a low density of the material, properly designed wood structures can span relatively large distances, with the 160 m diameter Tacoma dome built in 1981 as a well-known example. However, in order to utilise the benefits of the material, it is generally necessary to re-engineer the wood, for example, by means of endwise and flatwise joining. Thus, modern timber engineering relies largely on the use of adhesive technology. In some aspects, adhesives are used slightly differently in the field

Building and construction ± timber

329

of timber engineering compared with other fields. Adhesive technology not only supplies the means of joining elements together to form structural systems, but also plays a crucial part in obtaining new re-engineered wood-based materials and products, so-called engineered wood products (EWP). In fact, the majority of adhesives used in timber engineering applications are used in materials production, such as for the production of wood-based panels. Wood is a natural, eco-efficient material with obvious advantages related to recycling and the carbon economy. However, as an engineering material, it also has some obvious disadvantages, mainly related to the natural variability in properties, its strongly orthotropic nature and because it comes in limited sizes. Thus, in order to promote the use of wood, these disadvantages must be dealt with. Compared with other engineering materials, the variability found in the mechanical properties of wood is extremely large, not only within species from different regions, but also within a single log or within a board. The coefficient of variation for strength can be in the range of 15±30% for structural timber from commonly used species. As an example, typical tensile strength values for structural timber (spruce or pine) are found to be in the range of 15±40 MPa (Thelandersson and Larsen, 2003). The high natural variability, mainly explained by the presence of knots and deviations in grain directions, results in timber design strength values that are much lower than the expected mean strength of small, clear wood specimens (typically only approximately 15%). Clearly, any method making it possible to obtain a more homogeneous and less variable material is of great importance, not only from an engineering point of view but also from an economical and material saving (environmental) point of view. The highly orthotropic nature of wood, with typical ratios of the stiffness parallel to the grain to the stiffness perpendicular to the grain in the range of 20± 40, is another reason why engineered wood materials have been developed. By combining the orthotropic material in several layers or reducing it into small pieces and using a more or less random orientation when reconstituting them, a more homogeneous and less orthotropic material can be obtained. This is the principle behind many engineered wood products used today. The joining of smaller wood pieces to form a larger component, such as a beam or sheeting material, will also result in a product having more accurately defined dimensions and being less sensitive to moisture changes, showing less warping. Glued laminated timber, or glulam, is perhaps the most well known example and, together with plywood, also one of the oldest developed using adhesive technology. Other materials include laminated veneer lumber (LVL), laminated strand lumber (LSL) and parallel strand lumber (PSL) which are products made from thin plies (LVL) or strands (LSL and PSL) which are glued together to form a solid material. The veneers or strands are stacked on top of each other, all layers maintaining the same main fibre direction. Board materials such as oriented strand board (OSB), medium- and high-density fibreboards (MDF, HDF) and chip boards, are also prime examples of what can be achieved in

330

Adhesive bonding

terms of engineered materials. These materials are discussed in more detail in section 15.10. As mentioned, there is a need for obtaining timber products in any size, and not having to rely on the sizes available in nature. Large-grown trees are becoming more rare and, in order to obtain the larger cross-sections needed for heavy timber framing, adhesive technology is used. Obtaining large-sized structural members means not only obtaining large cross-sections but also members of arbitrary length. The lengthwise splicing of boards used as laminates for glulam production and for structural grade timber is of the utmost importance in today's modern timber engineering material supply. This lengthwise splicing is performed using finger-joints, which gives a strong and reliable joint. Apart from the above-mentioned materials, several types of engineered products or components have also been developed; examples include composite beams and columns, so-called I-joists, and stressed skin panels. Composite beams and columns have been manufactured since the late 1960s when the first I-joists were introduced. I-joists consist of flanges made from solid or laminated lumber (LVL), which is finger-jointed and stress graded. The web material is commonly OSB or structural grade plywood. Also, medium or high-density fibre boards are used and some companies produce open web joists with timber struts or steel webs. Stressed skin panels are obtained by using solid timber for the web and panels to form the skins on one or both sides. The connections are mechanically fastened, but adhesive is used to prevent squeaking, and to improve the stiffness and sometimes strength (Johansson et al., 2002). Wood can also be effectively combined with other materials by using adhesive technology. Apart from the use of wood-concrete composite structures, where the cement-based concrete can act as adhesive, glued-in steel plates, rods or bolts can also be effectively used for obtaining strong and stiff connections and beam-to-column foundations for glulam structures. Other applications include the use of technical textiles, e.g., glass-fibre reinforced polyester coatings as local reinforcement. Such coatings can be an extremely effective way of increasing the load-bearing capacity of notched beams and dowel-type fasteners, where high stresses perpendicular to the grain often cause failure. Also, reinforcing wood with other materials, such as glass or carbon fibres placed in the outer parts of laminated beams, or I-joists has been used to a commercially limited extent, although research has been extensive. In conclusion, adhesive technology is used in timber engineering mainly for overcoming the disadvantages related to the raw material and for obtaining strong and reliable: · joints ± finger-joints ± glued-in rods and plates

Building and construction ± timber

331

· materials ± wood-based panels ± glued laminated timber · products and components ± composite beams (I-joists) ± stressed skin panels.

15.3 Wood characteristics Bond formation and performance is influenced in numerous ways by wood properties. In order to fully understand this complex interaction, it is necessary to also give a short overview of the characteristics of wood. The overview here relates to softwood species most commonly used in Europe, such as spruce (Picea abies) and pine (Pinus silvestris). Wood can be considered as both a material and a structure depending on the scale at which the material is viewed. However, making it clear that there exists an ultra-structure makes it easier to explain many of the peculiarities of wood characteristics, such as its orthotropic nature and the difference in tensile and compressive strength. The ultra structure of wood also explains some of its surface characteristics. Wood is a natural cellular ligno-cellulose composite. Its cellular structure is arranged in annual growth rings, organised in a nearly concentric pattern, with the yearly growth adding new material in a shape close to cylindrical to the outer part of the log. This structure explains the main mechanical characteristics of wood as being a near cylindrical orthotropic material, its three principal material directions being denoted parallel (to the grain), radial and tangential. The wood cells of softwoods, known as fibres, have the dual task of both taking part in the transportation of liquids and acting as load carriers. Apart from the fibres, a small fraction of the wood consists of other types of cells with special tasks related to horizontal transport and food storage. The main constituents of wood are cellulose and hemicellulose, which are the main building blocks of the cell walls, lignin, which bonds the fibres together and extractives, a number of more or less soluble organic compounds including fats, tannins, oils, waxes, carbohydrates, acids, gums and resins (Marra, 1992). Figure 15.1 shows a close-up of two PRF-bonded pieces of pine (Pinus silvestris). During the growth season of the tree, the growth rate varies. In early spring, the growth starts suddenly and the cells formed during this time of year are large in cross-section with thin walls. During late spring and early summer, the growth rate diminishes, resulting in the cells formed during this time being smaller with thicker walls. The different types of cells formed during the season are termed springwood and summerwood. Sometimes the terms earlywood and latewood are used. In a living tree, different parts of the stem have different functions. In its

332

Adhesive bonding

Figure 15.1 A PRF adhesive bond in pine (Pinus silvestris). Note the crushed wood cells to the left.

outer parts the cells take part in the transport of liquids. This part of the stem is known as the sapwood and its cells are physiologically functioning. Closer to the centre of the stem, in what is know as the heartwood, the cells do not take part in the transportation of liquids, but may still store food. As the tree grows older, the cells make the transition from sapwood cells to heartwood cells. This transition means that the cells become infused with various materials such as waxes, oils and phenolic compounds, collectively termed extractives. In some species, this transition changes the colour, such that is it possible to distinguish the heartwood from the sapwood by the naked eye. For other species, this is not possible. During its lifetime, the tree grows in different ways, resulting in different characteristics of the wood formed. During its early years, the growth is fast, producing wood that is known as juvenile wood. This typically has a low density, due to its open structure. Older trees tend to grow slower, with diminishing annual ring width. The amount of heartwood also increases. A special type of abnormality found in many species, is so called reaction wood. The term is used for that wood formed as a reaction to stresses, which can occur in a tree that, for example, is leaning. Softwoods react to such situations by forming an extra amount of wood on the compressive side of the leaning tree or under a branch. This wood is known as compression wood and possesses many abnormal properties; excessive shrinkage, high density and, perhaps surprisingly, low strength.

Building and construction ± timber

333

15.4 Adhesive characteristics needed 15.4.1 Wood material, woodworking and assembly factors The properties of wood sets requirements for the adhesive that are somewhat different from adhesives for bonding of man-made materials like plastics, metal or concrete. Two principally important parameters are the need for a controlled penetration of the adhesive into the material and the varying wetting properties of the surface. As the wood is prepared for bonding, either by sawing, planing or sanding, the wood cells are damaged down to a depth of at least 0.1±0.2 mm from the surface (Stehr and Johansson, 2000). In order to produce a reliable bond line, the adhesive must penetrate through the damaged material and reach the undamaged wood. At the same time, the adhesive should not be absorbed too much, too deep a penetration will lead to an insufficient amount of adhesive left in the bond line to produce a strong bond line. A balanced level of penetration can lead to a gradual change from wood to adhesive, rather than an abrupt interphase surface. Such a gradual change is beneficial since it reduces stress concentrations and improves short-term strength of the bond line as well as its durability. If the adhesive also penetrates into the walls of the individual wood cells, a bond line that is highly resistant to climatic variations can be achieved. The surface properties of wood vary between different areas of a single piece as well as over time. The wood structure leads to differences in wetting properties over the surface also immediately after planing, where, for instance, heartwood commonly is more difficult to wet than sapwood. If the wood is not bonded immediately after surface preparation, the surface energy will be reduced due to the resins migrating to the surface as well as oxidation (Nussbaum, 2001). Hence, a wood adhesive must function over a great range of surface properties in order to produce strong bond lines with acceptable quality variation. In addition to the special needs set by the wood material, the structure of the woodworking industry also sets requirements. Gluing of wood is commonly done in factories with relatively simple production equipment. Therefore, the adhesives need to allow for the expected variations in the production process, such as varying temperature and air moisture, relatively long and varying assembly times due to large structures being assembled manually and limited possibilities to fully control the properties of the wood raw material. Traditionally, it has been a rule of thumb not to use on-site gluing. However, the need to repair and restore historical buildings as well as a demand for altered use of buildings has led to the development of several European systems for onsite bonding of wood and wood to metal, fibres or concrete. New European-level initiatives have been taken regarding both research (such as COST-action E34) and standardisation (CEN/TC193/SC1/WG11) to facilitate this type of

334

Adhesive bonding

application. This places new demands on the adhesive systems to be used, since basic requirements such as climate and surface contamination prior to and during gluing can no longer be controlled, or at least not controlled with a high degree of accuracy.

15.4.2 Mechanical properties and climatic factors The adhesive must have a well-balanced set of mechanical properties such as strength and stiffness in its cured state. For many wood applications, a cohesive adhesive failure should generally be regarded as proof of poor gluing. The wood material normally has a much lower strength than the adhesive in the modes of loading generally in question; shear parallel to the grain and normal tensile stresses perpendicular to the grain. Thus, failure should normally be in the wood-adhesive interface region or in the wood itself. A well balanced adhesive joint should also possess some ductility, making it possible to re-distribute stresses under increased loading through plasticity or fracture propagation, in order to avoid sudden and brittle failure modes. In timber engineering applications, duration of load (DOL) effects can be regarded as one of the most decisive in terms of design. Those adhesives traditionally used in timber applications perform extremely well in this respect. They are yet unchallenged by more modern formulations and, in fact, the main concern in the development and approval of new adhesives is often their DOLbehaviour. Another important factor when designing an adhesive to be used in timber engineering applications is its response to climatic changes. The adhesive itself must withstand any predictable climate, including elevated temperatures to approximately 60±80 ëC. Being highly hygroscopic, wood is known to react to changes in climate by shrinking and swelling when air humidity changes and its strength and stiffness is affected by moisture. It is thus of utmost importance that the adhesive can withstand these climate-induced deformations and preferably neutralise them by acting as a stress or strain equaliser.

15.4.3 Environmental factors Several environmental aspects must be considered for wood adhesives, the most obvious ones being related to potential health hazards for workers and to the aspect of recycling. Since many of the products made using wood adhesives can be found in our home environment, we are exposed to them for a large part of the day and night. Therefore, the emissions in the cured state from any potential wood adhesive should be thoroughly examined. The emission aspect has been intensely discussed during the last decades, mainly within the wood-based panels industry, where the emission of formaldehyde from particle boards has been the main concern. Apart from formaldehyde, emittable substances include

Building and construction ± timber

335

phenol and isocyanates from the use of PUR adhesives. Unreacted phenol can possibly be emitted both during and after production of wood-based boards, however to a very small extent (Dunky et al., 2002).

15.4.4 Fire Wood possesses favourable properties in terms of strength resistance when exposed to fire. During fire, the charcoal layer developed reduces the burning speed of the timber considerably, making it possible for a solid (one single piece or laminated) timber beam to withstand loading for a considerable length of time. The insulating effect of the charcoal layer in combination with cooling from evaporation of water from the interior of the wood keeps the temperature relatively low for a substantial time during the fire, as shown in Fig. 15.2. The charcoal layer and the resulting low temperature protect the adhesive and the bondline. The test results shown indicate that all adhesives that have properties suitable for timber engineering applications can match the requirements needed, so that the beneficial properties of timber in relation to other materials can be maintained (KaÈllander and Lind, 2002). However, the results arrived at were made with relatively large cross-sections of the wooden member. A small size beam would most likely show a different result.

Figure 15.2 Temperature history at different depths below the wood surface in a glulam beam made with PVA D3 adhesive during fire exposure (adapted from KÌllander and Lind, 2002).

336

Adhesive bonding

15.4.5 Aesthetics A final aspect, which should not be forgotten when describing the characteristics needed, is related to aesthetics. The traditionally dark brown phenolic adhesives used since the middle of the last century were challenged by light and wood coloured adhesives in the 1980s and 1990s thanks to a strong demand from the market for `invisible' adhesive joints in glulam. This demand led to the introduction of melamine-urea-formaldehyde (MUF) adhesives and Polyurethane (PUR) for this type of application.

15.4.6 Adhesives for structural timber purposes Adhesives that have been or are being used for structural timber applications can be organised in two groups: traditional ones such as animal/casein glues and formaldehyde resins on the one hand and new or at least non-traditional adhesives on the other (Davis, 1997). Traditional adhesives include: · natural adhesives ± animal glues ± casein glues · formaldehyde resins ± phenol-formaldehyde (PF) ± urea-formaldehyde (UF) ± melamine- and melamine-urea-formaledhyde (MF, MUF) ± resorcinol-formaldehyde (RF) ± phenol-resorcinol-formaldehyde (PRF). Non-traditional adhesives are: · polyurethanes (PUR) · emulsion-polymer-isocyanate (EPI) · epoxy resins (EPX). In addition to these, other wood adhesives have been used for non-structural and semi-structural purposes, such as finger-jointing of compression only studs or for furniture applications. In these applications, polyvinyl acetate adhesives (PVA) are commonly used. Animal glues are of historic interest and are not used for structural purposes due to their poor behaviour when exposed to moisture and to their creep behaviour (Davis, 1997). Casein glues were used until the 1970s for glulam production, although they cannot be used in outdoor exposed climates due to their poor behaviour in moist conditions. Casein adhesives are also vulnerable to mould and fungal attack because they are protein based. Casein is the name of the main compound used, and is a milk protein.

Building and construction ± timber

337

Polyurethane and especially epoxy adhesives have been used for combining wood with other materials, such as bonded-in steel rods and for the gluing of wood to glass. Epoxy and two-component PUR adhesives can also be used for gap-filling applications, with the potential of filling gaps of more than 2 mm width, making them suitable for repair applications. Because of their hardening chemistry, the one-component PUR adhesives are also suitable for gluing timber at high moisture content (MC), which is known as wet or green gluing. The onecomponent PUR adhesives cure in the presence of water and generally perform well at high MC. However, the glue line must be thin, preferably less than approximately 0.3 mm. This is because the reaction of isocyanate groups with water emits CO2, causing foaming of the adhesive. Pressure must therefore be applied during gluing and if too low a pressure is used, a thick glue line containing cavities will develop. These cavities cause a severe strength reduction. Although several types of new adhesives are available today, the introduction of these has been difficult. The primary reason for this is the lack of established test methods and approval procedures for adhesives other than the established aminoplastic and phenolic types. New test methods and approval procedures are presently being developed in order to allow for the introduction of new adhesive types as well as new production methods of wood products.

15.5 Surface preparation and bond formation 15.5.1 Surface preparation Surface preparation in the woodworking industry normally includes sawing, grinding, planing and possibly sanding. These methods, except for sanding, are primarily used to achieve the correct dimensions of the lumber and are not surface preparation techniques to promote bond formation in the traditional sense. Obviously, residues from the mechanical machining of the wood must be removed in order to obtain good bond performance. Such cleaning is usually done by brushing. For some types of adhesive, pre-treatment of the adherend with water to improve the curing can be used, although such surface preparation has more in common with the use of accelerators. As mentioned above, any mechanical surfacing will create layers of crushed material, so-called mechanically weak boundary layers (MWBL). These form a critical link in the strength chain connecting the adherends. MWBL consist primarily of loose wood fibres at the surface, damaged wood cells and residues, which form a poor substrate for adhesion. The concept of the MWBL is described in Stehr and Johansson (2000). Especially, the use of sanding is known to cause mechanically weak boundary layers, with a combination of loose fibre ends, and clogging of the porous structure of the wood (Marra, 1992). Other reasons for the appearance of a MWBL are dull or erroneously adjusted tools.

338

Adhesive bonding

Additional techniques, apart from the traditional mechanical surfacing techniques described above, have been reported (Dunky et al., 2002; Nussbaum, 1993). Chemical treatments investigated included the use of lithium and sodium hydroxides. These act as surface activators by lowering the surface tension of the wood, removing the extractives, and increasing the ability of the adhesive to dissolve the extractives. A serious drawback is that the chemicals may deteriorate the wood surface by breaking some of the hydrogen bonds of the cellulose, which can give poor wet strength. These treatments are therefore not suitable for outdoor use (Dunky et al., 2002). The chemicals may also swell the wood surface. Nussbaum (1993) investigated the effect of flame treatment on wood, which causes oxidation of the extractives. The flame treatment used clearly increased the wettability of a PVAc adhesive, but no increase in strength was found. A special technique based on laser ablation was described by Seltman (1995). The method reduces the MWBL by literally blasting of loose fibres and damaged wood using an UV-laser. The method and has been used to improve the adhesion to end-grain by Stehr et al. (1999).

15.5.2 Bond formation In the process of bond formation, a number of processes are involved. The adhesive must be able to wet the wood, to flow across its surface and to penetrate its porous structure. Following this, the adhesive must of course be able to form some kind of bond with the wood, be it a bond formed by mechanical interlocking, secondary forces or even chemical bonds. It is generally considered that such physical bonding mechanisms are predominant in wood applications, although mechanical interlocking will always contribute to some extent due to the porous nature of the material (Marra, 1992). The wood surface can be de-activated in several ways, by re-orientation of surface molecules, by contamination, or by closing the micropores of the cell walls. An important issue for the wetting performance is related to the fact that wood surfaces are actually self-contaminating by their extractive content, as described above. After surfacing the wood, a migration of natural extractives to the newly cut surface will occur. As a rule of thumb, gluing should generally be made within 24 hours. Other sources for the surface deactivation in wood bonding processes include contaminants from fire retardants and preservatives, over heating of the wood surface at drying, and air-borne contaminants from the bonding site. The cellular structure of wood is beneficial since it adds gluing area when the cells are opened by the surfacing employed. The wood density can act as a crude indicator for bondability since the thick-walled cell structure, which is typical for high-density wood, means that the adhesive has difficulty in penetrating the wood fibres. This prevents the mechanical interlocking mechanisms of the bond formation process developing.

Building and construction ± timber

339

According to tradition and to conventional technology, timber should be glued at a moisture content (MC) far below the fibre saturation point, preferably between 6 and 15% depending on its future use. The level of MC is usually chosen depending on the type of adhesive and on the working conditions of the final product. For example, furniture should be glued at 8±12% MC and structural timber at 15% MC. The present codes and standards for finger-jointing of timber prescribe both the maximum allowable MC of timber, and the limits for the difference in MCs between two pieces to be glued. A general argument for gluing at low MC is that an excessive penetration of the adhesive can occur at too high a MC and that the adhesive can be washed out or diluted by water. This causes a so-called starved glue line. Another reason is that high-MC wood, besides having a low strength, must be dried, which will subject the glue line to severe shrinkage stresses under the drying process. The grain orientation at the glued surfaces can play a decisive role on the bond formation process. The wood ultra-structure, with longitudinal cells, means that the penetration of the adhesive is much greater along the grain, i.e., at endgrain surfaces, than in the other two principal directions. An example where the along-grain penetration plays a role is when end joining structural lumber using finger-joints or scarf joints.

15.6 Strength and durability 15.6.1 Influence of wood and adhesive properties As mentioned, the failure in wood-adhesive bonds is normally located in the wood close to the bond line itself. A commonly made assumption in such cases is that the strength of the adhesive is regarded as adequate, and that no further attention needs to be paid to its mechanical behaviour. However, it is easy to show that not only the adhesive strength, but also its ductility play an important role for the joint strength. Increased ductility means that the bond line stresssmoothing capabilities increase, so that a more effective stress transfer can take place. The ductility can be in the form of the plasticity of the adhesive but, for the traditional brittle wood adhesives, the fracture energy of the wood-adhesive interface is a more adequate measure of the ductility. Being a highly orthotropic material, wood possesses markedly different properties in loading parallel to the grain as compared to loading perpendicular to the grain. As an example, the shear strength of small, clear, softwood specimens of spruce (Picea abies) is typically 15±25 MPa. The tensile strength for the same type of material perpendicular to the grain is 0.5±5 MPa, dependent on the specimen size. Thus, it is of extreme importance for wood applications that the joints are loaded in pure shear and that the stresses perpendicular to the grain are minimised. In addition, the orientation of the orthotropic adherends has a major influence on stress distribution. As an example the shear modulus in

340

Adhesive bonding

so-called rolling shear, i.e., shearing perpendicular to the grain, can be an order of magnitude lower than the shear modulus along the grain. The orientation of the wood in the adherends also plays a role as regards the moisture-induced deformations that occur in all wood components. The swelling and shrinkage of wood is different in different directions. This means that even drying a wood adhesive joint slowly, to avoid moisture gradients, will still induce deformations and stresses in the joint if the adherends are not properly orientated.

15.6.2 Duration of load (DOL) effects and influence of climate According to national and international building codes the most severe load case for timber structures and structures made from wood-based panels often includes the long-term loading from self-weight, and medium-term loading from snow. It is therefore of utmost importance that wood adhesives behave well for long-term loading, at least in relationship to the behaviour of the wood itself. On the other hand, wood is known to behave poorly under long-term loading. The climate to consider for timber structures will typically include elevated temperatures up to 50±80 ëC, and in extreme cases a relative humidity (RH) of the air of 100%. As an example of the severity of the influence of DOL and climate, the present European code for timber structures gives reduction factors for the strength values in the range of 0.20±0.6 for the case of permanent loads (dead weight). The lower values are used for wood-based panels, the higher values for solid timber and glued-laminated timber. Generally, the performance of traditional PRF adhesives is unquestioned, and it is well known that these perform excellently in terms of strength, DOL and fire. Their main deficiencies are their brittle behaviour and poor gap filling properties In contrast to the well-established PRF adhesives, the DOL-performance of PUR adhesives has been widely discussed in the timber engineering community. The long-term durability of PUR adhesives is not well known, since they have not been used in construction for as long as PRF adhesives. Accelerated tests have been carried out, and the deterioration of the joint after the tests can be severe. Vick and Okkonen (1998) compared four commercial one-component PUR adhesives with one conventional PRF adhesive. The conclusions were that the dry strength of the PUR-adhesives is at least as high as that of the PRF and, after a water-saturating process, the strength of the PUR adhesives was still high, but the wood failure percentage was very low and the delamination severe. Later research (Vick and Okkonen, 2000) has shown that coupling agents can improve the delamination resistance and the durability of PUR adhesives, making them equivalent to structural PRFs. Studies on creep properties of PUR adhesives have been made on small samples as well as laminated beams. Creep rupture tests in compression have identified PUR adhesives that show acceptable creep properties also at elevated

Building and construction ± timber

341

temperatures, with tests made at 50 ëC and 80 ëC. The test results indicate that the relative strength loss over time is similar for different batches of the same adhesives. At the same time the test results indicate that the initial bonding quality of one-component PUR adhesives is strongly influenced by the raw material and bonding process, with great variations in time to rupture between different test samples as well as between different batches of test samples made with the same adhesive brand. Typically the time to rupture can differ by a factor of 1000 at the same test climate and shear stress (KaÈllander and Bengtsson, 2002). Reports on the results from ten years of load experiments with paired laminated beams exposed to an outdoor sheltered climate have shown that beams produced with one PUR adhesive could match PRF glued beams while beams made with a second PUR did not. The tests were, however, conducted on a limited number of beams and it is unclear to what extent the creep deformations measured were related to the wood material and not to the adhesives (Radovic and Rothkopf, 2003).

15.6.3 Predicting wood-adhesive joint failure The strength and durability performance of wood-adhesive joints based on traditional brittle adhesives is largely influenced by the behaviour of the wood adherends. Thanks to the relatively brittle and stiff adhesives used in the thin bond lines, the joints formed with this type of adhesive can often be treated as solid wood from a mechanical modelling point of view. As long as the adhesion to the wood is good, the failure of such joints can be estimated by considering the geometry of the joint, including the sharp corners often present, but neglecting the thickness of the bond line and assuming that the adherends are perfectly bonded. Such an estimate of joint performance would thus include the wood material mechanical properties only. Since wood is a highly non-homogeneous, orthotropic material with different stiffness and strength properties for different modes of loading, with failure modes ranging from elasto-plastic in compression to quasi brittle in tension perpendicular to the grain and to brittle failure for tension parallel to the grain, this is, however, still a challenging task. For other types of adhesive or joints, the non-rigid connection between the adherends can be of importance even for the case when the adhesive bulk is as rigid as the PRF. This is the case for example when gap-filling adhesives are used, and where bond line thicknesses of up to 5±6 mm have been reported for bonded-in rod applications using epoxy adhesives. In order to predict accurately the behaviour of a wood adhesive joint, it is necessary to know its local bond line performance. This can be obtained by using shear and tensile specimens, recording the stress versus slip behaviour in different modes of loading. It has been shown (Wernersson, 1994; Serrano, 2000; Serrano and Gustafsson, 1999) that wood adhesive bonds are quasi brittle,

342

Adhesive bonding

Figure 15.3 Test results with specimens cut from finger-joints showing shear stress versus shear displacement behaviour for three different adhesives (Serrano, 2000).

with strain softening behaviour. This means that the bond line can still transfer load at increasing deformation even after the maximum stress has been passed, although the load transferring capabilities diminish with increasing deformation. The experimental evidence for this was first presented by BostroÈm (1992) for solid wood specimens and for wood adhesive bond specimens by Wernersson and Gustafsson (1987). Test results obtained with small specimens cut from timber finger-joints are shown in Fig. 15.3 (Serrano, 2000). Using non-linear behaviour, it is possible to obtain strength models that are applicable beyond the range of traditional linear elastic or elasto-plastic strength analyses. In Gustafsson (1987), a so-called quasi-nonlinear strength prediction model for overlap joints is presented. The model is based on the traditional Volkersen shear-lag theory but accounts for the fracture energy of the bond line. This makes it possible to predict the behaviour of joints ranging from brittle to ductile, depending on their material and geometrical parameters. Thus, this socalled generalised Volkersen theory includes the theories of elasto-plastic analysis and linear elastic fracture mechanics as special cases corresponding to the fracture energy of the bond line being infinite with a limited strength or the fracture energy being limited but the strength being infinite, respectively. Closed-form solutions have been obtained for several types of joints, single and double overlap, dowel-type joints, and tubular joints. For cases including complex geometries or loading conditions, it is generally not possible to obtain closed form solutions. In such cases, the finite element method can be used in combination with non-linear material models according

Building and construction ± timber

343

to Fig. 15.3. Examples of applications studied using non-linear FE-modelling include finger-joints and bonded-in rods (Serrano and Gustafsson, 1999; Serrano, 2001). These studies show that there is potential in PUR adhesives as structural adhesives, thanks to their higher ductility and fracture energy, as compared with the more brittle PRF adhesives. It is even possible to increase the joint strength by using a more ductile adhesive, even if this adhesive has a (moderately) lower strength (Gustafsson 1987, Serrano 2004). As an example of the finite element modelling performed, the element subdivision used and the joint geometry studied for a glued-in rod connection is shown in Fig. 15.4. Because of symmetry, only one-half of the length of the specimen as well as one-half of the width was analysed. To make it possible to trace post-peak-load behaviour, the pull-out loading of the rod was applied by increments of displacement. The adhesive layer was modelled with a non-linear softening model defined by bond line stress-slip relations similar to the ones depicted in Fig. 15.3. The resulting stress distributions in the bond line of the glued-in rod are shown in Fig. 15.5 for a linear elastic state and at maximum global load.

Figure 15.4 A finite element model of a glued-in rod.

344

Adhesive bonding

Figure 15.5 Stress distribution in the linear elastic state (top) and at ultimate load (bottom). ÐÐ shear stress. ---- peel stress.

Obviously, the local performance of the bond line is highly nonlinear. Table 15.1 gives the numerical values of the pull-out loads as predicted with the numerical model and as obtained in the tests. The agreement with the test results is in general good, except for the epoxy, which exhibited a larger amount of wood failure than the others, and such failure modes were not included in the study (Gustafsson and Serrano, 2002). Bearing in mind that the material parameters (input data for the FE-model) were obtained from separate tests with small specimens, only approximately 10 mm in glued-in length, and these test results were used without further calibration in large specimen models, the predictions of the FE-analyses are accurate.

Table 15.1 Simulated and tested pull-out loads, Gustafsson and Serrano (2002) Adhesive

PRF

PUR

EPX

Glued-in length (mm) l ˆ 160 l ˆ 320 l ˆ 640 l ˆ 160 l ˆ 320 l ˆ 160 l ˆ 320 Test (kN) 55.3 101.7 144.1 64.4 91.0 61.6 106.3 FEM-prediction (kN) 53.9 104.1 151.6 67.1 93.8 89.2 118.7

Building and construction ± timber

345

15.7 Common failures In general, two different reasons for the failure of wood adhesive are found in practice, failure due to the mechanical performance being poor in relation to the design loading, or failure which can be related to the process of bonding or manufacturing. Poor mechanical behaviour can often be attributed to the low perpendicular grain strength of the wood, which often is overseen by the engineer. Also, the DOL effect plays an important role here. The process of bonding is sensitive to various types of errors in production. Examples found in practice include neglecting to change or sharpen milling tools, using the wrong tool speed, thus creating burnt surfaces, poor wetting due to the migration of natural extractives to the surface, and neglecting to account for a short pot-life of the adhesive. The most common failures in glued wood members are related to the production process, and should generally be detected in the quality control system of the producer. · Wood with too high moisture content can lead to problems with RF-heating of the adhesive, leading to a risk of the gluing press being opened before the adhesive has cured. · Planing too long before bonding can lead to reduced wetability as well as dimensional changes of the material to be bonded. · Disturbances in the glue spreader can lead to lack of adhesive on the bonded surface or wrong mixing ratio between resin and hardener. · Long assembly times can lead to pre-curing of the adhesive. · Low temperature in the glue press or short press time can lead to delamination as well as reduced resistance to moisture. · Extremely cold weather in combination with too early transport from the factory can lead to halted post curing. The traditional adhesives show very good strength and durability once a proper bonding has been achieved. Damage to wooden structures is primarily linked to excessive loads or unfavourable stress patterns due to incorrect design or improper handling of the material. Wood is sensitive to stress perpendicular to the grain, and several failures have occurred when large beams have split at, for instance, holes or notches. The capacity to carry stress perpendicular to the grain direction can be greatly increased by means of reinforcement by glued-in rods or glass- or carbon-fibre glued to the surface of the beams. Larger glued wood members are commonly produced at a moisture content (MC) of the wood of approximately 12%. After the building has been finished, the wood in an indoor structure commonly dries to around 6%. If the wood is prevented from shrinking, checking can occur. Shrinking can also lead to additional stresses around notches or glued-in rods. Another reason for failures in glued wooden structures is a lack of fundamental understanding of wood as an engineering material. Laminated

346

Adhesive bonding

members in wooden structures commonly have a slender cross-section with a large depth in comparison to the width, leading to lateral instability unless the structure is properly stabilised. The risk of tilting is especially large during erection of the structure, before the lower frames of the members have been connected and several such failures have been reported. Legislation in Europe makes it difficult to join large wooden members on a building site by means of adhesive bonding. As opposed to welding of steel, no systems for licensed adhesive operators and on-site quality inspection of adhesive bonds exist. Thus, large wooden members are commonly joined by bolts. As the holes for the bolts are drilled, the cross-section of the beam is reduced which, in addition to stress concentrations around the bolts, has caused several failures. One type of failure in glued wood structures relates to the public view of wood as a simple material and the ease by which one can work the material. A fair number of representatives of the public have learned the hard way that a beam can lose part of its capacity to carry a load when the bottom flange is cut. Chemical damage differs from the normal failures above. Acid damage to the wood has led to failures of several large structures. Cold setting phenol adhesives with acid catalysts were earlier used in Europe for wooden structures. Use of cold setting phenol adhesives for load-bearing structures was stopped after two failures in Sweden in 1965. The failures in 1965 were primarily caused by poor craftsmanship of the manufacturer of the wooden beams, but acid damage from the resin was believed to possibly have influenced the damage. More than 20 years later, in 1987, buildings started to fail. The buildings had been inspected in 1965 and found to be without damage. Inspections of more than 110 buildings and analysis of samples from bond lines have later shown that the acid in the adhesive had catalysed a hydrolisation of the cellulose immediately close to the adhesive, with a total loss of bond strength as a result. Acid damage can lead to catastrophic failures since only the strength but not the stiffness of the beams is affected until a bond line actually fails (Hedlund, 1989, 1990).

15.8 Inspection, testing and quality control Due to the size of wooden structures and the variations within the wood material, actual inspection of the quality of bond lines on finished products or in service is very difficult. Although it is possible to use acoustic methods to detect hidden delaminations (bond line openings), these methods can generally only detect completely separated sections of the bond line, a weakened bond line with low strength is not detected. Apart from visual inspection where open bond lines are detected, bond line quality must normally be established by means of tests on samples cut from the structure. The difficulties above have led to quality assurance systems of the production process. Wood adhesives for load-bearing purposes are generally considered a raw material or input material to be used in the finished product rather than a

Building and construction ± timber

347

separate product that will be sold to the public. The systems for approval of wood adhesives and quality control of glued wood products are, in principle, similar in most countries of the world; the adhesive is approved for specific purposes after a series of tests according to local standard procedures. Once an adhesive has been approved for a specific purpose, quality assurance of the adhesive is generally controlled by the adhesive producer. The requirements for the adhesive is, in principle, set for each product and the quality assurance system covers the production process and final quality of the glued product, a system in which the quality of the adhesive used is controlled indirectly through tests of produced bond lines. The approval systems for load-bearing adhesives in Europe as well as North America have primarily been developed for phenolic and aminoplastic adhesives. Testing procedures and requirements have been developed based on many years of practical experience. These traditional adhesives show extremely good properties regarding resistance to creep, elevated temperatures, moisture and biological attack. In principle, in a properly produced product, both the adhesive and the bond line outlive the wood under all circumstances. The approval systems have thus been aimed at rough tests verifying that small samples made with the adhesive show acceptable bond strength after a series of relatively short climatic treatments. Long-term tests to verify the durability or creep properties of the adhesive are generally not required. The European system for approval of adhesives is used as an example: Eurocode 5 states that an adhesive that is to be used for a load-bearing wood structure shall comply with requirements set in European standard EN 301, with tests made according to procedures in EN 302 parts 1±4. The test procedure in Part 1 covers shear tests on small beech samples after various climatic treatments ranging from testing in dry state to testing wet after boiling and cooling. Part 2 is a delamination test where laminated members of spruce (or the actual species that the adhesive is going to be used with) are dried in severe conditions after impregnation with water. If the wood breaks, the adhesive has passed the test. If the bond lines fail, the adhesive fails. Part 3 is a test to control that the adhesive does not cause chemical damage to the wood fibres (like the acid damage described previously). Finally, Part 4 is designed to show that the adhesive properties allow for swelling and shrinking of the wood without failure in the bond line. In the USA, a corresponding standard for structural laminated wood is ASTM D2559. The test procedures as well as the requirements have proven efficient for phenolic and aminoplastic adhesives. However, the tests do not cover creep deformation under sustained load or effects of elevated temperatures, issues that are important for new adhesives such as polyurethane (PUR) and emulsified polymer isocyanate (EPI). These adhesives are expected to be more sensitive to elevated temperatures and can show creep under sustained load, and thus both requirements and test procedures for these properties are needed. Such systems

348

Adhesive bonding

are presently being developed for instance by CEN in Europe (KaÈllander and Bengtsson, 2002; KaÈllander, 2003), CSA in Canada (CSA, 2002) and ISO. All of the systems are in principle based on existing test procedures for phenolic and aminoplastic adhesives, with additional tests to cover the properties above. For some applications, such as glued-laminated timber or finger-jointed structural lumber, additional standards exist which prescribe specific production control procedures and requirements on the product. As examples, for gluedlaminated timber and wood-based panels, delamination testing and internal bond testing is performed in the process of approval and quality assurance.

15.9 Repair Repair of timber structures may be necessary for several reasons because of accidents, decay due to insect or fungus attack, design errors, overloading, moisture-induced deformations or altered use (Broughton and Hutchinson, 2001). The repair of timber structures can be performed using traditional carpentry methods, using mechanical fasteners to attach strengthening parts, or by resin methods. The use of resin methods is of special concern for the building preservation bodies, due to the less well-known behaviour of the adhesives in terms of material compatibility, and structural performance. The general principle behind the conservation of historical structures is that the method employed should possess authenticity in material and methods used, it should be reversible, and it should use a minimum of intervention. Fully authentic methods imply the use of large grown trees of less common structural timber species and hand labour using traditional tools, both adding significantly to the cost of these methods. The most reversible method is probably one based on the use of mechanical fasteners, although these are not always practical to use due to moisture-induced deformations, aesthetics, and increased weight. Resin methods are preferred for some applications since they often can be used in situ, which is important for delicate structures. Another advantage is related to the fact that resin methods often require a very small amount of loss of original material. Drawbacks of using resin methods include the over stiffening of joints and the unclear long-term behaviour (Wheeler and Hutchinson, 1998). Various commercial methods for resin repairs exist. The methods are similar and are mainly based on the use of epoxy adhesives. In repair and strengthening applications, the damaged wood is cut from the beam or column, and an epoxy grout is cast as replacement material. For stitching fissures and for columns and beam upgrades, bonded-in rods or plates are used.

15.10 Examples of use Some examples of the use of adhesive technology in timber engineering are given below. The examples chosen represent the development of engineered

Building and construction ± timber

349

wood products chronologically from the first engineered wood products such as glulam and plywood to the more recent applications such as LVL and PSL and also the use of adhesive in joints such as finger-joints and bonded-in rods. Glulam is basically what one obtains by stacking a number of boards or laminations on top of each other and gluing them together, so that they form a beam cross-section of the shape desired. For approximately a century, glulam has been used as a material with enhanced performance as compared with solid wood. Early examples of glulam structures still in use are found at the railway stations in MalmoÈ and Stockholm, Sweden, built in 1922 and 1925, respectively. Among the most often cited advantages of using glulam are: · freedom in the choice of size and geometrical shapes · improved strength and stiffness properties · improved accuracy of dimensions and stability of shape during exposure to moisture · possibility to match the lamination qualities in relation to expected stress levels. Theoretically, glulam can be produced in almost any size. For practical reasons related to transportation and factory layouts, the maximum length is commonly approximately 16±20 m. Another limiting factor on the size is the pot-life of the adhesive, extremely large or complicated beam lay-ups take too long to assemble. Glulam comes in various shapes, straight prismatic beams and columns being the most common, but also curved or tapered beams are in general use. Glulam is a highly engineered product which, due to the industrialised production method, allows for quality controls to be performed in the production process. The quality controls include bending or tensile tests of finger-joints, delamination tests, and shear tests of the glue line. Such quality control methods are important parts of glulam production, involving both internal control performed by the producer and external control performed by an independent third party. In the early days of glulam manufacturing the laminations were not lengthwise joined at all, but the introduction of the finger-joint in the early 1960s radically improved the performance of glulam. A finger-joint can be described as a multiple overlap joint, and is a very efficient way of splicing wood members (Fig. 15.6). Finger-jointed structural timber can be used in applications in the same way as solid timber. The finger-joint introduces an area reduction, which is proportional to the ratio of the width of the finger tips to the pitch of the fingers. This ratio should be kept small, and this can be achieved by using small tip widths, i.e., cutting the fingers as sharp as possible. Although the finger-joint obviously is to be regarded as a weak section, with a strength that is lower than that of the clear, knot-free wood, it is not always true that the joint is the weakest section. This is because other parts of the jointed members can include strength-

350

Adhesive bonding

Figure 15.6 A finger-joint cut in spruce wood and bonded with a phenolic resorcinol.

reducing anomalies such as knots and deviation of grain direction that are more severe in terms of strength reduction. In some applications, the finger-jointing is done merely to obtain laminations or studs of a specified length, thus optimising the use of the raw material. Defect (knot) elimination for improving strength and aesthetics are other reasons for using this effective joint type. Plywood is obtained using wood veneers stacked on top of each other in an odd number of layers, each layer having the grain direction turned 90ë. This produces a sheeting material with less orthotropy. Since the early 20th century, plywood has been industrially produced. For many years, it was used for the aircraft industry as the high-tech material of the time and object for extensive research. In the early days, production was based on natural adhesives but today hot-setting PF adhesives are normally used for producing structural or marine grade plywood. The process of cutting or even disintegrating the raw material into smaller pieces such as flakes or fibres has led to the development of flake boards, such as oriented strand board, OSB or high density fibre boards, HDF. In these types of product the re-orientation of the raw material leads to a lesser degree of orthotropy, as was the case for plywood. However, the lesser orthotropy is generally obtained at the expense of lesser strength as compared to the strength of solid wood loaded parallel to the grain. On the other hand, advantages obtained include improved dimensional stability and availability in large sizes. Adhesives used in the production of these materials include UF, MUF or MDI (isocyanate) (Dunky, 1998; Griffiths, 1995). Laminated veneer lumber (LVL) is obtained by gluing a number of veneers together using a structural adhesive, similar to the process of making plywood.

Building and construction ± timber

351

In LVL, however, all veneers are orientated with the same fibre direction. Sometimes, a few layers are orientated in the cross direction in order to improve the dimensional stability and the strength perpendicular to the main grain direction. The veneers are approximately 2.5±5 mm thick and are stacked on top of each other to obtain billets with thicknesses ranging from 20±90 mm. These are then cut into the desired shape, and the final products available include beams and headers of 65±1200 mm depth with lengths up to 25 m. The adhesive used in producing LVL is typically PF. The main advantages associated with the use of LVL include improved strength and less variability as compared to solid wood. In the mid and late 1980s two similar products known as laminated strand lumber (LSL) and parallel strand lumber (PSL) were introduced. PSL is produced by cutting veneers into strands and gluing with PF adhesive to form billets, which are then cut to beams and headers. Although similar to PSL, LSL is produced from strands, which are cut directly from the log, and then assembled into billets using MDI adhesive (isocyanate) (Lam and Prion, 2003). A final example of the effective use of adhesives in timber applications is bonded-in rod connections. By inserting steel rods, threaded bolts, or deformed reinforcement bars, or glass-fibre reinforced pultruded polyester rods (GFRP), it is possible to obtain strong and stiff beam-to-column connections or column foundations. It is also possible to use the bonded-in rod connector for reinforcing the wood perpendicular to the grain. This type of connector has been successfully used in the Nordic countries and in Germany since the 1970s. The adhesives used in these joints have been epoxies and PURs although even modified PRFs were used in a European research programme on glued-in rods (Bainbridge et al., 2002). The main advantages of using this type of connection are related to its strength and stiffness properties, in combination with the aesthetics obtained by the almost invisible connector. By embedding the steel parts in the insulating wood, good fire properties can also be obtained. The main disadvantage of using the bonded-in rod connector is perhaps the difficulty in obtaining a ductile joint, such that brittle failure is avoided.

15.11 Future trends and further reading The future trends regarding development of wood adhesives can be expected to follow four main lines: 1. 2. 3. 4.

improved environmental performance broadened fields of application improved possibilities to predict and control the properties of the adhesives and bondlines improved sorting and preparation of the wood material to be bonded.

352

Adhesive bonding

15.11.1 Environment One clear line of development is to reduce the need for non-sustainable raw materials. Efficient adhesive systems based on sustainable raw materials like tannins are likely to be developed. Existing adhesive systems are increasingly being mixed with sustainable raw materials, like soybean protein. Several cold setting adhesive systems have been introduced, challenging the traditional adhesives, which require heat for curing. Presently ammonia-accelerated systems are used for fingerjointing (Soybond, Greenweld) and one-component polyurethane adhesives are used both for finger-jointing and lamination. The cold setting systems not only reduce energy consumption, but also require smaller investments in machinery. The manufacturers of traditional formaldehyde-based adhesive systems have already greatly reduced the amount of formaldehyde in the adhesives, which in combination with advanced formulations of the adhesive and bonding systems greatly reduce the emissions during production and in the user stage. Improved methods to eliminate or seal formaldehyde in the products will probably be developed. This development will be accelerated by national legislation against emissions in the consumer stage in countries like Japan. The new wood adhesive systems based on isocyanates lead to a risk of introducing new health hazards during production. However, published results from measurements made in production plants indicate that the MDI-based adhesive systems presently used in the wood industry show extremely low emission rates. At the same time, the systems have a great benefit in the absence of emissions from the finished product. A step back concerning improvement of the environment in the production plants could be the ammonia-accelerated systems for cold and green fingerjointing that have been introduced. Efficient systems to protect staff from ammonia emissions need to be developed.

15.11.2 Broadened fields of application The ammonia-accelerated systems for cold and green gluing have primarily been used for finger-jointing. With the addition of one-component PU systems, laminations can also be made with green wood. There is a great interest in green gluing within the scientific world in Europe, and it can be expected that different systems for green gluing of laminations will be developed in the near future. On-site gluing will increase in importance and thus new products and methods will be developed. It is expected that the need for new housing will drop dramatically in Europe as well as in Japan during the next decades. As the production of new houses drops and existing houses become older, the need for efficient methods to alter existing buildings will increase.

Building and construction ± timber

353

The developments described above will also increase the need for bonding of other materials to wood. Fibre reinforcement is already used to strengthen and stiffen wood and strengthening of wooden floors by concrete slabs is currently used in Europe.

15.11.3 Improved prediction and control of product performance and adhesive formulation Structural wood adhesives are frequently optimised for a specific product and production process. With modern techniques for analysis and quality control of raw materials combined with improved systems for calculating and simulating the production process, adhesives can be tailor-made for each required strength and production process. Optimised quality of adhesives will be combined with improved methods for calculating and designing glued products, making it possible to adapt the properties of the adhesive to the needs of each product. For example, a product with a defined low stress load can be produced with a weak adhesive. Tailor-made adhesives with low variations in adhesive properties will allow reductions in environmental impact and cost as well as production. An adhesive designed for low stresses can probably be made cheaper, with increased production rates. Improved methods to analyse adhesives and bond lines, in combination with improved knowledge about the actual interaction between adhesives and adherend, will open up possibilities to develop fast and reliable methods to approve new adhesive types. Today, a new adhesive type requires prolonged studies of glued members in order to be approved for structural purposes. In the future, improved methods for determining adhesive properties and calculating long-term performance can greatly reduce the time and cost required to get acceptance for new products. With faster (and cheaper) approvals, the development of new adhesive products will benefit. Improved methods of calculation will also make it possible to create new advanced wood structures, structures that both can improve the competitiveness of wood against other raw materials and function as an inspiration for architects and property developers.

15.11.4 Improved material handling In addition to improved adhesive properties, the properties of the adherends, wood or materials glued to the wood, will be improved. When surface treatments like laser ablation, chemical activation or flame treatments are commercialised, it will be possible to use bonds with higher strength and durability in exposed positions, perhaps in combination with bond lines made with cheaper methods in less exposed areas. Improved methods of drying and conditioning of wood will reduce moisture related distortion of the finished product as well as the influence of moisture

354

Adhesive bonding

variations on the curing process of the adhesive. With improved control of the wood raw material, both production rate and product quality will improve. Recently developed methods for selection of the material to be bonded, such as automatic scanning and sorting equipment, will help to reduce production costs and improve the quality of the final product.

15.11.5 Further reading For further reading in the field of adhesive applications in timber engineering and wood-based products one can generally recommend two types of literature, literature on wood and timber and literature on adhesive bonding. The amount of literature available in these two fields is extensive. Classic work on the properties and structure of wood as an engineering material can be found in Kollman and CoÃte (1968). Recent literature in the field of timber engineering includes the book of Madsen (1992) and the more recent and research-orientated work of Thelandersson and Larsen (2003). The latter includes several chapters on adhesive joints and related areas. The amount of literature available for the specific field of wood adhesion and its application is more scarce. The list of references for this chapter should of course be consulted and especially the state-of-the-art reports by Dunky et al. (2002) and Johansson et al. (2002) should be mentioned. These reports, which were written within a European research action (COST), present an extensive overview of the field of wood bonding. Another work which deserves a special mentioning, is the extensive book on wood adhesion and its applications by Marra (1992).

15.12 References Bainbridge R, Mettem C, Harvey K and Ansell M (2002) `Bonded-in rod connections for timber structures ± development of design methods and test observations', Int. J. Adhesion and Adhesives, 1, 47±59. BostroÈm L (1992) Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model, PhD. thesis, Report TVBM1012. Lund Sweden: Division of Building Materials, Lund University. Broughton J G and Hutchinson A R (2001) `Adhesive systems for structural connections in timber', Int. J. Adhesion and Adhesives, 3, 177±186. CSA (2002) O112.9 Draft. Standard specification for evaluation of adhesives for structural wood products (Exterior exposure). Canadian standards organisation, 2002. Davis G (1997), `The performance of adhesive systems for structural timber', Int. J. Adhesion and Adhesives, 3, 247±255. Dunky M (1998) `Urea-formaldehyde (UF) resins for wood', Int. J. Adhesion and Adhesives, 2, 95±107. Dunky M, Pizzi A and van Leemput M, eds (2002) COST-Action E1. Wood adhesion and wood products. State of the art report. Working group 1: wood adhesives.

Building and construction ± timber

355

Griffiths D R (1995) `Wood-based panels ± Fibreboard, particleboard and OSB', in Blass H J et al. (eds) Timber Engineering STEP 1, Centrum Hout, Almere, The Netherlands. Gustafsson P J (1987) `Analysis of generalized Volkersen-joints in terms of non-linear fracture mechanics' in Verchery G and Cardon H, Mechanical Behaviour of Adhesive Joints, Paris, France, Edition Pluralis. 323±338. Gustafsson P J and Serrano E (2002) `Glued-in Rods. Local Bond Line Fracture Properties and a Strength Design Equation'. In Proc. of the International Symposium on Wood Based Materials, Wood Composites and Chemistry, 19±20 September, Vienna, Austria. Hedlund B (1989) `Acid deterioration of glulam beams in buildings from the early half of the 1960s', Proceedings. CIB W18A Meeting 22, Berlin, 1989. Hedlund B (1990) Syraskadade fenollimmade och staÊlarmerade traÈbalkar. Lokalisering och besiktning av byggnader med riskabla balkar. Utredning av skadornas orsaker och foÈrlopp (in Swedish). SP Rapport 1990:27. SP Swedish National Testing and Research Institute. BoraÊs, Sweden. Johansson C-J, Pizzi A and van Leemput M, eds (2002) COST-Action E1. Wood adhesion and wood products. State of the art report. Working group 2: glued wood products. KaÈllander B (2003) CEN/TC193/SC1/WG4 (One component polyurethane adhesives) Convenors report to SC1. Document CEN/TC193/SC1: N215. KaÈllander B and Bengtsson C (2002) `Creep testing wood adhesives for structural use' Proceedings. CIB W18 Meeting 35, Kyoto, 2002. KaÈllander B and Lind P (2002) Strength properties of wood adhesives after exposure to fire. Nordtest Project No 1482-00. SP-Report 2001:35, SP Swedish National Testing and Research Institute. BoraÊs, Sweden. Kollman F F P and CoÃte W A (1968) Principles of wood science and technology Vol. 1. Solid wood, Springer-Verlag, Berlin, Germany. Lam F and Prion H G L (2003) `Engineered Wood Products for Structural Purposes' in Thelandersson S and Larsen H J (eds), Timber engineering, John Wiley & Sons, Ltd. Chichester. Madsen B (1992) Structural behaviour of timber, Timber Engineering Ltd., Vancouver, British Columbia, Canada. Marra A A (1992) Technology of Wood Bonding. Principles in practice. Van Nostrand Reinhold, New York, USA. Nussbaum R M (1993), `Oxidative activation of wood surfaces by flame treatment', Wood Science and Technology, 27, 183±193. Nussbaum R (2001) Surface interactions of wood with adhesives and coatings, PhD thesis. KTH ± Royal Institute of Technology Department of Pulp and Paper Chemistry and Technology Division of Wood Chemistry. Stockholm, Sweden. Radovic B and Rothkopf C (2003) `Eignung von 1K-PUR-Klebestoffen fuÈr den Holzbau unter BeruÈcksichtigung von 10-jaÈhriger Erfahrung', Bauen mit Holz, 6, 36±40. Seltman J (1995) `Freilegen der Holzstruktur durch UV-Bestrahlung', Holz als Roh- und Werkstoff, 4, 225±228. Serrano E (2000) Adhesive joints in timber engineering ± modelling and testing of fracture properties, PhD Thesis, TVSM-1012, Division of Structural Mechanics, Lund University, Sweden. Serrano E (2001) `Glued-in rods for timber structures ± a 3D model and finite element parameter studies', Int. J. Adhesion and Adhesives, 2, 115±127.

356

Adhesive bonding

Serrano E (2004) `A numerical study of the shear-strength-predicting capabilities of test specimens for wood-adhesive bonds', Int. J. Adhesion and Adhesives, 1, 23±35. Serrano E and Gustafsson P J (1999) `Influence of bondline brittleness and defects on the strength of timber finger-joints', Int. J. Adhesion and Adhesives, 1, 9±17. Stehr M and Johansson I (2000) `Weak boundary layers on wood surfaces', Journal of Adhesion Science and Technology, 10, 1211±1224. Stehr M, Seltman J and Johansson I (1999) `Laser ablation of machined wood surfaces. 1. Effect on end-grain gluing of pine (Pinus silvestris L.) and spruce (Picea abies Karst.)', Holzforschung, 1, 93±103. Thelandersson S and Larsen H J (2003) Timber Engineering, John Wiley & Sons, Ltd. Chichester. Vick Ch B and Okkonen E A (1998) `Strength and durability of one-part polyurethane adhesive bonds to wood', Forest Products Journal, 11/12. Vick Ch B and Okkonen E A (2000) `Durability of one-part polyurethane adhesive bonds to wood improved by HMR-coupling agent', Forest Products Journal, 10. Wernersson H (1994) Fracture characterization of wood adhesive joints, PhD thesis, Report TVSM-1006, Lund, Sweden, Division of Structural Mechanics, Lund University. Wernersson H, Gustafsson P J (1987) `The complete stress-slip curve of wood-adhesives in pure shear', in Verchery G and Cardon H, Mechanical behaviour of adhesive joints, Paris, Edition Pluralis, 139±50. Wheeler A S and Hutchinson A R (1998) `Resin repairs to timber structures', Int. J. Adhesion and Adhesives, 18, 1±13.

16

Automobiles

K DILGER

16.1 Introduction `The future of structural adhesive bonding as a joining technique in automotive construction has only just begun. Modern lightweight design, safety and modular concepts can no longer do without adhesive bond joints and the strength they provide in a crash scenario. To date, one has relied on combined joining methods to increase rigidity, energy absorption capability, and fatigue strength of body structures. The technological trend, however, is a gradual one toward singular adhesive bond joints. An essential key to fast, process-reliable transfer of this technology to series application is the further development and use of powerful simulation tools.' Heinrich A. Flegel, DaimlerChrysler AG.

Indeed, the use of adhesives started decades ago with the bonding of windscreens for design purposes. Later, windscreens and rear windows became a structural part of the car by the use of high modulus adhesives. At the same time adhesives were used to fill gaps in spot welded flanges to prevent corrosion. When the engineers noticed that a side effect of the enhancement of the corrosion resistance was the stiffening of the car body by the adhesive bonding, they started to use high modulus adhesives to optimise the torsion stiffness of the car. In the middle of the 1990s new adhesives with a high strength and a high energy absorption in crash situations were developed and the use of adhesives as a structural element in body construction started. The use of adhesives started about 40 years ago as a sealer. After that adhesives were used to prevent corrosion. The first structural adhesives were to enhance the stiffness of the car body. Now adhesives are being used as a real structural part of a car even in crash situations. The trend to a lightweight design by the use of different materials like high strength steels, Al- and Mg-alloys, sandwich structures and fibre-reinforced plastics accelerates the use of adhesives by car manufacturers. The new S-Class Coupe of DaimlerChrysler has more than 100 m of structural bonds in body in white applications; in the series 7 BMW there are more than 10 kg of structural adhesives applied.1±8

358

Adhesive bonding

Today the results of using adhesives in an automobile structure are · · · · ·

robust and sure processes lightweight construction crash resistant structures design, Class A-surface modular design, platform strategies.

16.2 Basic needs 16.2.1 Materials used in modern cars Until the end of the 1980s cars were produced mainly from mild steels. There were some exceptions where high strength steels, aluminium alloys or reinforced plastics were used in small quantities. Because the objective of the use of these materials normally was not to optimise the structural weight and the number of cars with these materials was very low, there was no need for an optimised joining technique, so that the parts that could not be welded were screwed together. Regarding the high number of all-steel cars, spot welding was an efficient, well known method to achieve car bodies with acceptable properties in robust processes. With the production of all-aluminium cars in the early 1990s this scenario changed. The aluminium alloys used could not be as easily spot welded because of the low electrical resistance of the aluminium, the very stable and not conductive oxide layer and the trend of the aluminium to interact with the spot weld electrodes. This impacts on the shelf life of the electrodes and leads to ineffective expensive processes. On the other hand, the strength of the welded parts is reduced by the effect of heat during the welding process of the aluminium. Especially regarding fatigue, the negative effect of the heat during the welding process is very relevant. To avoid the heat disadvantage cold processes such as riveting and adhesive bonding had to be established. In modern cars materials are used in a so-called multi-material design. In this concept lightweight materials like aluminium and magnesium alloys, fibre reinforced plastics and high strength steels are used where they fulfil the purpose best. Steel is used for pillars, longitudinal and cross beams which are absolutely crash relevant. Aluminium can be used for large parts like front bonnet, boot lid, roof or doors. Magnesium is often used for internal parts because of its poor corrosion resistance. Plastic is used for parts with complicated shapes because of its good formability and for bumpers and other parts that must have good elasticity if they are not to be damaged in accidents up to 15 km/h. For different reasons it is not possible to weld the mixed materials so that mechanical joining and adhesive bonding have to be used.3 In Fig. 16.1 the multi-material design used in the S-Class Coupe of DaimlerChrysler is shown.

Automobiles

359

Figure 16.1 Multi-material design in the S-Class-Coupe¨ of DaimlerChrysler.3

16.2.2 Process chain in automobile construction To determine the basic requirements for adhesives used in automobiles the process chain in automobile construction has to be described. Depending on the application of adhesives in different sequences of the process chain, different groups of adhesives with different properties during application and manufacturing and even during the use of the car can be defined. The classical process chain in automobile production can be divided into the following steps: · · · ·

press shop body shop paint shop trim assembly.

For corrosion resistance and for a better formability in the press shop the materials have to be lubricated. Normally different mineral oils are used for that purpose. Recently so-called dry lubrications ± highly viscous oils and waxes ± are used for the purpose. To bond these parts in the body shop the adhesive has to be applied on the oily surface. After the joining of the parts they have to be fixed for handling until the adhesive is cured. Mainly this is done by so-called hybrid joining, combining the adhesive bonding with spot welding or mechanical joining like riveting or clinching. A typical joint in body in white is shown in Fig. 16.2. After being jointed, the body has to be painted. Therefore the panels have to be washed to remove the oils and lubricants. After washing the body has to be

360

Adhesive bonding

Figure 16.2 Typical adhesive joint in body in white.3,9

pre-treated e.g. by phosphating. During these processes it is very important that the adhesive is not washed out of the joint. Washing out the adhesive would lead to leakage, lower strength and the contamination of the process fluids. To avoid this, commonly highly viscous adhesives that have to be applied warm are used in the body shop. Because the use of these materials could cause problems regarding the pressability of the bead and the weld through or clinch through properties often it is better to use a lower viscous material in the body shop that has to be precured in a furnace at, for example, 125 ëC to achieve a sufficient washout stability. After washing and pre-treatment the electrophoresis process follows. The applied paint is cured in a furnace at 180 ëC for about 30 minutes. This process step is used to cure the adhesive. In the next step final paint is applied and cured. Then the painted body goes to the trim assembly where the windscreen and the windows as well as trim strips and emblems are bonded onto the painted sheets. In Fig. 16.3 a typical adhesive joint made in the assembly is shown.3 Because heat treatment of the assembled automobile is not possible, cold curing adhesives with good adhesion to the paint have to be applied in the assembly. Figure 16.4 gives an overview of the adhesives used in a modern car. In another process the power train is assembled. In the power train adhesives are used for shaft to hub connections, for sealing purposes and as a thread lock.

Figure 16.3 Bonding on paint in the assembly line.3,9

16.2.3 Adhesives in the body shop As described earlier in the body in white adhesives are applied on oiled sheets. During the application of the mostly highly viscous adhesives it has to be

Automobiles

361

Figure 16.4 Adhesive applications in a modern car.10

ensured that the adhesive bead sticks to the panel and does not drift away on the oil film. In most cases warm applied highly viscous materials with a viscosity up to some 1000 Pas are used to ensure the washout stability of the adhesive and to ensure the right position and geometry of the bead during the process. In the joining process the adhesive bead has to be pressed within the two parts without deforming the parts due to the forces resulting from the high flow resistance of the adhesive. Regarding the application process, it is very important that the right amount of adhesive is applied at the right place. If there is not enough adhesive applied, leakage and insufficient strength are the result. An over dosage leads to the adhesive being squeezed out and pollution of the jigs and tools. Because the adhesive normally stays viscous until it is cured in the paint oven, it is necessary that immediate handling strength is obtained by other means. In an automated series production it is not economical to use mechanical fasteners. To achieve a sufficient immediate handling strength, adhesive bonding is combined with spot welding, riveting (self-piercing rivets) and clinching to a hybrid joining technique. In these hybrid joints the mechanical joints should supply only the handling strength and if needed compensate for peel loads; strength is obtained by the adhesive bond. There is no need to have two structural joining techniques. In Fig. 16.5 the behaviour of a part joined with different (hybrid) joining techniques is shown.10±12 Adhesives in an automobile body are used to avoid corrosion, to achieve higher body stiffness and to enhance resistance to fatigue and crashes. In Fig. 16.6 the different applications of adhesives in a car body are shown. To avoid corrosion in flanges and hem flanges adhesives have to fill the gaps to avoid the penetration of water. If there is no need to enhance the stiffness or the crash

362

Adhesive bonding

Figure 16.5 Behaviour of parts joined with different techniques (schematic).11

resistance of the parts, plastisols based on a PVC or an acrylic chemistry are used. Because of production restrictions the gaps are not filled completely and water could migrate into the joint. Therefore a cosmetic sealer is often used to prevent the water intrusion. These sealers are also on a PVC plastisol basis. An adhesively bonded hem flange with a cosmetic sealer is shown in Fig. 16.7.2 If organic precoated, galvanised or hot dip Zn-coated steel sheets or aluminium sheets with a conversion layer are used, it is in some cases possible to get rid of the cosmetic sealer. To enhance the stiffness of attached parts and outer panels and to reduce vibration and noise of these parts so called anti-flutter adhesives are used. These adhesives have to compensate tolerances and to guarantee a first-class surface of the outer panels. Therefore they have to fill up wide gaps, should have a low shrinkage and a comparatively low modulus. They have to have good adhesion

Figure 16.6 Different applications of adhesives in body in white.2

Automobiles

363

Figure 16.7 Bonded hem flange with cosmetic sealer.

on oily steel sheets and be cured in the paint oven. Mainly rubber-based adhesives that are sometimes modified by epoxies are used as anti-flutter adhesives.13±15 The structural stiffness of a body can be enhanced by 15 to 30% depending on body construction. In Fig. 16.8 the enhancement in stiffness of adhesively bonded structures compared to only spot welded structures is shown. Especially regarding aluminium, a higher fatigue strength compared to spot- and MIGwelding, clinching and even laser welding can be achieved. Figure 16.9 gives an example for the enhancement of fatigue properties of bonded aluminium parts

Figure 16.8 Structural stiffness of adhesively bonded bodies compared to only spot welded structures (DaimlerChrysler).3

364

Adhesive bonding

Figure 16.9 Comparison of different joining techniques for aluminium parts under fatigue load: WPS+Adhes. (spot welding and adhesive bonding), MAG (arc welding), SN (riveting), Laser (laser welding), WPS (spot welding).

compared to other joining techniques. Structural adhesives for stiffening the body with a good adhesion on oily steel sheets and degreased aluminium sheets are hot curing adhesives on an epoxy or rubber-based chemistry. A new generation of epoxy-based adhesives with a high energy absorption at high velocities is used in new car concepts to improve the crash behaviour of the body. These adhesives have a high shear strength under crash load, a low crack propagation and a high energy absorption under shear and peel load at high velocities. Because of this high strength and toughness the joints remain stable in crash situations and the bonded part can absorb high crash energies. In Figs 16.10 and 16.11 the behaviour of samples and real bonded parts in a crash situation is shown. According to references 16±18 a crash optimised adhesive, highly cross-linked thermoset resins such as catalytically cured epoxy resins, can be toughened without sacrificing strength. Figure 16.12 shows the matrix of such an adhesive layer. Recently other formulations with an equivalent behaviour of the adhesive layer under crash loads have come onto the market. Because body shop adhesives are cured in an electro-coat oven, it is necessary to create the handling strength of the body by joining techniques such as riveting (self-piercing rivets) or spot welding. Therefore the weld-through ability or rivet-through ability must be given. Washout stability is necessary if no pre-curing in the body shop furnace or by induction takes place. Warm applied one-part epoxy adhesives fulfil these demands and have been standard in automobile production for a number of years.

Figure 16.10 Behaviour of a Z-profile with a single overlap bonded joint at high testing velocity.

366

Adhesive bonding

Figure 16.11 Behaviour of a longitudinal beam bonded only with adhesive under crash load.

Figure 16.12 Crash-resistant adhesive layer according to refs 16±18.

Automobiles

367

16.2.4 Bonding in the trim assembly Bonding in the trim assembly means bonding onto the electrophoretically applied paint or onto the final paint. As well as the trim strips and emblems, with lower demands on strength, that are bonded to the painted sheets by pressuresensitive adhesive tapes or cyanoacrylates, the windscreen and the windows are bonded into the painted body. Because of the bonding of the windows the stiffness of the body increases up to 40%, strongly depending on the body design. The bonding of the screens has a structural (or semi-structural) character. The achievable degree of stiffening by the screen bonding depends on the modulus of the adhesive. In Fig. 16.13 the influence of the adhesive modulus on the torsion stiffness of a car body is shown. Because of the high tolerances of the screens the gap between the body and the screen can vary from less than two to more then 15 millimetres. One- and two-part polyurethanes used as semi-structural adhesives in a modern direct glazing application have a stake of about 95%. In some applications so-called silane modified polymers are used for windscreen bonding. These polymers do not contain isocyanide and have, for example, a polyoxypropylene or polyurethane backbone and reactive silane groups, which are responsible for the cross-linking and adhesion. Due to these silane groups these polymers have good adhesion on different surfaces and good UV stability. The mechanical properties still do not match those of the polyurethane adhesives. Another problem is that these different materials are not compatible and they do not

Figure 16.13 Influence of the adhesive modulus on the torsion stiffness of a car body.

368

Adhesive bonding

adhere to one another. Due to this, to ensure quality in repair, it makes sense to use only one kind of material for screen bonding.19 To guarantee the structural integrity of the body/glass assembly and to prevent leakage, as well as the adhesion and cohesion of the adhesive, the right application is also of importance. The application must ensure the presence of the material, the dimensional accuracy and the continuity of the bead. The highly viscous material is applied to the glass with a triangular cross-section. Dimensions of a typical bead are 8 mm at the base and up to 16±18 mm in height. This triangular shape is needed to compensate tolerances and to reduce joining forces. As well as application criteria such as sag resistance, stringing, skin formation time and pumping stability, physical properties such as adhesion on painted sheets, modulus and strength, the elongation at break and heat stability are very important and have to be guaranteed. Bonding to a painted sheet means that the adhesion of the adhesive to the body varies with the kind of paint used. There are big differences in adhesion between solvent and water-based paints and powder coatings. The mechanical properties of the joint vary even with the colour of the paint. Because the paint is a structural element of the joint, the adhesion and cohesion strength of the coating is important for the strength of the joint. Especially in a crash situation the bonding on a red paint will have different properties from those on a silver metallic paint. For this reason tests have to be performed considering the different colours. To avoid these tests, some automobile manufacturers tend to bond to a more constant surface. They cover the bond line after the electro coating, so that they bond to the electro coat and not to the final paint. In many cases a primer is used to enhance adhesion to the paint. The polyurethane adhesives used for direct glazing are very sensitive to UV radiation. To prevent the adhesive layer being destroyed by UV radiation there is a ceramic layer on the glass surface with a very low UV transmission. To minimise the UV transmission and to optimise adhesion to the ceramic layer some automobile manufacturers use black primers for the pre-treatment of the ceramic layer. In order to be able to move cars immediately after the glazing operation, a high positioning tack and a high green strength is necessary. To achieve this, direct glazing adhesives with a so-called quick fix character are available. These adhesives have a high viscosity at room temperature and a crystalline structure. They are applied at temperatures of between 50 and 80 ëC. At elevated temperatures the viscosity of these adhesives is comparable to conventional systems. While cooling down after joining the screen to the body, the viscosity rises which leads to a handling strength. Another possibility to improve handling strength and to support the hardening process of the moisture curing material ± even when the relative humidity is very low (e.g. in northern countries in winter) ± one-part polyurethanes are mixed with a water paste in static mixers during the

Automobiles

369

Figure 16.14 Sagging behaviour of different glazing adhesives.20

application process. This ensures the presence of water and accelerates the curing process due to shorter diffusion distances. New generation systems have a higher modulus to improve the stiffness of the body and a lower electrical conductivity, which is very important if electronic components like radio antennae are integrated into the screens. The lower conductivity is reached by the reduction of carbon black in the formulation. This reduction of carbon black leads to a change of the rheology of the adhesive. The sagging behaviour of the adhesive bead changes (Fig. 16.14). A new trend in car manufacturing is the integration of painted substructures like complete roofs (with the complete package) into a painted body. In this case two painted parts have to be joined in the assembly line in a cold process. The needs of this process are very similar to the screen bonding, so that the same technology as described for the screen bonding is used to bond these parts into the car body. Sometimes however, the requirements on the safety of these parts are higher than those of the bonded screens, so that the crash performance of the system sheet-paint-adhesive-paint-sheet has to be proved.19

16.2.5 Adhesives in the power train In the power train adhesives are used for shaft to hub connections, for sealing purposes and as thread locks. For shaft to hub connections mainly anaerobic acrylates are used. In most cases the adhesive bond is supported by a shrinking

370

Adhesive bonding

Figure 16.15 Bonded shrink fit.22

of the hub to the shaft. By this hybrid joining process a high axial strength and a high torque can be achieved. A secondary effect of the shrinking is residual stresses in the parts, which can lead to a reduction of the fatigue strength. Especially for rigid materials like sinter metal this can cause problems, so that the shrinkage should be reduced and the adhesive part of the strength should be increased.21,22 To produce these bonded shrink fits the hub should not be heated to more than 200 ëC to avoid damage to the adhesive and the shaft should not be cooled to avoid condensed water. Figure 16.15 shows a bonded shrink fit in a gear-shaft connection. Thread locks are made either by anaerobic or micro encapsulated adhesives. These adhesives have a defined initial breakaway torque. Silicones are often used in the power train in place formed gaskets. To enhance the curing time, there are UV curing products on the market. It is important that, because of the bad paint adhesion on silicone contaminated surfaces, silicones are used only for power train applications and it has to be ensured that no contamination of body parts is possible.

16.2.6 Loads and exposure to detrimental effects Loads and exposure to heat, moisture, etc., during production have to be distinguished from the expected loads and conditions during the service life of the vehicle. Loads during production During production the static and dynamic loads are not very high but it has to be considered that the adhesive is not cured or not cured completely. If the parts are not stiff enough during production or the flanges tend to open, the adhesive has

Automobiles

371

to be cured (precured) in body in white in a furnace or by induction heating. A critical process is the heating in the paint furnace where the adhesive is cured. On the one hand it is possible that due to handling problems, the body rests much longer in the oven than the defined 30 minutes. An over cure can occur and the properties of the adhesive layer may change. In tests prior to the use of the adhesive it has to be shown that the adhesive is tolerant enough to remain viable even when it is over cured. On the other hand, due to the thermal expansion of the parts and the shrinkage of the adhesive, stresses that may lead to residual stresses or even the destruction of the joint can be introduced here. A material mix with different thermal expansion rates especially causes problems. The so-called delta alpha problem is one of the main restrictions on the use of a multi-material design in body in white. To avoid these restrictions, there is a trend to shift bondings from the body shop to the trim assembly. Loads during use During use there are static, dynamic and impact loads. The strength of the bonded joints is influenced by weather conditions and other effects like UV radiation and detergents from washing. In the case of static and dynamic loads, the stiffness of the car body is important to improve the handling of the car. The fatigue strength of the bonded joints is normally higher than the fatigue strength of the parts. Under crash loads the shear and the peel strength as well as the energy absorption of the adhesive layer are relevant. Special adhesives with good crash properties, like some toughened epoxies, can be used. The change in properties due to ageing in a warm and humid atmosphere has to be considered. Here the change in properties of the boundary layer is as, or even more, relevant as the change in properties of the bulk material.

16.3 Adhesive characteristics required 16.3.1 Required characteristics during production In the series production of automobiles adhesives are mainly applied automatically by robots and adequate dispensing systems. The adhesive is applied to the sheets in different positions and with different bead geometries. The position of the bead and the geometry must be consistent. Therefore the adhesive has to have a high initial tack to stick to the panel. To have a stable shape, the adhesive should have a high viscosity when it is applied. Because highly viscous materials are not easy to pump and to dispense, shear thinning materials are used because these materials have a lower viscosity during application and build a robust bead after application. The process is often supported by heat application at temperatures over 35 ëC. The warm material has a lower viscosity and the temperature over 35 ëC ensures that the temperature of

372

Adhesive bonding

the adhesive is higher than the ambient temperature which ensures constant temperature of the applied adhesive and therefore a constant viscosity. The material is sometimes pumped over a long distance and the shear rate in this process can change the rheology, which means that the pumping distances should be in a certain range and the adhesive should be robust enough for the defined process. To have a clean application the stringing of the adhesive is of importance. A high position tack is necessary to ensure the position of the bead. Especially on oily surfaces this is not easy to achieve. Depending on the application position, the stiffness (viscosity) of the bead and the application speed (and the application radius) the bead can drift away on the oil layer. To avoid this, the amount of oil is limited to 3±4 g/m2. Body shop adhesives are based on one-part epoxies for structural applications, one-part butyl rubber (modified) for anti-flutter and one-part PVC-plastisol for hem flange bonding and cosmetic sealer cured at 180 ëC during 20±30 minutes in the electro coat furnace. To enhance washout performance, a precure step at 120 ëC is carried out by some manufacturers. If induction heating is used, which is very useful to obtain a quick handling strength, the adhesives should be adapted to the high heating gradients. Due to production discontinuities the adhesives can stay in the oven for a longer period. A dramatic change of the adhesive properties in this situation should be avoided.12,23±26

16.3.2 Required adhesive characteristics during use of the vehicle As described above, adhesives in automobile construction are used to prevent corrosion, to enhance stiffness of the car body under static and dynamic load and to improve the fatigue and the crash properties of the car. For the different objectives different adhesive properties are required. Corrosion protection For corrosion protection different types of adhesives are used to seal the flanges. Actually hem flanges are sealed by the hem flange adhesive and the cosmetic sealer. To achieve good corrosion resistance it is necessary that the hem flanges be filled with adhesive and sealer in the right quantity at the right place. There should be no gaps or bubbles in the sealing bead. To achieve this, the application of hem flange adhesive and cosmetic sealer has to be controlled. If water were to enter the gap, it should be possible for it to run out. This can be achieved by a wide hemming radius that is not filled up with adhesive and open at the end, to give the water the possibility to run out. Flanges that are not filled up completely with adhesive (which is the industrial standard) contain air. This air expands during the curing process in the oven. If the air cannot leave the flange by a

Automobiles

373

predefined path, the expansion leads to bubbles and leakage in the cosmetic sealer. Any bubbles in the sealing bead can produce a porous surface which water can penetrate and lead to corrosion. A good way of avoiding these failures is to apply the sealer while the flange is heated, so that the air is expanded when the sealer is applied. One way to do this is to apply the sealer while the hem flange adhesive is cured by induction heating. In the same process the applied sealer can be cured directly. Another premise for good corrosion resistance is good adhesion of the adhesives on the (oily) substrates and a sufficient durability of the adhesion under the various conditions during the use of the car. These properties should be tested in advance by shear and peel tests in the initial state of the bonding and after ageing, e.g., by the VDA 621-415 test, which is used in the German automobile industry. Another method to determine adhesive properties is the Boeing wedge test. When using a hybrid bonding it is important that not too much of the adhesive is squeezed out of the flange during the joining process. If there is a spot welding through the adhesive layer, it has to be ensured that no corrosive crack products are introduced with the welding heat, therefore PVCplastisols should not be used for weld trough applications.27±28 Stiffness To enhance the stiffness of a body under static and dynamic loads, high modulus adhesives should be used. Depending on the geometry of the body, the stiffness can be enhanced by more than 40%. The enhancement of stiffness by the use of adhesives3 was illustrated in Fig. 16.8. Usually hot-curing one-part epoxy adhesives with good adhesion to oily sheets are used for this application. However, a new generation of stiff rubber-based adhesives have recently been developed as an alternative. Crash Standard epoxy adhesives used to increase stiffness have insufficient energyabsorption properties which also mean reduced shear strength and peel resistance under impact. With these adhesives it is not possible to create a joint that is able to keep the parts together in a crash situation. In the middle of the 1990s a new generation of toughened epoxy adhesives came onto the market, which has a very high strength at high velocities and a high energy absorption. This behaviour was achieved by rubber particles that were finely dispensed in the epoxy matrix (Fig. 16.16). In Fig. 16.17 the good behaviour of bonded parts after a crash is shown.16±18 Crash-resistant adhesives can be used to enhance stiffness also because they have a comparable Young's and shear modulus but if the crash performance is not requested, the use of these adhesives is not economic.29

374

Adhesive bonding

Figure 16.16 Morphology of a crash-resistant adhesive.16

Figure 16.17 Bonded part after a crash test (DaimlerChrysler).9

Automobiles

375

16.4 Surface preparation For body shop applications steel, aluminium, magnesium and plastic surfaces have to be considered.

16.4.1 Steel For body shop applications on steel a surface preparation is not economic and not necessary. Available hot-curing epoxy, rubber-based and plastisol adhesives show a good adhesion on oily surfaces on the steel sheets as supplied. It is necessary to limit the amount of oil to less then 3±4 g/m2, mainly to avoid the slipping away of the uncured adhesive. After curing in the oven at 180 ëC, these adhesives show a good adhesion to the substrates. Cold curing adhesives cannot be used on oily steel surfaces. High-alloy steels have to be pretreated by corundum blasting, sanding or etching.30,31

16.4.2 Aluminium The available body shop adhesives show a good adhesion on aluminium. The adhesion on oiled aluminium surfaces is not satisfactory. Because there is no need to protect the aluminium from corrosion during the production process, there is no need to handle oiled sheets. The lubrication for the pressing process must be given by adhesive-compatible organic layers or the lubricant has to be washed off after the pressing process. Because aluminium surfaces as supplied with an undefined oxide layer are not stable to a corrosion attack, a conversion layer has to be applied in an anodising process. The bondings on chrome and chrome-free conversion layers, based predominantly on fluorozirconates show good bonding strength and good durability. A schematic composition of a coil passivation layer and a dry film lubricant on aluminium is shown in Fig. 16.18.32

Figure 16.18 Schematic composition of coil passivation layer and dry film lubricant.33

376

Adhesive bonding

16.4.3 Magnesium Before bonding, the surface of the magnesium sheets have to be passivated by a chrome VI or a chrome-free conversion layer. Figure 16.19 shows the strength of bonded samples with different pretreated bonded Mg-sheets after 60 days in a climate corrosion test.34

Figure 16.19 Force-displacement behaviour of bonded Mg sheets after different pretreatments.34

16.4.4 Plastics Plastics must be separated into duromer and thermoplastic materials. Duromers Duromers normally used as fibre-reinforced composites, show good adhesion bonded with polyurethane, epoxy or acrylic adhesives. Residues from the forming process of the parts like mould release agents must be removed prior to bonding. A smooth mechanical pretreatment, e.g., by brushing, enhances adhesion strength. Damage to the fibres in the pretreatment process should be avoided, because this leads to a decrease of the strength of the substrates.

Automobiles

377

Thermoplastics In automobile manufacturing many different thermoplastic materials are used. Most of them need a pretreatment to assure good adhesion. Because of their low costs and good environmental characteristics polyolefins are particularly popular in the automobile industry even though they have to be pretreated. Flame and corona treatment are common during production and lead to reproducible and adequate results. In addition to the treatment in most cases, for example, a primer on amino basis is applied. A new generation of adhesives based on acrylics has been available for some years and shows good adhesion even on untreated polyolefins. Due to the higher costs and to occupational safety and health pretreatment is still favoured.35±38

16.4.5 Precoated/painted panels A new trend in automobile manufacturing is to use coil-coated sheet metals for better corrosion resistance and to improve pressing or to avoid part paint processes. Precoatings have to be differentiated in thin weldable precoatings, electro coatings and final paintings. Weldable organic coatings are 3±5 m thick and consist usually of zinc filled epoxy. The electro coated and the final painted sheets are similar to the painted parts, which implies that they cannot be joined by welding. Organic coatings enhance corrosion resistance and show a good reaction to adhesive joining. Because the electro coated and final painted sheets cannot be welded and the corrosion resistance is reduced by mechanical joining, adhesive bonding is the preferred way to join these substrates. In the case of bonding coated sheets it should be remembered that the coating becomes a structural element, which means that the mechanical properties of the coating must be adequate.36,39

16.5 Strength and durability 16.5.1 Strength In an automobile there are high structural, semi-structural and low structural adhesive bondings produced in the body shop and in the trim assembly. Static, dynamic and crash loads have to be considered. Structural adhesive bondings Epoxy and rubber-based high-strength adhesives are used for structural joints in body in white. These adhesives have a bonding strength under quasi-static load of about 10±30 MPa depending on the kind of adhesive and the kind and geometry of the substrate. The failure shear strain is less than 10%. These

378

Adhesive bonding

Figure 16.20 Fatigue strength of spot-welded and spot-weld-bonded steel joints.

adhesives have a Young's modulus of about 1500 MPa and a G-modulus of about 500 MPa.40 The glass transition temperature should be higher than the temperature during use. A usual value for Tg is about 80±100 ëC. When the joints are designed correctly, fatigue loads normally lead to a failure of the substrate. In Fig. 16.20 the fatigue behaviour of spot-welded- and spot-weldbonded steel is shown. Regarding crashes, classical structural adhesives used for body stiffening have a too low energy absorption for crash situations. A new generation of modified epoxies has much higher energy absorption and a higher bonding strength at high deformation velocities as already mentioned. The impact energy of classical and improved structural adhesives is shown in Fig. 16.21.

Figure 16.21 Impact energies of classic and improved structural adhesives.29

Automobiles

379

Semi-structural adhesive bondings Anti-flutter adhesives are used to form soft elastic joints, e.g., between inner and outer panels on bonnets or sides to reduce vibration and noise. The adhesives are usually based on reactive elastomer mixtures, which are cross-linked when cured. The strength level in lap shear is about 0.5±3 MPa. The Young's- and Gmoduli are low and the elongation at break can be more than 200%. Screen bonding/direct glazing One-part moisture curing polyurethanes, which are used for screen bonding, are applied in relatively thick layers up to 20 mm. They have a Young's modulus of about 3±15 MPa and a G-modulus of 1±5 MPa. The higher modulus adhesives lead to a stiffer body, the limit of the stiffness (the modulus) is ± besides others ± given by the strength of the screen. The lap shear strength of the actual screen adhesives is between 3±6 MPa. Regarding the strength, it is to be noted that the screen bonding takes place on painted panels and therefore the maximum load can be limited by the mechanical properties of the paint layer.

16.5.2 Durability The listed properties can be affected by the influence of moisture and salt at different temperatures. If there is an irreversible decrease of the main properties like strength or failure strain the decrease must be determined and a minimum value for the application must be defined. To see the effects of different climates in the automobile industry different accelerated ageing tests are performed. They all use the time temperature superposition principle to shorten the time of the test. To simulate corrosion attack NaCl ± mainly in water solution ± is used in a so-called salt spray. Factors affecting the long-term performance of adhesive joints are the substrate, the surface quality, the process of application and cure and the type of adhesive. Usually the influence of these parameters on the failure mode, lap shear strength, T-peel resistance or impact energy absorption is tested on bonded samples in an initial state and after the application of different ageing methods. A common test is the salt spray test for a period from several hours up to some thousand hours. In many cases this test is performed for 480 hours. The effect of humidity can be determined by an immersion test, where the samples are immersed in deionised water at an elevated temperature, e.g., 80 ëC or in the so-called cataplasma test, where the samples are stored for 21 days at 70 ëC and for 16 hours at ÿ30 ëC in a very humid environment. To simulate real conditions during the use of a car, several combined ageing methods are used. They consist of periods with low temperature, periods with high temperature and high humidity and a salt spray period. Examples are the VW P-1200 and the

380

Adhesive bonding Table 16.1 Climate test according to VDA 621-4158 24 h 6h 3h 6h 2h 5h 66 h

Salt-spray ± test according to DIN 50021 Condensation water ± test according to DIN 50017 Storage at room temperature (RT) Storage at 100 ëC 3 times Storage at RT Storage at ÿ30 ëC Storage at RT

g

climate test according the German VDA 621-415. The VDA-test is described in Table 16.1. There are only very few data available about real-life ageing. But on the other hand, while there are millions of cars with bonded doors, hoods, etc., only very few serious problems with debonding and corrosion are known.

16.6 Common failures To ensure a good quality of the bonded joint with the pre-defined properties, it is imperative that the right adhesive that was qualified in the run up with the right bead geometry, be in the right place on the right material with the defined surface. After joining, subsequent processes like handling and washing have to be endured by the uncured adhesive layer. The parameters of the curing process like curing time and temperature have to be within the defined process window. Common failures during this cycle occur when: · There is too much oil on the surface ± especially in the lower areas of formed parts ± which causes slipping of the adhesive bead and/or bad adhesion. · Too much adhesive is applied; the surface and the tools are contaminated by squeezed out adhesive. If the surface is not cleaned afterwards, the quality of the painted panel is reduced. · Not enough adhesive is applied and the flange is not filled up; leakage and low strength are the results. · The adhesive is not in the right place. This is the same as too much or not enough adhesive, depending on the position. · The curing temperature is not reached during a sufficient period. Because of the design the temperature in the oven is not constant for the entire body. Regions where the air flow is blocked do not get enough energy to cure the adhesive completely. · One-part polyurethanes are not completely cured because of a low relative humidity, e.g., in winter. · The surface of supplied plastic parts is contaminated by mould release agents or other surface contaminations. This results in bad adhesion.

Automobiles

381

16.7 Inspection, testing and quality control As listed above, it is necessary that a proper amount of adhesive is applied, the adhesive bead has the right shape and is in the right position. To ensure reliable application, process control is required. The flow rate of the adhesive has to be controlled online. The shape and the position of the bead can be determined with optical sensors and CCD cameras and pattern matching software. The quality of the adhesive joints produced under the circumstances of the actual process chain can be approved by samples that accompany the parts in the processes. The samples should be tested under conditions similar to those experienced during service life. Another possibility for quality control is non-destructive testing of the parts after manufacturing. Regarding adhesively bonded joints, the existence or the absence of adhesive can be proved online by ultrasonic methods or by thermography. Ultrasonic testing has the disadvantage that the flange has to be scanned completely which, with certain geometries, is difficult and takes a long time. For these reasons ultrasonic testing is rarely used in the automobile industry. Thermography as an optical method gives the possibility to control whole parts contact free. That is the reason why some automobile manufacturers have now started to use thermography as an online NDT method. With the above-mentioned methods it is not possible to determine the strength and the durability of a bonded joint. For that reason destructive testing cannot be dispensed with in the near future.25

16.8 Repair and recycling Repair and recycling means that adhesively bonded joints have to be disbonded. Regarding recycling, the joint can be destroyed without taking into account the resulting surface and the resulting geometry. Concepts for disbonding for recycling purposes use heat to soften or decompose the adhesive to achieve separation of the parts. Other concepts embrittle the adhesive layer at low temperatures. The common method is to shred the car and to separate the fractions. Repair of windscreen bonding is state of the art. The thick adhesive layer is cut by an electric knife or a hot wire. The remaining adhesive layer is removed except for a thin layer of adhesive that serves as an adhesion surface for the newly applied adhesive bead. For repair purposes often two-part polyurethanes are used to enhance curing time. Concepts for body repair provide disbonding by elevated temperature and/or mechanical means and the use of two-part epoxy adhesives as a cold-curing repair adhesive.

16.9 Other industry-specific factors Automotive production is a serial production with short cycle times, not especially skilled labour, large tolerances, etc. Due to these circumstances the

382

Adhesive bonding

adhesives used and the processes have to be very robust. This is the only way to ensure the high quality that is needed for an adhesively bonded car in daily use. The demands on adhesive bonds in a vehicle are very high. Regarding fatigue and crash resistance, one must also remember that lives depend on the quality of the joint.

16.10 Examples of use As listed above, modern cars like the E-class and the S-class of DaimlerChrysler have more than 50 m of structural bondings. The situation is similar for other high-performance cars (Figs 16.22 and 16.23).

Figure 16.22 Adhesive bonding in a Mercedes A-class (DaimlerChrysler).

Figure 16.23 Adhesive bondings in the Lotus Elise.41

Automobiles

383

16.11 References 1. Kalaygian, M. `The road more traveled'. Adhesives Age 46 (2003), 2, 12±13. 2. Walther, U. `FunktionalitaÈt des Klebens fuÈr das Automobil'. Stahl (2002), 3, 63±65. 3. Flegel, H. `Die Zukunft des Klebens als FuÈgetechnik im Automobilbau'. Swiss Bonding 02, Kleben 2002 ± Grundlagen, Technologie, Anwendung 16. Intern. Symp., Erfolg durch strukturiertes Denken und Kleben (2001), 27±39. 4. Vollrath, K. `Kleben fuÈr den Leichtbau bei Kraftfahrzeugen'. Der Praktiker 54 (2002), 7, 222±226. 5. Maurer, B. `Strukturelles Kleben im Automobilbau'. Konferenz-Einzelbericht: 3. Industriekolloquium `Fertigen in Feinblech', Leichtbau durch innovativen Werkstoffeinsatz (2002), 191±202. 6. Haldenwanger, H.G., Walther, U. `Verhalten von geklebten und kombiniert gefuÈgten Verbindungen bei unterschiedlichen Beanspruchungen'. Konferenz-Einzelbericht: Praxis-Forum, Arbeitskreis Automobil, 15 (1999), 147±157. 7. Hopf, B., Haepp, H. J., Biemans, C. Potenziale und Risiken bei der Produktion von gewichtsreduzierten Fahrzeugen in Mischbauweise. Konferenz-Einzelbericht: Innovationsquelle Leichtbau, Fakten ± Trends ± Visionen. Dresdner Leichtbausymposium 2000 (2000), 1±5. 8. Habenicht, G. Kleben, Grundlagen, Technologien, Anwendung, 4, Auflage, Springer Verlag, Berlin, Heidelberg, New York (2002). 9. Jost, R. `Klebtechnik bei DaimlerChrysler', Firmeninformation DaimlerChrysler (2000). 10. Haldenwanger, H.G., Walther, U. `Klebverbindungen in und an der Fahrzeugstruktur'. Kunststofftechnik (2002), 85±97. 11. Bullivant, C.P., Robberstad, F. `Strukturkleben mit System'. Automobiltechnische Zeitschrift ± ATZ 97 (1995), Sonderausgabe Fertigungstechnik 95/96, 38±40. 12. Schenkel, H. `Adhesive bonding in car bodies'. DVS-Berichte Band 218 (2001), 163±172. 13. Meschut, G., Eis, M. `Minimierung von Bauteildeformationen beim Kleben'. Konferenz-Einzelbericht: Ber. a. d. Fertigungstechnik (2000), 135±152. 14. Frensch, M., SchuÈrholz, F. `Einfluss der Prozesskette auf Klebstoffapplikationen und Darstellung eines prozessoptimierten Klebstoffapplikationsverfahrens im Karosserie-Rohbau'. Praxis-Forum, 51±69. 15. Greiveldinger, M., Jacquet, D., Verchere, D., Shanahan, M. E. R. `Adhesion of oilcovered substrates: Behaviour in the interphase during cure'. Institute of Materials 920 (1999), 141±146. 16. MuÈhlhaupt, R. `Flexibility or Toughness? ± The Design of Thermoset Toughening Agents', Chimia, 44 (1990), 43±52. 17. US Patent 5.278.257 (1994): Phenol-terminated polyurethane or polyurea (urethane) with epoxy resin. 18. European Patent 0307666 (1988): Composition of butadiene/polar comonomer copolymer, aromatic reactive end group-containing prepolymer and epoxy resin. 19. Terfloth, C. `Einsatzpotentiale innovativer Hybridklebstoffe im modernen Fahrzeugbau'. Adhesive Bonding in Automobile Production. Sixth Annual and 3rd European Expert Automobile Conf., Proc., Bad Nauheim, Germany, 17±18 October 2002, 31±42. 20. BaÈr, C. `Scheibenklebstofftypen ± Einfluss auf die Fertigung'. Praxis Forum (1997), 73±85.

384

Adhesive bonding

21. Anon. `Gewindesicherung auf der Basis der Mikroverkapselungstechnologie'. AdhaÈsion, 36 (1992), 1±2, 18±19. È bermaûpassungen an 22. Mayer, W. `LastuÈbertragung und Berechnung bei geklebten U hochfesten Welle/Nabe-Verbindungen'. Konferenz-Einzelbericht: Kleben, 6. Int. Symp. Swiss Bonding, Fachseminar: LeistungsfaÈhigkeit der modernen Klebtechnik (1992), 311±327. 23. Keller, H. `Neue Generation reaktiver Butyl-Klebstoffe. Ein aufeinander abgestimmtes Klebstoff- und Anlagenkonzept setzt die Automobilindustrie ein'. AdhaÈsion ± Kleben & Dichten, 37 (1993) 7±8, 34±35. 24. Hirthammer, M. `Advanced adhesives for direct glazing applications'. KonferenzEinzelbericht: `Glass Processing Days', the 5th International Conf. on Architectural and Automotive Glass, Now and in the Future, Conf. Proc. (1997), 363±365. 25. Davies, B. L., Razban, A., Forrest, A. K. The use of automated systems in dispensing adhesives. Konferenz-Einzelbericht: Proc. of the 28th Int. MATADOR Conf., UMIST, University of Manchester (1990), Apr., 61±68. 26. Budde, L. `Synergieeffekt durch Verfahren. DurchsetzfuÈgen und Kleben ergaÈnzen sich'. Industrieanzeiger, 118 (1996), 34/35, 48±49. 27. Hilla, W. `Arbeitsschutz beim Einsatz von Klebverfahren im Automobilbau'. Konferenz-Einzelbericht: Kleben, 7. Int. Symp. SWISSBONDING, Interkantonales Technikum (1993), 498±509. 28. Wang, P. C., Chisholm, S. K., Banas, G., Lawrence, F. V. `The role of failure mode, resistance spot weld and adhesive on the fatigue behaviour of welded-bonded aluminium'. Welding Journal, New York 74 (1995), 2, 41s±47s. 29. Symietz, D. LeistungsfaÈhige FuÈgetechnik Kleben im Automobilbau mit hochfesten Klebstoffen. Konferenz-Einzelbericht: Zulieferer Innovativ, Jahreskongress mit Fachausstellung, BAIKA, Bayern Innovativ, AUDI Forum (2001). 30. LuÈbbers, R., Kordisch, T., HaÈrtel, W., Rostek, W. FuÈgetechnologien im Leichtbau. Konferenz-Einzelbericht: FUKA-PFT, Wissenschaftliche Berichte. Forschungszentrum Karlsruhe Technik und Umwelt 210 (2002), 205±213. 31. Eicher, C., Brockmann, W., Deutscher, O., Fata, D., Hennemann, O. D., Neeb, T., Possart, W., SchaÈfer, H., Schlett, V. Untersuchungen der langzeitbestaÈndigen Klebbarkeit von nichtrostendem Stahl im Automobilbau. Konferenz-Einzelbericht: DVS-Berichte Band 222 (2003), 21±25. 32. Pfestorf, M., MuÈller, P. Application of aluminium in automotive structure and hang on parts. Konferenz-Einzelbericht: Materials Week 2001, Internat. Congress on Adv. Materials, their Processes and Applications, Proc. (2001), 1±8. 33. Gehmecker, H. `Chemical pretreatment of multi-metal and all-aluminium car bodies'. Konferenz-Einzelbericht: Aluminium 2000, 4th World Congress on Aluminium, Conf. Proc. (2000), 342±352. 34. Meschut, G., Walther, U. Kleben von Magnesium. Konferenz-Einzelbericht: Automotive Circle Internat. Conf. (2001), 335±344. 35. Dorn, L., Hofmann, F. Festigkeitsverhalten und Anwendungspotentiale von Kunststoff-Metall-Klebeverbindungen fuÈr den Fahrzeugleichtbau. KonferenzEinzelbericht: DVS-Ber. Band 668 (2001), 187±197. 36. Liebing, G., Temme, U. Klebbarkeit thermoplastischer Kunststoffe. KonferenzEinzelbericht: Konstruktives kleben im Maschinen-, Anlagen- und Automobilbau. 3. Klebetechnische Tagung, UniversitaÈt (GH) (1990), 24±37. 37. Altmann, O. Kleben von Kunststoffen im Automobilbau. Konferenz-Einzelbericht:

Automobiles

385

DVS-Berichte Band 147 (1992), 40±44. 38. Valersteinas, P., Bonari, R., Vitali, M. Composite material technology for the mass production of large automotive structural components. Konferenz-Einzelbericht: Florence ATA 1994, New Design Frontiers for more Efficient, Reliable, and Ecological Vehicles. Proc. of the 4th Internat. Conf. 1 (1994), 57±62. 39. Koll, T., Eggers, U., HoÈfemann, M. Leaner manufacturing with precoated high steels. Konferenz-Einzelbericht: SAE-P, Band P-369 (2001), 171±175. 40. Tokar, G. `Punktschweiûkleber ± Eigenschaften und Berechnungsmethode fuÈr lineare Karosseriesteifigkeiten'. Konferenz-Einzelbericht: VDI-Berichte Band 1559 (2000), 549±575. 41. Kochan, A. `Lotus: Aluminium extrusion and adhesives'. Assembly Automation, 16 (1996), 4, 19±21.

17

Boats and marine

M HENTINEN

17.1 Introduction Adhesive joining is gaining more popularity also in marine applications. In this chapter, the status of adhesive bonding in boats and ships is discussed. The main focus is in structural joints between large parts, like hull and deck, stiffeners and hull shell, bulkheads, etc. The main types of connections are presented, and some typical applications are shown in section 17.2. The materials used in marine applications vary widely, but in most cases the adhesive joints are between two FRP parts (polyester, vinylester or epoxy matrix) or between FRP and aluminium. While the calculation methods to determine the stresses and strains in the joint do not differ from similar methods in other applications, some typical load cases are demonstrated. The required characteristics of adhesives, like moisture, heat and UV-resistance are discussed in section 17.3. Surface preparation is also a vital part of the bonding process in marine applications. Its significance is even emphasised because good long-term characteristics and resistance to moisture are needed. Pre-treatments that are suitable for use at boat- or shipyards are discussed in section 17.4. Strength and durability, and some aspects of designing for strength are discussed in section 17.5. The realistic possibilities to inspect the joints in marine applications, as well as common failures are introduced briefly in the following sections. Two very different examples of use are reported in section 17.9. The first one is a hull-deck joint of a small FRP-boat, the design of which is typically based on experience. The other example is a design case from shipbuilding, where an adhesive joint between aluminium deck and FRP-sandwich structure is compared with a bolted joint with thorough analysis. Finally, some future trends are given in section 17.10.

17.2 Basic needs There is a large variety of different needs of adhesive bonding in boats and marine applications. The bonds may be critical structural bonds, or the main

Boats and marine

387

purpose of the bond is sealing with only a secondary structural role. Drawing a line between these is often difficult, especially in bolted-bonded joints. Sandwich structures are common in marine applications. Bonds between core and faces are comparable with sandwich structures in general, and are mentioned only briefly in the following sections. The large size of the adherends and quite difficult environmental conditions are typical for marine applications. Service life of the ships and boats is normally several decades, and structural failures may be critical for overall safety. Reliable robustness and failsafe behaviour of the joints is thus essential.

17.2.1 Typical materials of adherends Typical materials used in hulls and decks in marine applications are fibrereinforced plastics (FRP) with thermosetting matrix (orto- or isophtalic polyester, vinylester or epoxy), marine grade aluminium (AlMg3, AlSi1Mg for example) and steel (standard shipbuilding steels, or stainless steels like AISI 304 and AISI 316). Wooden structures are becoming more and more rare in hulls and decks (except in some one-off boats), but are used in bulkheads (typically plywood) and other parts of interiors and, of course, in teak decks. In sandwich structures the core material is typically cross-linked PVC or endgrain balsa. Honeycombs are quite exceptional, but used in some high-end applications. Faces are normally of FRP, but other materials are often used for interior or decorative panels. As a consequence, structural adhesives are typically used in joints between · · · · ·

FRP±FRP FRP±(ply)wood FRP±PVC (foam) FRP±metal (aluminium or steel) aluminium±aluminium.

As in the automotive industry, the adhesive bonding of windscreens and other windows is gaining more and more popularity in marine applications, especially in recreational boats. Glass-FRP joints and glass-aluminium joints are thus added to the list above. Acrylic windows can also be bonded. Secondary joints and sealing can include a wide selection of materials; for example, many fittings are made of thermoplastics.

17.2.2 Types of connections From the structural element point of view, the following kinds of attachments can be listed: · frames, stiffeners and bulkheads to shell panels · inner liners to hull

388

Adhesive bonding

· deck to hull · heavily loaded items (chain plates, ballast keels, rudder bearings), hull or other structures · FRP-substructures to metal structures · hull panels together · windows to their frames. The first three types are typical of boat and ship hulls constructed of FRP. Adhesive bonding is also used in aluminium hulls to attach frames to hulls, especially if appearance and surface smoothness is important and distortion under welding is a problem. Deckhouses of large aluminium yachts are good examples of these. Some heavily loaded parts like chain plates or rudder bearings can also be adhesively bonded to hull or bulkhead. Good quality control and design for longterm durability is then essential. Additionally, some kind of mechanical securing is usually used in these critical joints. For example, ballast keels are firmly bolted to the bottoms of sailboat hulls. The joint is typically sealed with polyester or epoxy putty, which largely carries the load between keel and hull during normal sailing. However, the keel bolts are dimensioned to carry alone the loads from the keel in the extreme case of a yacht capsizing. Connecting FRP substructures and components to metal hulls is typical in larger ships. The weight savings achieved with sandwich parts can be vital especially on upper decks. The joint between FRP substructures and metal is typically bolted. In the case of very large parts this is no longer cost-effective. Adhesive bonding then becomes a competitive alternative, although the shipyard environment is normally not suitable for adhesives in terms of cleanliness, temperature and humidity. Adhesive bonding vs. laminating in FRP hulls Especially when building boats in batches there are three types of joints where adhesive bonding is replacing laminating (see Fig. 17.1): 1. 2. 3.

Deck to hull joint: the connection can be purely bonded, riveted-bonded, screwed-bonded or bolted-bonded, or overlaminated. Bulkhead to hull shell or deck joints: the connection can be a combined bolted-bonded joint. Compared with a laminated joint, adhesive bonds can be beneficial from the strength point of view (Burchardt, 1997). Inner liner to hull or bulkhead connection: inner liners can be primary or secondary structures and the joint is often partly laminated, partly bonded.

In conventional hand lay-up, as well as spraying, the bonds between frames and hull shell are most often done by laminating. The bonding laminate consists of

Boats and marine

389

Figure 17.1 Typical joints and their loadings in boats.

part of the stiffener web laminate, or extra strips of laminate are added. A similar attachment principle is possible with pre-pregs or when using vacuum infusion methods. Adhesive bonding of frames and stiffeners instead of laminating is also gaining popularity, especially in high-end products. Stiffeners with bonding flanges are moulded separately, and bonded into their places using a layer of adhesive. The advantages of adhesive bonding are especially relevant when using vacuum infusion or pre-pregs, because vacuum bagging is easier and more reliable if the hull or deck is flat and clean. Added structural strength is also possible. In small boats the main reason for using separately moulded, adhesive bonded frames is the smooth surfaces visible to the end-user. Adhesive bonding vs. bolting in FRP substructure ± metal hull connections Joining large FRP parts to the steel or aluminium structures of a ship in a reliable and economical way is a demanding task. It has been shown that bolting, which is common practice, is not cost-effective as the number of bolts increases in extensive parts (Hentinen et al., 1997). Also, the static strength of the connection can be clearly increased with adhesive bonding. On the other hand, to perform adhesive bonding in a metal shipyard environment can be very difficult. The requirements for temperature, humidity and cleanliness are quite different for adhesive bonding than for welding. An alternative solution to overcome these problems is to use prefabricated joint elements which are adhesively bonded to the sandwich panel by the FRP manufacturer. The shipyard can then weld the FRP part to the ship in a similar manner as corresponding metal parts.

390

Adhesive bonding

Figure 17.2 Potential applications for FRP sandwich substructures.

Possible FRP-substructures are shown in Fig. 17.2 (Hentinen et al., 1997): · Whole sections built of fibre-reinforced plastics: load-bearing structures of some public and residential rooms. · Bulkheads: interior bulkheads and B-class structures as the lightest alternative, load-bearing water-tight bulkheads as the heaviest alternative. · Decks: decks in public and residential rooms, car decks. · Rails, windshelters and stairways: `visible' structures for which aesthetics as well as ease of service is important; equipment on the upper decks, where light weight is important regarding the centre of gravity. · Shelters and lockers: intakes for ventilation and air-conditioning, toilets, equipment rooms. · Masts: radar masts. · Funnels: covering shells of funnels.

17.2.3 Load characteristics Load determination is an essential part of joint design. In marine applications the loads or load combinations are usually divided into local and global loads. Local loads directly affect a limited area of the structure, while global loads result from bending, twisting or shearing of the hull girder or hulls in the case of multihull vessels. Local loads may normally be dominant for a single panel, but the effect of the global loads is pronounced in sailing yachts and in large ships. When joining dissimilar materials together, the stresses due to different heat expansion can be significant if the heat expansion coefficients are very different from each other. Typical values for these are (Engineered materials handbook 1987): stainless steel, 18
10ÿ6 Kÿ1 ; aluminium: 23
10ÿ6 Kÿ1 ; E-glass reinforced epoxy, 10 ÿ 12
10ÿ6 Kÿ1 (0/90ë, 42 vol-%). If the other adherend material is FRP, the stresses in the joint due to global loads and heat expansion usually remain moderate; this is because of the relatively low in-plane stiffness of FRP laminates. Global loads Global loads affect the design loads in joints between hull and deck, bulkheads and hull or deck, superstructure (or other extensive structures) and deck or hull.

Boats and marine

391

In some cases the joints between stiffeners and hull or deck shell are also loaded by global bending of the hull girder. Hull girder loads are normally not critical for small motor vessels having a low length-depth ratio. The limit for `small' is not precise, but for example the DnV rules for high-speed light craft (HSLC) states that the minimum strength standard is normally satisfied for scantlings obtained from local strength requirements for craft with L/D < 12 and length less than 50 m (Hentinen and Hildebrand, 1995). However, also smaller fast planing boats jumping from one wave crest to another have had problems with the global strength. Hull girder loads in motor vessels are normally regarded to be: · longitudinal bending, shearing and axial loads ± still water moment ± wave bending moment ± crest/hollow landing in waves ± beaching and docking · twin hull loads (in catamarans) ± transverse vertical bending moment and shear force ± pitch connecting and torsion moments. The parameters involved in hull girder loads in normal sailing conditions are wave height and period, main dimensions of the ship, mass distribution, and hull shape or factors describing it. For example, in high-speed vessels, in which FRP structures are gaining popularity, the global bending is a function of acceleration at the centre of gravity (acg). According to DnV HSLC, the longitudinal bending moment is  
ls (kNm) 17:1 MB ˆ …g0 ‡ acg † ew ÿ 4 2 where
is weight of the vessel, ew is 0.25 times length L, and ls longitudinal extension of slamming reference area. Shear force calculated from longitudinal bending is Qb ˆ

MB 0:25L

(kN)

17:2

Sailing boats are subjected to significant global bending due to rig forces (see Fig. 17.3). Transverse rig loads are dependent on the righting moment of the yacht; in typical keelboats the loadings from the shrouds are of the same magnitude as displacement, and can be derived from (Hentinen and Holm, 1994) PT ˆ

45RM1 rf bri

17:3

where RM1 is righting moment at 1ë of heel, rf is displacement at full load, ri is displacement during the inclination test, and b is chainplate distance from

392

Adhesive bonding

Figure 17.3 Global bending induced by the rig forces.

centre line. The tension in the fore-and-aft stay inducing a longitudinal bending moment in the hull girder, results in a bending force of approximately 85% of the displacement (Larsson and Eliasson, 2000). Local loads Local loads have a direct effect on a limited area of the structure. For hull plating the normal design load is water pressure, which is a combination of hydrostatic pressure, hydrodynamic pressure due to motion of the ship, and hydrodynamic pressure due to waves. These pressures can be relevant high above design waterline, and due to green water also on weather decks and superstructures. Hydrostatic pressure is also relevant in watertight bulkheads. In tanks, sloshing and impact loads have also to be taken into account. The internal loads due to mass inertial effects from equipment or solid cargo are controlled by the accelerations of the hull at that location. Other typical local loads are fender loads during berthing, beaching and docking loads and possible ice loads. The parameters involved in local loads depend very much on the load type, but because accelerations are derived from the ship's motion, wave height and period, main dimensions of the ship, mass distribution, and hull shape or factors describing it are relevant parameters as in the previous section. The available methods for load estimation can be divided into the following groups: · appropriate standards and rules of classification societies · own (semi)empirical formulas and measurements · direct calculation. For example, according to the NBS-VTT extended rule (Furustam, 1996), the slamming pressure acting on the bottom of a boat is Pslam ˆ kl
kv
k
kdisp
Pbase…V ˆVmax †

17:4

where maximum pressure Pbase is a function of speed and length, and kl, kv, k and kdisp are correction factors as a function of longitudinal and vertical

Boats and marine

393

location of the panel, deadrise, and displacement. The design pressure is then Pslam times an area reduction factor, which takes into account the locality of the slamming load. Typical design pressure for a bottom panel of a small (5 m) planing boat is 20±25 kPa.

17.2.4 Other requirements Depending on the application, other required characteristics for the joint may include (Hentinen et al., 1997): (i) heat insulation; an FRP sandwich panel is an effective insulator and this can also be required from the joint. (ii) Corrosion resistance; low maintenance costs are one reason to use FRP structures in ships and the joint should be in line with this. The long-term strength of the joint also requires corrosion-resistant materials. (iii) Coating; similar coatings that are used for metal structures or FRP panels should also be suitable for the joint. (iv) Dismantling; replacement of a damaged panel should be possible. If the metal profile is easily accessible, the joint can be dismantled by cutting the profile.

17.3 Adhesive characteristics required Some specific requirements for adhesives are typical in boat and marine applications: · · · · · ·

moisture resistance heat resistance UV resistance tolerance to joint thickness variations strength and stiffness ease of application to large structures.

These requirements are described below.

17.3.1 Moisture resistance Moisture resistance is one of the governing factors when choosing a suitable adhesive for marine applications. Direct immersion is expected only in underwater parts, bilges and tanks, but humidity is usually high in any case, and both spray and drain are expected also on upper decks. Fortunately, a wide selection of adhesives is capable of resisting moisture and suitable for marine use. Joint weakening through water penetration between the adhesive and the adherend is normally more critical. This can happen in the following ways: 1. 2.

The adhesive is applied to a humid surface. Water diffuses through the cured adhesive.

394 3. 4. 5.

Adhesive bonding Water penetrates into the joint along the boundary layer. Water penetrates into the joint through a porous adherend. Water penetrates into the joint by capillarity along possible cracks in the adhesive.

Good surface treatment (see section 17.4) is essential to avoid point 3. Shielding of the joint with a sealant can reduce all these problems, but especially point 5.

17.3.2 Heat resistance Ship structures are typically subjected to temperatures between ÿ30 ëC and ‡30 ëC (Hentinen and Hildebrand, 1995). Even higher temperatures can be expected on upper decks in direct sunshine. At a dark FRP surface it is possible to exceed ‡60 ëC. This must be taken into account especially if the difference in the thermal expansion coefficients of the materials is large. Also, the strength and stiffness values of adhesives can change significantly in these temperatures. The concept of using `weldable' FRP sandwich panels (see section 17.2.2) leads to a considerable temperature load in the joint area during the welding process in the shipyard. Hence, the heat resistance of the adhesive materials must be suitable for the concepts, as the temperature during welding must be sustainable without damage. Typical values of heat resistance of structural adhesives are shown in Table 17.1. However, it has to be noted that there are differences in heat resistance within the different adhesive types, certain products having been developed specifically for that purpose. The heat resistance of epoxy adhesives also strongly depends on the curing or post-curing temperature.

17.3.3 UV resistance Resistance against sunlight is relevant in all areas on decks and freeboard. Typically, polyurethane and epoxy adhesive bonds have to be shielded from direct sunlight. In normal boatbuilding practice, many joints are shielded in any Table 17.1 Typical heat resistance values for different adhesive types (Hentinen and Hildebrand, 1995) Adhesive type 1-component polyurethane 2-component polyurethane Epoxy

Heat resistance (ëC) long time (short time without mechanical loads) 50 ± (200) 50 ± (100) 50 ± 120 ± (180)

Boats and marine

395

Figure 17.4 Protecting the adhesive bond in windows.

case for the sake of appearance. Hull-deck joints covered with rubber or aluminum profiles are a good example of this. Windows and other transparent substrates are of course problematic, when protecting their adhesive bonds against sunlight. This is even more important in marine applications than in cars and road vehicles, which is due to UV-reflection from water, and because there is no shelter or shadow area at sea, in ports or in marinas. The most effective way to protect the bond is to use a ceramic screenprinted border with a light transmittance value of 0.1% or less (Burchardt et al., 1998) (see Fig. 17.4). Cover trims, opaque paints or black primers are other alternatives.

17.3.4 Joint thickness It is difficult to keep tolerances small in extensive parts, especially FRP parts. This applies to both shape and thickness tolerances (see Table 17.2). The variation of the joint thickness may be large when joining these parts together; in some cases even 10±15 mm. The adhesive should be able to bridge the gap between the adherends, and the characteristics of the adhesive bond should not be very sensitive to joint thickness. The characteristics of flexible polyurethane adhesives are typically insensitive to thickness. This has been shown for example by (Marttila and Holm, 1991) (see Figs 17.5 and 17.6). Due to thick joints the consumption of adhesive can be high, which makes material price of the adhesive an important factor.

396

Adhesive bonding

Table 17.2 Typical thickness tolerances of different FRP panels (Hentinen and Hildebrand, 1995) Number of Nominal Max measurement points (mm) (mm) Single-skin hand/wet Single-skin hand/wet Balsa sandwich hand/vacuum Honeycomb sandwich hand/vacuum PVC sandwich pre-preg/vacuum Al honeycomb hand/vacuum

12 18 15 15 8 15

6.0 12.1 54.8 42.4 51.7 55.5

8.1 15.8 55.5 40.7 52.9 54.2

Min

Mean

(mm)

(mm)

5.6 11.6 54.6 40.3 52.5 53.8

6.3 13.1 55.0 40.5 52.7 54.0

Figure 17.5 Effect of joint thickness to yield strength. Single lap joint.

Figure 17.6 Effect of joint thickness to ultimate strength.

Boats and marine

397

17.3.5 Strength and stiffness Strength of the adhesive as such is not necessarily a very critical value in boat and marine applications because the geometry and area of the bond can usually be altered to increase the load-carrying capacity of the joint. Instead, finding a realistic strength value for dimensioning (see section 17.5) can be a problem. The nominal strength of an adhesive is sensitive to load type, environmental conditions, production parameters, etc. Reduction factors can be very significant, especially for elastic adhesives. For example, the creep strength of one-part polyurethane adhesive can be less than 10% of the short-term strength. Also for other types of adhesives, it is common practice to use no more than 10% of the ultimate strength as the allowed stress value. This should allow uncertainties due to stress concentrations, fatigue, etc., if detailed analyses cannot be done. Stiffness of the adhesive and the real need for a stiff joint should be considered carefully. In fact, only few marine applications require very high stiffness of the adhesive. A certain amount of elasticity is often beneficial, see Marttila and Holm (1991). Also, low stiffness often relates to good vibration damping properties.

17.3.6 Curing time and viscosity The manufacturing of large parts often sets requirements for pot-life of the adhesives. Depending on the methods of adhesive application and adherend positioning, the required pot-life varies largely. When attaching the frames, bottom stiffeners or inner liners, a typical pot-life necessary is about 30 minutes. In hull-deck joints, the required time is normally longer but is strongly dependent on the size of the hull. The demand for a short curing time is most evident in typical series boat production. However, their production procedures do not normally include curing in elevated temperatures. Instead, hulls and decks of higher performance yachts and boats are more often cured at 40±70 ëC. Viscosity of the adhesive should allow application on vertical surfaces. Pastelike adhesives are usually used if the gap between adherends is large: the adhesive layer must be thick enough to bridge this gap while the temperature during curing should remain acceptable.

17.4 Surface preparation Surface treatment of the adherends and shielding of the joint have two different purposes: to improve adhesion between the adhesive and adherend and thus obtain higher maximum strength of the joint; to improve long-term strength and resistance against humidity. The latter is especially important in a marine environment. It is not reasonable to manufacture adhesively bonded joints without surface treatment of the adherends.

398

Adhesive bonding

The general principles of surface treatment for adhesive bonding are relevant to marine applications also. The methods shown for example in (Adams et al., 1997) can be applied. However, the large size of the adherends limits the use of methods requiring immersing (like chromic or phosporic acid anodizing for aluminium alloys). The cost of surface treatment in terms of materials and manhours is also an important factor because of the large areas in question. The methods of surface treatment can be divided into four main groups: 1. 2. 3. 4.

cleaning and degreasing mechanical treatments chemical treatments physical treatments.

Cleaning and degreasing is carried out with organic solvents. Mechanical methods include sanding, brushing and grit blasting. Various chemical methods are suitable if the number of joints is high or if mechanical treatment is otherwise uneconomic. Physical methods, like corona treatment, are very rare in boats and marine applications. FRP adherends may also require additional shielding if they are thin in comparison with the bondline length. Thus, moisture migration to the adhesive may be much faster than in metal joints because of the shorter path, assuming the diffusivities for the adhesive and composite are of the same order of magnitude. Matrix damage in the composite may even provide direct access of moisture to the adhesive layer by capillary wicking (Liechti et al., 1987). In boatbuilding surface preparation typically varies from nil to the use of primers. In joints between FRP and metal more effort is normally expended on surface treatments.

17.4.1 An example of surface treatment on joints between FRP and metal The substantial effect of different surface treatments on the shear strength of a lap joint between FRP and metal is shown, for example, by Peltonen (1991). Epoxy-, polyurethane- and acrylic-based adhesives were tested (Araldite AV138, Foss Than 2K750, Loctite Multibond 330). The FRP specimens were 3 mm thick glass-polyester laminates (chopped strand mat + Neste A300) and carbon-epoxy laminates (UD 0ë/90ë + SP 110/120). The 1.5 mm thick metal specimens were of aluminium (AlMg3) and stainless steel (AISI 316). The surface treatments included sanding (roughness of the abrasive cloth 180), grit blasting (Al2O3-grit 50±100 m) and grit blasting + etching (for AlMg3 a 12 min. immersion in 65±70 ëC natriumdichromate solution, for AISI 316 a 10 min. immersion in 85±90 ëC sulphur oxalic acid solution). Grease removals were carried out with acetone (FRP) and trichloroethylene (metals).

Boats and marine

399

Joints between aluminium and FRP The results for the shear strength of a lap joint between aluminium and FRP are shown in Figs 17.7 and 17.8 as relative to the highest achieved value.

Figure 17.7 Relative shear strength of aluminium-FRP single-lap joints with different surface treatments and adhesives (Peltonen, 1991). Chopped strand mat ± polyester laminate.

Figure 17.8 Relative shear strength of aluminium-FRP single-lap joints with different surface treatments and adhesives (Peltonen, 1991). Carbon-epoxy laminate.

Joints between AISI 316 and FRP Adhesion to stainless steel is good because of the adsorption layer that can easily be modified. The results for joints between AISI316 and FRP are shown in Figs 17.9 and 17.10 relative to the highest achieved value.

17.5 Strength and durability Structural design is normally divided into three areas:

400

Adhesive bonding

Figure 17.9 Relative shear strength of stainless steel-FRP single-lap joints with different surface treatments and adhesives (Peltonen, 1991). Chopped strand mat ± polyester laminate.

Figure 17.10 Relative shear strength of stainless steel-FRP single-lap joints with different surface treatments and adhesives (Peltonen, 1991). Carbonepoxy laminate.

1. 2. 3.

Load determination ± magnitude, frequency. Material characteristics ± strength and stiffness. Calculation of response ± strains, stresses, and displacements.

In marine applications, loads are often in the area where the uncertainties are largest. The complicated material characteristics of adhesives, especially in the long term, do not make life any easier for a designer. Numerical analysis of stresses at a bonded interface is very useful when improving joint geometry, but purely theoretical estimates of joint strength are normally unacceptable as a basis for design. This is because of uncertainty about imperfections and local stress concentrations within the connection (Smith, 1990). Quality control to the same degree as in the aerospace industry is not common or even possible in the marine industry. Higher safety factors as well as test data obtained from a larger structure under static and dynamic loading is therefore essential when developing new solutions for bonding in ship scale.

Boats and marine

401

17.5.1 Designing for strength Efficient design of structural connections requires a clear understanding of the function of each type of joint, together with a quantitative knowledge of the forces and moments to be transmitted. The designer should strive for a joint location and geometry that gives a beneficial stress distribution from the joint type point of view. However, this is a demanding task: for example, it is widely known that peel stresses should be avoided, but in practice, it is often impossible to do so. Designers just have to live with and try to reduce peel stresses, and evaluate them carefully. This is especially true for many FRP adherends, as the peel strength of FRPs is low compared with their in-plane strength. Peel stresses can be efficiently reduced by an appropriate joint design and choice of adhesive (see Fig. 17.11). Tip shape It is important to evaluate carefully the local phenomena occurring at the edges of the joint, where failure is normally initiated. The shape of the ridge tip is an important parameter, which quite easily can be worked on in boat- and shipyards. The effect of different tip shapes for FRP-metal joints has been analysed (Adams et al., 1997; Hildebrand, 1994). Some results are shown in Tables 17.3±17.5. It is interesting to note that in these cases chopped strand matpolyester FRP is always the critical part, but with pultruded glass-polyester the critical part is normally adhesive.

Figure 17.11 Different possibilities to reduce peel stresses in typical marine applications.

402

Adhesive bonding

Table 17.3 Materials used for the joints (Hildebrand, 1994) Adherends

Aluminium: Steel: FRP:

Adhesives

Epoxy: Epoxy:

± AlMg3H32 ± AISI316 ± CSM-UP: Resin transfer-moulded chopped strand mat-polyester (fibre content 25 vol.-%) ± Tape-pultruded glass-polyester (60 vol.-%) ± Pultruded carbon-vinylester (58 vol.-%) ± 3M Scotchweld 9323 B/A ± Ciba Araldite AV138 / HV998

Table 17.4 Results of CSM-UP/AlMg3H32 joints. Adhesive: AV138. Adhesive thickness: 0.1 mm. Joint strength (in N/mm) and location of failure (critical part) of differently shaped single-lap joints (Hildebrand, 1994) Joint

Joint strength

Critical part

C

78

FRP

F

102

FRP

G

144

FRP

J

134

FRP

K

135

FRP

N

135

FRP

O

135

FRP

Table 17.5 Results of tape-pultruded glass-UP/AlMg3H32 joints. Adhesive: AV138. Adhesive thickness: 0.1 mm. Joint strength (in N/mm) and location of failure (critical part) of differently shaped single-lap joints (Hildebrand, 1994) Joint

Joint strength

Critical part

A

69

Adhesive

C

182

Adhesive

F

252

Adhesive

G

323

Adhesive

I

375

Metal & adhesive

K

375

Metal

L

375

Metal

M

375

Metal

Boats and marine

403

Analytical methods for calculating the strength of adhesive-bonded joints Adhesive joints can be analysed with present analytical methods quite well if the geometry of the joint, the load case and behaviour of the adhesive are simple. The general use of analytical methods is thus restricted, but some of them can be used successfully for instance during preliminary feasibility and design studies. Analytical methods have been extensively treated in the literature (Adams et al., 1997; Hart-Smith, 1973) and therefore they are not further revised in this work. However, in order to optimise a given joint and to obtain reliable results for joint strength, numerical methods are needed. Finite element techniques and solution methods Finite element methods (FEM) have been widely used for the analysis of adhesive joints. Some special points to consider in the Finite element analysis (Hildebrand, 1994): · The adhesive layer is thin compared to the thickness of the adherends, but due to the high stress gradients, it is necessary to use 4±6 elements through the thickness to achieve reliable results. · It is often possible to simplify the problem by using two-dimensional models for certain types of joints. In many joints, especially in wide ones, joint behaviour can be assumed to be of plane strain-type. In the case of axisymmetric joints, simplifications can be made to a similar extent. · Non-linear behaviour has to be taken into account. Geometric non-linearity (changing geometry under loading) is usually significant in asymmetric joints, for instance single-lap joints. Additionally, it is important to take the material non-linearities of adhesives and adherends into account. · When choosing the model for the adhesive layer (linear elastic, linear elastic ± ideally plastic or test data), it should be checked what kind of data is available. The situation is most difficult with many polyurethane adhesives, but even for the `high-end' epoxy adhesives, reliable strength data is not always provided by the manufactures.

17.5.2 Testing The standard test methods for adhesive materials are normally suitable also for adhesives in marine applications. If ageing of the test specimens is required, for example ASTM D 1183 ± 70: `Standard Test Methods for Resistance of Adhesives to Cyclic Laboratory Ageing Conditions' is a suitable standard to simulate marine environments, which also includes low temperatures. (See Table 17.8 on page 411.)

404

Adhesive bonding

17.6 Common failures Some common failures in boat and marine applications are caused by too large tolerances of adherends, selection of too brittle an adhesive and dirt or other failures in surface preparation. Because the dimensions of parts are normally large in marine applications, the absolute values of tolerances are also quite large. Shape distortion due to shrinkage during curing can be difficult to manage especially in corners. Thickness variations and imperfections due to substandard work can be other sources of too large tolerances. Locally insufficient pressure to clamp the adherends together is often a consequence of the previous, i.e., too large tolerances of adherends. Dirt or other failures in surface preparation are common in shipyards, where the environment is more suitable for metal cutting and welding, not for adhesives.

17.7 Inspection, testing and quality control Unlike the aerospace industry, quality control in marine applications does not normally include NDT. Manufacturing flaws (if not visually detected) are more common and those have to be accepted and tolerated to a certain extent. Periodic inspections for ships in use follow the culture of metal structures and welding. This means that the tolerance for cracks is quite high. Some parts of the hull can be accessed only in dry dock or at a shipyard or other locations where the vessel can be taken out of the water. Such facilities may also have available a range of NDE equipment and specialist operators. However, relatively few shipyards or docks have experience with adhesives or FRP composites, so it may still be necessary to bring in specialist personnel and equipment. For boats there is not a common procedure for periodic inspection of structures. However, during winter storage and other servicing there is the possibility to examine the structures, and docking of boats is usually not a problem.

17.7.1 Inspection methods Visual inspection is normally not adequate for adhesively bonded joints. Ultrasonic techniques can be successfully used to inspect adhesive joints with careful selection of the transducer and frequency. The main problems with marine applications are related to dissimilar adherend materials, large areas to be scanned, as well as accessibility to the joint. Because it is not normally easy to access both sides of the joint, the pulse-echo technique is preferred instead of through-transmission techniques. Immersion of large parts is usually impossible, and contact techniques have to used. This means that scanning of large areas is time consuming and inspection should be restricted to the most critical areas only.

Boats and marine

405

Infra-red thermography is based on measuring the change in the distribution and flow of heat due to a defect in the specimen. A combination of a heat lamp and infra-red video camera is the most common method to detect defects. The heat source and the camera can be placed either on the same side or on opposite sides of the object. The latter is recommended if the adherends or adhesive layer is thick. Fairly large areas can be inspected quickly, which is important in marine applications. However, small defects are difficult to find with thermography. The method should usually not be used alone; critical areas have to be inspected with ultrasonic techniques.

17.8 Repair Repairing adhesive joints is normally laborious, and it is thus worthwhile to categorise the damage or defect as: · negligible (not critical) · repairable (cost-effectively and technically) · non-repairable (must be renewed). The criteria for assessing the damage using one of these categories are very different depending on the requirements for the structure at the location in question. If damage is considered as repairable, the next step is to decide how, when and where the repair shall be carried out. For larger ships the two latter questions may have a substantial effect on the costs if the ship has to be drydocked before a normal service interval. For smaller boats the costs are dominated by the repair method and extent. Bonding a new patch onto the defect can be mechanically an efficient way of repairing, provided there is no requirement for flush shape or aesthetic appearance. High-quality adhesives can be used because material price is normally not a critical factor in repairs. However, the compatibility between repair materials and the existing structure should always be checked. If repairs have to be carried out in field conditions, the environment may be the controlling factor for choosing the repair method and materials. For example, the adhesive or resin should be capable of curing at low temperatures and adhering to surfaces that are not completely dry or clean.

17.9 Examples of use Two very different design cases are introduced below. The first one is a hulldeck joint of a small FRP-boat, and the other example is from shipbuilding, where an adhesive joint between aluminium deck and FRP-sandwich structure is compared with a bolted joint.

406

Adhesive bonding

17.9.1 Hull-deck joint in small FRP boats A typical hull-deck joint in small FRP boats has outward-facing flanges that are attached using pop rivets and adhesive (see Fig. 17.12). Although there are some global loads affecting the joint, the critical load is clearly local bending due to fender loads or minor impacts of a quay edge or similar. Unfortunately, the joint is loaded in just the way that induces cleavage in the adhesive bond (Fig. 17.13). To calculate the impact force during a smooth collision with a quay, we assume the following typical input values:

Figure 17.12 Typical hull-deck joint and transom attachment of a 5 m motorboat.

Figure 17.13 The critical load of 5.6 N/mm affecting the joint.

Boats and marine · · · ·

407

weight
of the fully loaded boat 1100 kg collision force and speed 1.0 kN (0.5 m/s) length of the fender (contact length) 0.5 m the fender flexing 100 mm during the impact.

Using a simple equation of harmonic motion, the maximum force during the impact is 2.8 kN, which in this case corresponds with 5.6 N/mm. Further, if we assume that · the material of the adherends is GRP (mat/rowing with orthophthalic polyester matrix) · the thickness of the laminates is 7 mm · the thickness of the adhesive joint is 2 mm · the breadth of the bond is 40 mm · the surfaces of the adherends are sanded then the results of Marttila and Holm (1991) can be applied (see section 17.3.2). The results show that flexible polyurethane adhesives can be used without additional strengthening, if this kind of impact is regarded as an unusual maximum dynamic load. Depending on the adhesive, the factor of safety is between 3 and 5. Without additional strengthening, the joint strength with stiff adhesives or wet CSM will not be sufficient. If pop rivets are used, a factor of safety of about 6 is achieved.

17.9.2 Replacing bolts with adhesive bonding in a joint between aluminium hull and FRP-sandwich deckhouse (Hentinen et al., 1997) The starting points for design are as follows: · the joint length per application exceeds 100 m · panel size 2  3 m, lateral pressure load (34 kPa derived from DnV HSLC; global loads not critical in this case) · panels are constructed of glass-epoxy skins (0ë/90ë, thickness 2.5 mm) with an end-grain balsa core (thickness 50 mm) · the metal joint profile is joined to the sandwich panel in conjunction with the laminating process · even outer surface required (asymmetric joint profile) · epoxy adhesive, thickness of the adhesive layer 0.5 mm. Two-dimensional (plane strain) analyses of the joints were performed. The analyses allow for both geometrical and material non-linearities. In the present concept, however, the most significant non-linearities are due to plasticity in the aluminium profile. The experimental part of the work included different flexural and tensile tests. Various load cases were analysed. The first three load cases

408

Adhesive bonding

Table 17.6 Analysed load cases (Hentinen et al., 1997) Load case 1 `Lateral pressure, simply supported'

2 `Lateral pressure, rotations fixed' 3 `Lateral pressure, fixed'

4 `Tensile test'

5 `Flexural test'

6 `Panel compression'

correspond to a laterally loaded panel under different boundary conditions. The loading in the joints of these cases was compared with the loading obtained in flexural and tensile tests. Table 17.6 shows the loads and boundary conditions of the analysed cases. The analysis results indicate that the critical stress concentrations in the FRP faces and the adhesive near the aluminium profile can be decreased by improving the shape of the aluminium profile. Additionally, the thickness of the end of the aluminium profile should be increased in order to avoid plastic deformations, which in turn generate high stress concentrations in the FRP faces at location B (Fig. 17.14). The flexural test (load case 5) produces a joint loading which, in the most critical areas of the joint, is similar to that in the laterally loaded panel with simply supported edges. The joint loading generated by the tensile test is completely different from that of the laterally loaded panel. The behaviour of the joint in in-plane compression was also studied. In order to obtain a reference magnitude for the load, the stability of the corresponding panel under in-plane compression is analysed under different boundary

Boats and marine

409

Figure 17.14 Critical areas of the overlaminated joint.

conditions. The results of the analysis show that the stress distribution in the joint under compressive in-plane loads is similar to the one resulting from transverse pressure load of the panel. Tensile and three-point bending tests were performed under different conditions. Some of the test specimens were tested after undergoing a weathering cycle. Some of the specimens were tested in room conditions, some at elevated temperature and humidity (40 ëC/85% RH) and some at high temperatures (200 ëC). The weathering cycle was performed according to the standard ASTM D 1183 ± 70: `Standard Test Methods for Resistance of Adhesives to Cyclic Laboratory Ageing Conditions'. The standard (exterior/marine use) and the performed cycles are shown in Table 17.7. The specimens to be tested at 40 ëC and relative humidity of 85% were first kept from seven to eight days in these conditions so that the specimens were fully exposed. The tests were carried out in a special `box' which was designed and built to keep these conditions steady during the tests. Table 17.7 The standard method and performed cycles in the weathering tests. The first performed cycle was applied to the overlaminated and clevis joints. The second cycle was applied to the flexible joints (Hentinen et al., 1997) Standard ASTM D 1183 ± 70: `Standard Test Methods for Resistance of Adhesives to Cyclic Laboratory Ageing Conditions' Period Temperature (h) (ëC) 48 48 8 64

Relative humidity (%)

71  3 < 10 23  1.1 Immersed in substitute ocean water 57  3 approximately 100 23  1.1 Immersed in substitute ocean water

Performed first cycle

Performed second cycle

Period Tempera- Period ture (h) (ëC) (h)

Temperature (ëC)

48 48

71 23

48 54

71 23

161

ÿ50 . . ÿ58 23

91

ÿ50 . . ÿ60 23

65

64

1. Test specimens were within the standard regime for 8 hours The relative humidity in both test series was in the regime given by the standard

410

Adhesive bonding

Figure 17.15 Specimen sizes (overlaminated joint) in the tensile test. The specimen dimensions for the clevis and flexible joints are similar.

In many marine structures, fire safety must be taken into account in the dimensioning and selection of materials. In the present case, the heat conducted to the joint via the metal profile was considered critical. Therefore, static strength tests were carried out for specimens after heating them to a temperature of 200 ëC in an oven. Figures 17.15 and 17.16 show the sizes of the tensile and flexural test specimens. The average test results are shown in Table 17.8. The first failure occurred at location B (see Fig. 17.14). The lower face debonded from the profile over a width of around 30±40 mm from the laminate end. However, the debonded area did not grow further during the tests. Ultimate failure occurred at location A, the failure type being delamination between the FRP face and core, and subsequent core shear failure. The effect of the different surface treatments on strength is considerable. The silane primer (Alb) led to a 2.5 times higher strength at first failure and to an increase of 17% in maximum strength compared with specimens without primer (Ala). An interesting observation is that the effect of specimen conditioning and testing conditions on the joint strength is only moderate: the decrease in maximum strength after weathering is below 10%. The 40 ëC/85% RH conditions lead to a decrease in maximum strength of 12%. After exposure to high temperatures (200 ëC), most of the panels were clearly damaged (faces debonded from core). As expected, the strength values were

Figure 17.16 Specimen sizes (overlaminated joint concept) for the three-point flexural test. The specimen dimensions for the clevis and flexible joint concepts are similar.

Table 17.8 Average static test results (Hentinen et al., 1997) Joint type

Condition

Strength at first failure (N/mm)

Maximum strength

Stiffness

Toughness

(N/mm)

Deflection at maximum strength (mm)

(N/mm/mm)

(N/mm/mm)

Dry + RT

48

121

4.7

28.2

569

Dry + RT

123

142

6.1

27.4

889

Weathering + RT 40 ëC/85% 200 ëC

108 112 ö

132 127 1

6.6 9.6 0.5

22.5 21.5 ö

893 1225 0.5

Al a

Al

b

Table 17.9 Fatigue test results, R ˆ 0.1. The number of cycles at first failure (Nff) has been approximated from the cycles-deflection curve at a point where deflection starts to increase (Hentinen et al., 1997) Joint type

Nmean Nffmean Nmin Nmax Failure type

Conditions

Load

Nmean

Nffmean

Nmin

Nmax

Failure type

Dry/RT

80%

18775

553

17600

19400

Weathering/RT

80%

7450

671

350

11000

Weathering/RT

70%

18700

633

2050

28800

Weathering/RT

60%

50100

575

50100

81550

Laminate Aluminium Balsa Aluminium Balsa Aluminium Laminate Aluminium

average number of cycles to failure average number of cycles to first failure lowest number of cycles to failure (`weakest specimen') highest number of cycles to failure (`strongest specimen') balsa = shear failure in the core, aluminium = failure in the extruded profile, laminate = flexural failure in face laminate

1 4 1 4 2 3 1 4

Boats and marine

413

Figure 17.17 Fatigue test results, R ˆ 0.1.

very low (about 1±5 N/mm) and the strength of the adhesive joint could not be measured. More suitable test methods should be developed to simulate the heat conduction. Aluminium structures are known to have relatively poor fatigue strength as compared to FRP laminates. The fatigue strength of the joint concepts was examined with fatigue tests for weathered and dry specimens. All fatigue tests were performed with the ratio one to ten (R ˆ 0:1) between minimum and maximum force. The frequency was 1 Hz. Maximum force levels between 60% and 90% of the equivalent average static strengths were used. The test results are shown in Table 17.9. The number of cycles at first failure was observed from the cycles-deflection curve. The results are shown in Fig. 17.17. Compared with the static strength values, the fatigue strength can be considered high. The failures occurred in the aluminium profile and balsa core (see Fig. 17.18) or, in two specimens, in the laminate. The correlation of the bending deflection between the flexural tests and the analysis is good. However, after the first failure, the stiffness of the joint is slightly reduced. This effect is not modelled in the analysis (Fig. 17.19). The force at which ultimate failure of the specimen occurs lies at the same level as where the analysis results suggest the beginning of a plastic hinge in the aluminium profile. Hence, the ultimate joint strength is at a

Figure 17.18 Typical failures during the fatigue tests.

414

Adhesive bonding

Figure 17.19 Correlation between analysis and flexural test results of the aluminium-FRP-sandwich joint.

level above which it cannot be increased without changing the thickness of the aluminium-profile root.

17.9.3 Comparison of the joint with a similar bolted joint type The joint concept can be compared with a similar joint type where the FRP sandwich is directly bolted to the metal structure (Fig. 17.20). The bolted joint also uses an adhesive in order to achieve water tightness. This bolted joint type is common practice in shipbuilding today, but it is known to be rather costintensive due to the large amount of man-hours involved in the bolting. The comparison is the most interesting regarding the amount of labour involved in both joint types (Table 17.10). The manufacturing costs of the bolted joint are known from joints used in current structures, whereas the manufacturing cost of the adhesively bonded joint have been determined during the manufacture of the test specimens. The test specimens of the bolted joints differed in two aspects from the adhesively bonded joint. Firstly, the core material in the flexural test specimens was aluminium honeycomb, with the exception of the sandwich where end-grain balsa was used. However, this is not considered to have an influence on the flexural strength, because the shear stiffness of aluminium honeycomb and endgrain balsa is similar. The bolted specimens failed at the panel edge, where the

Figure 17.20 Geometry of the comparable bolted joint type.

Boats and marine

415

Table 17.10 Comparison of both strength and manufacturing costs of the adhesively bonded joint concept and a similar bolted joint type (Hentinen et al., 1997) Adhesively bonded joint concept Strength: Flexural test ± force per width at first failure Flexural test ± force per width at break Tensile test ± force per width at break Manufacturing costs: Panel edge preparation Joining to ship

Bolted±bonded

48 N/mm 121 N/mm 537 N/mm

18 N/mm 18 N/mm 507 N/mm

100% 100%

100% 220%

core material was balsa. Secondly, in the tensile test of specimens of the bolted joint, the core thickness was only 25 mm, the core material being end-grain balsa.

17.10 Future trends At boatyards there is a growing trend to use adhesives in joining large fibrereinforced plastics (FRP) parts together. As a large majority of boat hulls and decks are of FRP, use of polymers is common practice at boatyards. It is thus not a large step to move from laminating to adhesive bonding when attaching the bulkheads to the hull shell, for example. The driving force to change from laminating to adhesive bonding is mainly productivity. Fastening bulkheads or inner liners with laminating is labour intensive compared with adhesive bonding. As closed mould techniques are becoming more common at boatyards, adhesive bonding better suits the character of the production. In some cases, like the attachment of stiffeners, benefits in strength can be a motivation for adhesive bonding. In aluminium boats progress towards adhesive bonding seems to be slower. The advantages of adhesives are clear in terms of vibration damping and appearance (smoothness of the plating) and some boatyards reap the benefit of using them. However, welding and riveting probably maintain their position as a main method for joining in aluminium boats. In shipyards the need for adhesive bonding is connected to multimaterial applications. The principle of `correct material for correct use' is becoming increasingly important, especially in high-speed applications. In commercial ships structural polymers are problematic in fire, because rules for fire resistance and safety are developed for steel structures. New rules are under development, and they will probably be applied first to high-speed craft. In naval ships the situation is different, because they are not tied to class rules.

416

Adhesive bonding

There is a growing need to save weight in ships and boats without reducing comfort or luxury. This is an area where a growing demand for a new kind of sandwich plate can be seen, for example, a marble face has been bonded onto FRP-honeycomb plate. Similar parts for interior or decoration are a potential area for adhesive bonding. A general increase of confidence in adhesive bonds will filter through to boat- and shipyards. Long-term reliability in humid environments is emphasised in marine applications, and is the area in which further research is mostly needed.

17.11 References Adams R D, Comyn J and Wake W C (1997), Structural adhesive joints in engineering, 2nd edn. London, Chapman & Hall. Burchardt C (1997), `Bonded sandwich T-joints for maritime applications' in BaÈcklund J, Ê stroÈm B, Composites and sandwich structures. Stockholm, Zenkert D and A Engineering Materials Advisory Services Ltd. Burchardt B, Diggelmann K, Koch S and LanzendoÈrfer B (1998), `Elastic bonding'. Landsberg/Lech, Verlag moderne Industrie. Engineered materials handbook. Vol. 1. Composites. ASM International, USA 1987. Furustam K-J (1996), `NBS-VTT Extended Rule'. Espoo, Technical Research Centre of Finland. Hart-Smith L J (1973), Adhesive bonded single-lap joints. Long Beach, CA, Technical report NASA CR 112236. Hentinen M and Hildebrand M (1995), `Joints between FRP sandwich and metal ± primary design'. Espoo, Technical Research Centre of Finland. Hentinen M and Holm G (1994), `Load measurements on the 9.4 m sailing yacht Sail Lab'. Proc 13th Int Symp. Yacht Design and Yacht Construction. Amsterdam. Hentinen M, Hildebrand M and Visuri M (1997), `Adhesively bonded joints between FRP sandwich and metal. Different concepts and their strength behaviour'. Espoo, Technical Research Centre of Finland. Hildebrand M (1994), `The strength of adhesive bonded joints between fibre-reinforced plastics and metals'. Espoo, Technical Research Centre of Finland. Larsson L and Eliasson R (2000), Principles of Yacht Design, 2nd edn. London, Adlard Coles nautical. Liechti K, Johnson W S and Dillard D A (1987), `Experimentally determined strength of adhesively bonded joints', in Matthews F L, Joining fibre-reinforced plastics. Essex, Elsevier Science Publishers Ltd. Marttila K and Holm G (1991), `Joints in large reinforced plastics parts', Espoo, Technical Research Centre of Finland (in Finnish). Peltonen J (1991), Surface pretreatments in metal-plastic composite joints, Oulu, Oulu University (in Finnish). Smith C S (1990), Design of marine structures in composite materials, Essex, Elsevier Science Publishers Ltd.

18

Shoe industry J M M A R T IÂ N - M A R T IÂ N E Z

18.1 Introduction Since the 1950s adhesive joints have been used in shoe manufacturing as an alternative to sewing or application of nails, staples or tacks to bond the upper to the sole. The introduction of adhesives technology in shoe manufacturing provides several advantages: (i) more flexible and homogeneous joints are obtained; (ii) the applied stresses are similarly distributed all along the joint; (iii) improved aesthetic properties are obtained, and new design and fashion issues are favoured; (iv) automation during manufacturing is feasible. However, as a limitation, bonding with adhesives needs great control of all the steps involved in the formation of the joints to avoid adhesion problems. Figure 18.1 shows the main parts of a shoe. Adhesive joints are used to bond several of these parts (Table 18.1). Although all adhesive joints in shoe manufacturing are critical, there is no doubt that the joining of the upper to the sole is the most important and difficult to carry out in the shoe industry. If sole to upper adhesion is poor, sewing has to be done or the adhesive has to be completely removed to produce a new joint. This chapter will be mainly devoted to upper-to-sole bonding. Depending on the type of shoe, different bonding performance is required. Due to fashion issues, bonding of casual and leisure shoes is a relatively low priority in the sense that the required durability is not too high (five years as maximum). However, sports shoes are extremely demanding of bonding as low weight, high performance under impact and flexural stresses, high comfort, and high durability are mandatory. Safety shoes are the most demanding in terms of bonding because they have to work in the presence of solvents, high temperatures or high stresses, and all of these for a long time. The adhesion between the upper, the adhesive and the sole surfaces must be properly optimised to produce adequate joints. Therefore, the adhesion in shoe bonding can be increased by surface modification of the upper and/or sole materials (by application of adequate surface treatments and/or primers), by modifying the adhesive formulation (incorporation of adhesion promoter), or

418

Adhesive bonding

Figure 18.1 Schema showing the main parts of a shoe.

both. Considering that most of the upper materials are porous and the adhesives used in shoe bonding are applied as liquids, an acceptable penetration of the adhesive into the upper is expected and then mechanical adhesion is generally favoured. However, because the different sole materials are non-porous and have in general a relatively low surface energy, both mechanical (for example, roughening) and chemical (for example, halogenation) surface preparations are necessary and enhanced chemical adhesion is required. Therefore, the main mechanisms of adhesion involved in shoe bonding are mechanical and chemical. Furthermore, in the bonding of some rubber soles the weak boundary layers produced by surface contaminants and/or by migration of anti-adherend moieties to the interface must be removed before joint formation. Several different materials are used in the shoe industry depending on fashion and required performance. New sole materials appear for each season which makes it particularly difficult to standardise the adhesive bond formation. Surface preparation procedures for these materials must be quickly developed and the validity of these treatments is generally too short. Table 18.1 Adhesives used in shoe bonding Shoe operation

Adhesive

Mounting Heel covering Heel attachment to sole Box toe bonding Shank and cushion bonding Lift attachment Sticking of socks lining Upper to sole bonding

Cement (rubber adhesive) Polyurethane Polyester hot-melt Polyamide hot-melt Waterborne polyurethane Polyester hot-melt Polychloroprene Polyurethane, polychloroprene

Shoe industry

419

Most of the upper and sole materials used in the shoe industry cannot be directly joined by using the current adhesives (polyurethane and polychloroprene adhesives) due to their intrinsic low surface energy, the presence of contaminants and anti-adherend moieties on the surface, and the absence of an adequate surface chemistry. Therefore, the surface preparation of upper and sole must remove contaminants and weak boundary layers, and roughness and chemical functionalities able to produce adequate bond strength should be created. In this section, an extended review of the main materials used in shoe bonding as well as their current surface preparation procedures will be considered. In the last section of this chapter, alternative surface treatments for upper and sole materials are described.

18.2 Upper materials in shoes Full-chrome, semi-chrome and vegetable tanned leathers are still the main source and most common upper material in shoe manufacturing. In general, the porous nature of leathers facilitates their bond with adhesives in solution, mainly solvent-borne polychloroprene adhesives. To obtain good adhesion, the weak grain layer of the leather must be removed by rotating wire brushes (roughening) to expose the corium (the part of the leather with higher cohesion of the collagen fibres) to the adhesive.1 In general, deeper roughening of upper leather produces a stronger bond than light roughening. Furthermore, two consecutive adhesive layers are generally applied on the roughened leather. First, a low viscosity adhesive solution is applied to fill the pore entrances of the corium fibres and to facilitate the wettability of the next adhesive solution. At least five minutes later (or alternatively when the adhesive layer becomes dried) a second high viscosity adhesive solution of the same nature as the previous one is applied; diffusion of the polymer chains between the two adhesive layers is produced and a homogeneous and high cohesion adhesive layer is created on the upper leather surface. Three main difficulties arise in the bonding of the leather upper. The first is the existence of finishing and pigments on the upper leather.2,3 Good adhesion can be obtained by mild surface scouring followed by application of a primer. This primer must be able to penetrate into the leather grain layer increasing the cohesion of the collagen fibres and also it has to be compatible with the adhesive. Several primers for thin leathers (water-dimethyl ketone blends, aqueous solutions of 8.5 wt% nonylphenol polyoxyethylene with 8.5 mol of oxyethylene, lyotropic agents ± urea, CaCl2) requiring the application of a thermal treatment at about 140 ëC have been proposed,4,5 but in general some degree of leather shrinkage is produced. Recently6±8 different low viscosity reactive solvent-based polyurethane primers containing about 10% free isocyanate groups have been developed. These reactive primers react with moisture and collagen in the leather and they do not need thermal treatment after

420

Adhesive bonding

application. Thus, leather shrinkage is avoided, but these primers contain solvents which may cause environmental threats. The second difficulty is greasy leather.9,10 Fatty acids are especially troublesome with semi-chrome leathers because they need more fat than full chrome leathers to make them mellow and because they are usually made from skins which are liable to contain a higher percentage of fatty acids in the fat. Amount (11±15 wt%) and type of fat (mainly unsaturated fatty acids in concentrations of 2.7±5.5 wt%) in leathers cause adhesive bond trouble. Polyurethane adhesives are more tolerant to greasy leather than polychloroprene adhesives, but in general amounts of grease larger than 11 wt% cause adhesion problems. Roughening of the leather is always necessary and the addition of 5 wt% isocyanate to the adhesive just before application helps it to make good adhesive joints. The third main difficulty is the use of polyurethane adhesives. Solvent-borne polychloroprene adhesives are excellent for bonding leather because of their high wettability and permanent tack (heat activation is not needed). However, these adhesives have poor performance with most of sole rubbers and with several synthetic uppers. In these cases, polyurethane adhesives provide better performance. When using polyurethane adhesives the application of two consecutive adhesive layers is necessary and heat activation is generally needed.11 It has also been suggested12 that the combined use of polyurethane adhesive on the upper and polychloroprene adhesive on the sole caused compatibility problems and improper film coalescence was found. Further, with PVC uppers migration of plasticisers is not inhibited. Several synthetic upper materials such as canvas, textiles, nylon, PVC on woven or non-woven base, and poromerics are also used in shoe manufacturing. As for leather, most of these materials need roughening (to remove finishing) and/or solvent wiping, followed by application of two consecutive adhesive layers, to produce good adhesive joints. PVC uppers may cause lack of adhesion to polychloroprene adhesives due to the migration of plasticiser into any material with which the PVC comes into contact.13 Also, the plasticiser seems to favour the incompatibility between the adhesive and the sole material. To avoid this lack of adhesion, a light roughening is necessary or alternatively a methyl ethyl ketone (MEK) wiping can be applied.3 Further, the application of polyurethane adhesives containing 5 wt% isocyanate ensure adequate adhesion and durability.14 Alternatively, application of a primer based on nitrile rubber may perform adequately in service followed by application of a two-component polychloroprene adhesive.11 PVC-coated fabrics may have acrylic or urethane coatings on top of the thick PVC. These coatings must be removed before cementing to avoid the formation of a weak layer in the final bond.15 Acrylic coatings are readily soluble by MEK wiping, whereas urethane coatings need tetrahydrofurane wiping. Polyurethane-coated fabrics can be bonded using polyurethane adhesives. Mild scouring using a fine rotary wire brush must be used to completely remove

Shoe industry

421

the coating. Poromerics (non-woven fabric supporting a microporous urethane layer) also need to be roughened into the microporous layer for adequate bonding. Without special care during roughening, the weft fibres may become damaged which would reduce the sole bond strength. When nylon, polyester or cellulose interlayers are present, isocyanate primers promote adhesion.15 Nylon fabric uppers are generally anti-adherend and show serious adhesion problems. The best bonding is obtained by mild roughening followed by applying isocyanate primer (about 2 wt% isocyanate is sufficient). Application of twocomponent polyurethane adhesive is also necessary.16

18.3 Sole materials in shoes Several kinds of sole materials are currently used in shoe manufacturing. A few soles derive from natural products, such as leather or cork. Leather soles are not difficult to bond because they are porous and easily wetted by adhesive solutions. Therefore, their bond strength is increased by enhancing the mechanical adhesion, i.e., roughening exposes the corium to the surface and allows adhesive interlocking. A primer is required to produce a good bond with solvent-base polychloroprene adhesives. The use of polyurethane adhesives is also feasible and needs the application of a low viscosity polyurethane primer to which 2.5 wt% isocyanate must be added. Cork soles are widely used for sandals. Cushion type insoles composed of a fibreboard layer wrapped in PVC-coated fabrics are common and are joined with polychloroprene adhesives. In some cases, a thin EVA midsole is attached to the cork to improve its abrasion resistance by using two-component polychloroprene adhesive.17 Rubber soles are by far the most common in the shoe industry. Vulcanised or unvulcanised (thermoplastic) rubber soles are used. In general, a bond is produced with one- or two-component polyurethane adhesives and a surface treatment is always required to produce good adhesive joints. Thermoplastic (TR) rubber soles generally contain polystyrene (to impart hardness), plasticisers, fillers and antioxidants; processing oils can also be added. Table 18.2 shows typical composition of a TR rubber sole. Due to their nature, TR rubber soles have a low surface energy, so to reach proper adhesion to the polyurethane adhesive a surface modification is needed. Special adhesives have been developed to avoid surface preparation but they have poor creep resistance. Although natural rubber (crepe) and nitrile rubber soles (common in the manufacturing of safety shoes due to its chemical inertness) are used, sulfur vulcanised styrene-butadiene (SBR) rubbers are the most common sole materials. Table 18.3 shows a typical composition of a SBR rubber. The vulcanisation system contains sulfur and activators (N-cyclohexyl-2-benzothiazole sulphenamide, dibenzothiazyl disulfide, hexamethylene tetramine, zinc oxide)

422

Adhesive bonding Table 18.2 Typical composition of a TR rubber sole. Composition expressed as parts per 100 parts rubber Ingredient

Percentage (phr)

SBS rubber Paraffin plasticiser Polystyrene Diesteramide Antioxidant (Irganox 565) Calcium carbonate Carbon black

100 10 20 0.1 0.2 10 1.1

and fillers (silica and/or carbon black, calcium carbonate) are added to control hardness and abrasion resistance. Antioxidants (zinc stearate, phenolic antioxidant) and antioxidant (microcrystalline paraffin wax) are also necessary to avoid degradation processes and early ageing. In general, vulcanisation is carried out in a mould within a hot plate press at about 180 ëC, where some chemical reactions affecting adhesion are produced. Because of the thickness of the SBR rubber sole, the part in contact with the mould is substantially hotter than the bulk, producing overvulcanisation on the surface leading to high stiffness. Furthermore, the zinc oxide and the stearic acid react to produce zinc stearate during vulcanisation, which is combined with hexamethylene tetramine to form an unstable complex. After vulcanisation, contact with moisture or many solvents apparently causes the breakdown of this complex with the appearance of the zinc stearate that migrates to the SBR rubber sole18 which acts as an antiadherend material to polyurethane adhesive.15 On the other hand, during cooling after vulcanisation, the paraffin wax migrates to the SBR rubber surface also giving adhesion problems.19±22 Table 18.3 Typical composition of SBR rubber sole. Composition expressed as parts per 100 parts rubber Ingredient SBR 1502 Precipitated silica Sulfur Cumarone-indene resin Zinc oxide Stearic acid N-cyclohexyl-2-benzothiazole sulphenamide Phenolic antioxidant Dibenzothiazyl disulfide Microcrystalline paraffin wax Hexamethylene tetramine Zinc stearate

Percentage (phr) 100 42 2.0 5.0 1.5 2.4 2.0 0.5 2.5 0.8 1.0 5.4

Shoe industry

423

The application of surface treatments to TR and SBR rubber soles should produce improved wettability, creation of polar moieties able to react with the polyurethane adhesive, cracks and heterogeneities should be formed to facilitate the mechanical interlocking with the adhesive, and an efficient removal of antiadherend moieties (zinc stearate, paraffin wax, processing oils) have to be reached. Several types of surface preparation have been proposed to improve the adhesion of vulcanised SBR rubber soles. These include: · · · ·

solvent wiping23 mechanical treatments8,15,20,24 cyclisation25±29 chlorination30±35

Amongst these methods, chlorination with solutions of trichloroisocyanuric acid (TCI) in different solvents is by far the most common surface preparation for TR and SBR rubbers. There are some other materials used for shoe soles. EVA (ethylene-vinyl acetate block copolymer) has a low surface energy and therefore is difficult to bond. The higher the vinyl acetate content, the less difficult to bond EVA soles. The lightweight microcellular EVA soles can usually be adequately bonded after scouring using polyurethane and polychloroprene adhesives; isocyanate wiping can also be helpful.15 The injection-moulded EVA is more difficult to bond although roughening followed by application of two-component polychloroprene adhesive can give a moderate bond strength.36 Recently36 corona discharge has been shown to be useful in improving the peel strength of injection-moulded EVAs containing 12 and 20 wt% vinyl acetate/polychloroprene adhesive joints. Treatment with sulfuric acid also provided improved adhesion of injection-moulded EVAs with vinyl acetate contents between 9 and 20 wt%, because this treatment produces sulfonation and creation of oxygen moieties on EVA.37 Oxygen and other low gas pressure plasmas are very effective in improving the peel strength of joints produced with EVA and two-component polyurethane adhesive.38 Finally, treatment with UV/ ozone is being currently developed to enhance the bond strength between EVA sole and polyurethane adhesive.39 Some EVA soles containing important amounts of LDPE (low density polyethylene) ± `Phylon'-type soles ± are especially difficult to bond. Good results have been obtained by applying extensive solvent wiping, followed by application of a UV-activated primer and by using two-component polyurethane adhesives.40 Alternatively, the application of a flame treatment to a `Phylon' sole followed by application of an isocyanate primer and by using two-component polyurethane adhesive, provides good bonding.40 Only polyurethane adhesives should be used to bond PVC soles. The adhesion problems of PVC derived from the presence of plasticisers and stabilisers (stearate type) whose migration to the surface impede contact with the

424

Adhesive bonding

adhesive (creation of weak boundary layers). A solvent wiping the PVC surface is usually effective in improving adhesion and solvents such as MEK are adequate. Stronger solvents such as THF are often mixed in to increase the cleaning action.15 After solvent wiping, the adhesive must be applied within the hour.41 To remove stabilisers, treatment with lactic acid solutions is beneficial. In some cases, application of isocyanate primer is adequate to increase bond strength. Polyurethane soles usually contain silicone mould release agents that prevent adhesion. To remove them, mild roughening is necessary followed by solvent wiping. Application of an isocyanate primer is very useful before a polyurethane adhesive is applied. To avoid roughening, two consecutive solvent wiping applications on polyurethane soles are adequate.15 Nylon and polyester soles should be solvent wiped or roughened before applying an isocyanate primer. Polyurethane adhesive provides adequate performance.15

18.4 Types of adhesive used in shoes Contact adhesives based on one- and two-component polychloroprene (neoprene) and polyurethane are the most commonly used in the shoe industry to bond upper to sole. These adhesives are bonded themselves by autoadhesion in which the adhesive is applied to both surfaces to be joined; diffusion of polymer chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. To achieve optimum diffusion of polymer chains, two requirements are necessary: (i) high wettability of smooth or rough substrate surfaces by the adhesive; (ii) adequate viscosity and rheological properties of the adhesive to penetrate into the voids and roughness of the substrate. Both requirements can be easily achieved in liquid adhesives. In the past, polychloroprene adhesives were more extensively used in upper to sole bonding, but nowdays polyurethane adhesives are preferred. Polychloroprene adhesives have better tack and improved wettability than polyurethane adhesives, but these are not compatible with many halogenated rubber soles and cannot be used to joint PVC soles. Therefore, polyurethane adhesives show better versatility on a higher number of substrates and also have lower oxidative degradation in time.

18.5 Solvent-borne polyurethane adhesives Elastomeric polyurethanes are the main components in solvent solution adhesives for the shoe industry. These polyurethanes are generally prepared in the form of pellets or chips by reacting an isocyanate (such as MDI ± 4,40 diphenylmethane-diisocyanate), a long chain diol (polyester or -caprolactone type), and a chain extender (glycol, diamine) to produce a linear polymer with negligible chain branching and relatively low molecular weight (Mw ˆ 200,000±

Shoe industry

425

350,000 daltons).42,43 Due to their polarity and ability to form hydrogen bonds, the polyurethane adhesives are able to join several different substrates. Elastomeric polyurethanes are industrially produced by a bulk polymerisation process at about 120 ëC during 1±2 hours. Higher temperatures favour biuret and allophanate formation which increase the mechanical properties of the polyurethane but decrease solubility in solvents. Once the correct molar mass is achieved (usually determined by the viscosity value) the polymer is rapidly quenched and further annealed. To maintain good solution properties in the common ketonic solvents, the isocyanate to hydroxyl molar ratio is usually kept near 1, thus producing a polymer with terminal hydroxyl groups. From a polymer physics point of view, the configuration of the elastomeric polyurethanes corresponds to a segmented structure (block copolymer of (AB)n type) consisting of soft and hard segments44,45 (Fig. 18.2). Typically the soft segments are composed of a rubbery polymer (mainly the polyester) the glass transition temperature (Tg) of which is located well below ambient temperature and contributes to the crystallinity of the final polyurethane. The hard segments are generally produced by the reaction of the isocyanate and a short chain glycol (chain extender), and have a rigid and crystalline structure. The non-polar low melting soft segments are incompatible with the polar high melting hard segments. As a result, phase separation (segregation) occurs in the polymer network and, thus, the polymer matrix will consist of flexible soft and rigid hard domains.

Figure 18.2 Scheme of the segmented structure of an elastomeric polyurethane.

426

Adhesive bonding

Typical elastomeric polyurethanes used as adhesives have a relatively low content of hard segments, and their properties are mainly determined by the soft segments. Therefore, these polyurethanes will be elastic in the range of temperature between the glass transition temperature (generally located between ÿ30 and ÿ40 ëC) and the softening temperature of the elastomeric domains (50± 80 ëC).46 The low melting point permits the elastomeric polyurethane to be softened at relatively low temperature for the adhesion of several substrates, with sufficient thermoplasticity (loss of cohesion at moderate temperature) and surface tack to ensure a correct bond. Figure 18.3 shows the typical rheological behaviour of a elastomeric polyurethane. The cross-over between the elastic and storage modulus is an indication of the thermoplasticity and the heat activation temperature necessary to produce adequate adhesive joints. For frequencies higher (or temperatures lower) than that of the cross-over, the storage or elastic modulus (G0 ) is dominant and the polyurethane shows high cohesion; however, for frequencies lower (or temperatures higher) than that of the cross-over, the loss or viscous modulus (G}) is dominant and the polyurethane flows allowing good wetting properties. Solvent-borne polyurethane adhesives are generally prepared by dissolving the solid elastomeric polyurethane pellets or chips in a solvent mixture. Because the polyurethanes have a linear molecular structure, the solid polymer does not need mastication prior to dissolving. Preparation of solutions is carried out in

Figure 18.3 Typical plate-plate rheology curve (modulus vs. reduced frequency) of an elastomeric polyurethane. G0 ˆ storage or elastic modulus; G} ˆ loss or viscous modulus.

Shoe industry

427

closed tanks with slow-running agitators; absence of metal oxide is important to avoid undesirable crosslinking. On the other hand, moisture should be avoided and tanks and agitators must be earthed to avoid electrical charging, because of the flammability of the solvents. Solubility of elastomeric polyurethanes in ketone solvents is not mainly governed by residual crosslinking but by the degree of crystallinity in the polyester soft segments. Crystallinity can be varied by selection of the reactants and by controlling the molecular weight of the polyol. A small amount of branching can significantly improve the tensile strength of the polymer without affecting solubility. Generally, elastomeric polyurethanes with low crystallisation rates have long open times but poor peel strength and heat resistance. Those with high rates of crystallisation have short open times, but high peel strength and improved heat resistance. For this reason, mixtures of low and high crystallisation rate polyurethanes are used to balance the open time and adhesion. Solvents determine the viscosity and solubility (Table 18.4) of the elastomeric polyurethane, its storage stability, its wetting properties and its evaporation rate when applied on a substrate. The most common solvents are the aromatic (toluene, xylene), cyclic ethers (tetrahydrofurane, dioxane, cyclohexanone), some esters (ethyl acetate, butyl acetate), and various ketones (acetone, methyl ethyl ketone). Commonly two or more solvents are employed, a low-boiling and a high-boiling solvent. The low-boiling solvent assures rapid flash-off of the majority of the solvent after the adhesive solution is applied on the substrate. The higher-boiling solvent helps to control the crystallisation kinetics of the soft segments, thereby helping to extent the open time of the adhesive. In fact, once crystallisation of the soft segments (phase separation) occurs, the open time expires. Another function of the high-boiling solvent is to keep viscosity of the adhesive low (this is why they are sometimes called diluents), allowing the adhesive solution to wet the substrate by enabling the polyurethane to penetrate the substrate surface, thus improving mechanical interlocking. On the other hand, the addition of toluene avoids gel formation of the adhesive solutions during storage. Table 18.4 Brookfield viscosity (25 ëC) of solutions containing 15 wt% elastomeric polyurethane in various solvents. Brookfield viscosities were measured using spindle no. 3 at 50 rpm Solvent Acetone MEK Ethyl acetate Tetrahydrofurane Cyclohexanone Dioxane

Brookfield viscosity (mPa.s) 380 435 1020 1360 4140 4225

428

Adhesive bonding

A formulation of solvent-borne polyurethane adhesives may include several additives such as tack and heat resistance modifiers, plasticisers, fillers, tackifiers, antihydrolysis agents and crosslinkers. Carboxylic acid as an adhesion promoter can also be added. A typical formulation of a solvent-borne polyurethane adhesive is shown below: polyurethane pellet fumed silica methyl ethyl ketone ethyl acetate

18 2 60 20

wt% wt% wt% wt%.

The spotting tack and/or improved heat resistance of elastomeric polyurethane adhesives may be extended by adding various low miscible resins (alkyl phenolic, epoxide, terpene phenolic, coumarone) or polymers (low crystallising polyurethane, acrylic, nitrile rubber, chlorinated rubber, acetyl cellulose).47 To improve adhesion together with heat resistance, reactive alkyl phenolic resins, chlorinated rubber or other chlorine-containing polymers can be added. One way to increase the strength of elastomeric polyurethanes is the addition of tackifiers,48,49 mainly rosins or hydrocarbon resins. Whiting, talc, barite, calcium carbonate, attapulgite or quartz flour have been suggested as fillers to lower the price of the solvent-borne polyurethane adhesive, improve joint filling and reduce the loss of adhesive during setting.50±53 Conductivity of elastomeric polyurethanes can be reached by incorporating carbon black.54 Pyrogenic (fumed) silicas can be used as fillers, reinforcing and/or rheological additives of polyurethane adhesives.51,55±61 When bonding highly porous materials (leather, textiles), fumed silicas are added to prevent undesirable penetration of the adhesive. Two patents62,63 showed the improved adhesion obtained in joints produced with solvent-borne polyurethane adhesives containing small amounts of different carboxylic acids.

18.6 Waterborne polyurethane adhesives Waterborne polyurethane adhesives are an environmentally friendly alternative to the solvent-based polyurethane adhesives. These are the more logical choice to replace the solvent-borne adhesives because they can be processed on existing machines, their performance in many applications is just as good as that of solvent systems and they can be used economically despite their higher raw material and processing costs. Use of waterborne polyurethane adhesives needs some minor changes in the current existing technology, essentially the additional heat required to remove water before joint formation. Furthermore, the waterborne polyurethane adhesives show additional limitations: (i) lack of tackiness at room temperature (heat activation is needed); (ii) poor wettability of several substrates, particularly greased leathers; (iii) polyurethane dispersions are thermodynamically unstable and therefore they show relatively poor stability

Shoe industry

429

Figure 18.4 Scheme of the structure of a waterborne polyurethane dispersion.

(dispersion collapses in the presence of metallic contaminants at low temperature (generally below 5 ëC) or by applying high stresses). Polyurethane dispersions are constituted by urethane polymer particles wholly dispersed in water. Dispersion of particles is possible because the presence of ionic hydrophilic groups chemically bonded to the polymer chains which are orientated to the surface of the particles (Fig. 18.4). These dispersions are based on crystalline, hydrophobic polyester polyols (such as hexamethylene polyadipate) and aliphatic isocyanates (such as H12MDI ± methylene bis(cyclohexyl isocyanate) ± or IPDI ± (isophorone diisocyanate). There are several routes to produce polyurethane dispersions, the two most common are the so-called acetone and prepolymer methods. In the prepolymer method, two basic steps are needed in order to produce a waterborne polyurethane dispersion, a prepolymer step and a chain extension step (Fig. 18.5). High shear rates should be used and some organic solvents may be necessary to assist this process, which are latterly removed to produce a solvent-free polyurethane dispersion. In general, aliphatic isocyanates are used to provide better adhesion characteristics and higher resistance to fading under light exposure. The prepolymer obtained by reacting the polyol and the isocyanate is modified through an internal emulsifier containing hydrophilic

430

Adhesive bonding

Figure 18.5 Reactions involved in the preparation of waterborne polyurethane dispersions.55

groups ± cationic, anionic or non-ionic ± thereby eliminating the use of external emulsifiers. Anionic internal emulsifiers (such as dimethylolpropionic acid or diethylamine sodium sulphonate) are the most commonly used. Great attention should be paid to the amount (minimum level needed) and type of surfactant used to stabilise the polyurethane dispersions. An excess of internal surfactant can cause moisture sensitivity and reduce the ability to bond of the waterborne polyurethane dispersions. The prepolymer is reacted with a tertiary amine (such as triethylamine) to increase the length of the chain and consequently increase the molecular weight of the polyurethane (Fig. 18.4). The amine reacts with the pendant carboxylic acid groups, forming a salt that under adequate stirring allows the dispersion of the prepolymer in water. The chain extension step takes place in the water phase. Hydrazine and ethylene diamine are commonly used chain extenders because of their faster reaction with isocyanates than with water. Therefore, polyurea linkages are formed and a high molecular weight polymer is obtained. The waterborne polyurethane dispersions generally have a pH between 6 and 9, and higher solids content (35±50%) and lower viscosities (about 100 mPa.s) than the solvent-borne polyurethanes. They are high molecular weight linear polyester urethanes constituted by small rounded spherical particles of about 0.1±0.2 m diameter (Fig. 18.4). Waterborne polyurethane dispersions for adhesives show good crystallisation rates and adhesion on various surfaces (PVC, ABS, polyurethane, leather, wood, fabrics).

Shoe industry

431

The adhesive characteristics of the waterborne polyurethanes are mainly defined by the melting point and the crystallisation kinetics of the polymer backbone. It is highly desirable to activate the adhesive at room temperature, but most of the waterborne polyurethane adhesives have a melting point above 55 ëC, and need reactivation. The crystallisation kinetics (measured by following the evolution in Shore hardness) defines the open time of the adhesive. On the other hand, unlike solvent-borne adhesives, the viscosity of waterborne polyurethane adhesives is not dependent on the molar mass of the polymer but on the solids content, mean particle size of the dispersion and the existence of additives in the formulation. Adhesion properties of the waterborne polyurethane adhesives are greatly determined by the polymer, so formulation is generally simple and generally includes only small amounts of a thickener and an emulsifier. When formulating the adhesives, the application conditions (heat activation, contact bond time, pressure) and substrate type must be taken into account. Thickeners (polyvinyl alcohol, polyurethanes, polyacrylates, cellulose derivatives) are added to increase the low viscosity of the dispersion and thus avoid excessive penetration in porous substrates. Emulsifiers (surface active agents) assist to stabilise the pH of the dispersion and to decrease its surface tension to obtain improved wettability. The emulsifiers are always orientated on the interface between polymer particles and the aqueous phase. Small amounts must be added to avoid loss of bonding characteristics. On the other hand, to achieve particular performance, dispersions of several polymers (vinyl acetate, acrylic ester) and resins (hydrocarbon resin, rosin esters) can be added. Special attention must be paid to the compatibility with the polyurethane to avoid dispersion collapse. In general, fillers are not added to waterborne polyurethane adhesives. To produce an adequate bond, the waterborne adhesive solution is applied to the two substrates to be joined and the water is evaporated at room temperature for about 30 minutes, or by heating at 50±60 ëC for a few minutes using hot air or infra-red lamps. When the water is evaporated, the polyurethane viscosity rises and a continuous film is formed on the substrate. The dry film is then heat activated and melting of the crystalline polyester backbone takes place imparting tackiness to the film. The heat activation process of waterborne polyurethanes is affected very strongly by the substrates to be bonded and by the process parameters such as temperature, pressure and contact bond time. Increasing the adhesive film temperature, raising the pressure and extending the time of contact all have a similar effect, i.e., an increase of the actual contact areas is produced and a better bond is obtained. After heat activation, the decrystallised polyurethane film is an amorphous viscoelastic melt with good flow properties. The decrease in viscosity after heat reactivation allows the adhesive film to wet the substrate and joining to a second substrate under pressure is produced. Once the adhesive joint is formed, the bulk viscosity and the modulus of the adhesive increase, initially by cooling and

432

Adhesive bonding

Figure 18.6 Scheme of the formation of crosslinked waterborne polyurethane films.

afterwards by the re-crystallisation of the polyurethane (Fig. 18.6). The kinetic and level of crystallisation can be influenced through the chemical structure of the polyurethane and the cooling rate. If an isocyanate is added as a crosslinker of waterborne polyurethane adhesive, the bond formation occurs much slower than with only the polyurethane but a further increase in viscosity is obtained. The crosslinking reaction does not initially bring about any increase in peel strength at room temperature but increases the heat resistance of the bond. The two-component waterborne polyurethane adhesives are similar in nature to the one-component ones, although they may have higher levels of carboxylic acid salt stabiliser that by reaction with the crosslinker increase the solvents and moisture resistance. The most common crosslinkers are polyisocyanates (adequately modified with surface active agents to become emulsifiable in water), although azyridines, polycarbodiimides and epoxies can also be used. Desmodur VK (Bayer) is one commonly used water-base isocyanate in the shoe industry; it contains 4,40 diphenylmethane diisocyanate and polymeric isocyanates and has about 75 wt% solids and 30% free NCO. The crosslinker is normally added to the dispersion and stirred in before application of the substrates. Once the crosslinker is added, drying is important because reaction with moisture reduces its effectiveness. Depending on the formulation, these adhesives can be applied over a period of 4±10 hours without affecting the adhesion properties. The isocyanate crosslinker generally takes several days before the crosslinking

Shoe industry

433

reaction is completed. Unlike solvent-borne adhesives, the pot-life of the adhesive dispersions is not associated with an increase in viscosity. Bond strength of waterborne polyurethane adhesives is particularly dependent on the substrate. Results obtained with surface chlorinated vulcanised rubbers and PVC are generally good, but adhesion to TR rubber may be poor or variable.64 Adhesion to leather is sometimes insufficient and roughening must always be carried out; adhesion to greasy or waterproof leathers and unroughened polyurethane coatings may also be difficult.

18.7 Polychloroprene (neoprene) adhesives Polychloroprene adhesives are the most common contact adhesives based on synthetic rubber. The diffusion process in polychloroprene rubber adhesives is mainly affected by the solvent mixture of the adhesive (which determines the degree of uncoiling of rubber chains) and by the ingredients in the formulation (mainly the amount and type of tackifier). Furthermore, polychloroprene rubber possesses similar characteristics to natural rubber and also has higher polarity. The chemical nature and molecular weight of polychloroprene greatly determine its adhesive properties. The polychloroprene adhesives show the following specific features in assembly operations: 1.

2.

3. 4.

Broad range of substrates for assembly. Polychloroprene adhesives bond to almost any high-polar surface, as well as many low-polar surfaces in a temporary or permanent way. Although in general curing (i.e., crosslinking or vulcanisation) is not necessary to provide high strength and, mainly, heat and chemical resistance to the joints produced with these adhesives, vulcanisation is mandatory. The most common room temperature curing agents are zinc oxide and isocyanates. High peel strength. The intrinsic properties of rubbers (ability to produce high elongation under stress) impart adequate strength to the joints under peeling forces. Polychloroprenes compounded with phenolic resins develop much higher peel strength, particularly after a few hours. However, polychloroprenes show poor resistance to shear stresses. Versatility of formulation. Several grades of polychloroprenes and different chemical modifications (e.g., grafting) can be achieved to impart specific properties to the joints. High green strength. This is one of the most important properties of polychloroprene adhesives in the shoe industry. The green (immediate) strength can be defined as the ability to hold two surfaces together when first contacted and before the adhesive develops its ultimate bonding properties when fully cured. The green strength can be modified by changing the solvent composition (for solvent-borne adhesives) and/or by incorporating ingredients in the formulations (mainly tackifiers).

434 5.

Adhesive bonding High resistance. Polychloroprene adhesives have great resistance to moisture, chemicals and oils, excellent ageing properties and excellent temperature resistance.

Polychloroprene elastomers are produced by free radical emulsion polymerisation of 2-chloro-1,3-butadiene monomer. The monomer is prepared either by addition of hydrogen chloride to monovinyl acetylene or by the vapour phase chlorination of butadiene at 290±300 ëC. This latter process was developed in 1960 and produces a mixture of 3,4-dichlorobut-1-ene and 1,4dichlorobut-2-ene, which have to be dehydrochlorinated with alkali to produce chloroprene. The emulsion polymerisation of chloroprene involves the dispersing of monomer droplets in an aqueous phase by means of suitable surface active agents, generally at a pH of 10±12. Polymerisation is initiated by addition of free radical catalyst at 20±50 ëC. During emulsion polymerisation a high conversion of monomer to polymer produces crosslinked rubber that is insoluble. To obtain a high conversion in the polymerisation reaction and a processable polymer, suitable polymer modification should be made. The addition of sulfur, thiuram disulfide or mercaptans allow this goal to be achieved.65 The properties of polychloroprene can be altered by modification of the conditions and experimental variables during polymerisation. During polymerisation, the monomer can add in a number of ways, the trans-1,4 addition is the most common. Crystallinity in the polychloroprene is produced by trans-1,4 addition and reaction conditions are usually selected to maximise this. In crystallising polychloroprene grades there is more than 90% trans-1,4 addition whereas non-crystallising grades generally contain 80±85% trans-1,4 addition. As a result of crystallisation, the cohesive strength of polychloroprene is much greater than that of the amorphous polymer. Crystallisation is reversible under temperature or dynamic stresses. Thus, for a temperature higher than 50 ëC uncured polychloroprene adhesives lose their crystallinity and upon cooling the film re-crystallises and cohesive strength is regained. The increase in crystallinity improves the modulus, hardness and cohesive strength of polychloroprene adhesives but decreases their flexibility, their resistance to oil swelling, and their resistance to permanent set. Polychloroprene polymers also vary in the degree of branching in the polymer. Polychloroprenes with little or no branching are called sol polymers, whereas those with considerable branching are referred to as gel polymers. Sol polymers are soluble in aromatic solvents. Most of the solvent grade polychloroprene polymers are sol polymers. The gel content in the polychloroprene affects the cohesive strength, resilience, elongation, open tack time, resistance to permanent set, and oil swell. A typical composition of a solvent-borne polychloroprene adhesive is as follows: polychloroprene, 100 phr; tackifying resin, 30 phr; magnesium oxide,

Shoe industry

435

4 phr; zinc oxide, 5 phr; water, 1 phr; antioxidant, 2 phr; solvent mixture, 500 phr. Although there are several manufacturers of polychloroprene elastomers, DuPont probably has the broadest range of polychloroprene grades in the market. The sulfur-modified Neoprene AC and AD polychloroprenes are the most commonly used in shoe adhesives, mainly Neoprene AD because it has superior viscosity stability. For difficult-to-bond substrates, methacrylic graft polymers (Neoprene AD-G or AF) show better performance. When specific properties (e.g., increased tack, improved wetting, increased peel strength) need to be met, blends of Neoprene AC or AD with Neoprene AG provide adequate performance. Metal oxides provide several functions in solvent-borne polychloroprene adhesives: · Acid acceptor. This is the main function of metal oxides in polychloroprene adhesive formulations. On ageing, small amounts of hydrochloric acid are released which may cause discolouration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergetically in the stabilisation of solvent-borne polychloroprene adhesives against dehydrochlorination. · Scorch retarder. Magnesium oxide retards scorch during the milling processing of polychloroprene adhesives. · Curing agent. Zinc oxide produces a room temperature cure of solventborne polychloroprene adhesives, giving increased strength and improved ageing resistance. · Reactant for t-butyl phenolic resins. Magnesium oxide reacts in solution with t-butyl phenolic resin to produce an infusible resinate which provides improved heat resistance. Resinate has no melting point and decomposes above 200 ëC. Although oxides of calcium, lead and lithium can also be used, they are not as efficient as magnesium oxide and also tend to separate from solution. The addition of resins to solvent-borne polychloroprene adhesives serves to improve specific adhesion, increase tack retention and increase hot cohesive strength. Para-tertiary butyl phenolic resins are the more common for solventborne polychloroprene adhesives. Amounts between 35 and 50 phr (parts per hundred parts of rubber) are generally added. In general, addition of 40±45 phr provides an adequate balance of tack and heat resistance. In general, tack decreases by increasing the phenolic resin content in the polychloroprene adhesive, and bond strength reaches a maximum at about 40 phr, decreasing for high amounts of phenolic resin.65 A good antioxidant should be added to polychloroprene adhesives to avoid oxidative degradation and acid attack of substrates. Derivatives of diphenyl amine (octylated diphenyl amine, styrenated diphenyl amine) provide good

436

Adhesive bonding

performance but staining is produced. To avoid staining, hindered phenols or bisphenols can be added; 2 phr antioxidant is sufficient in solvent-borne polychloroprene adhesive formulations. Solvents affect adhesive viscosity, bond strength development, open time, cost, and ultimate strength. Blends of three solvents (aromatic, aliphatic, oxygenates ± e.g., ketones, esters) are generally added, and in their selection environmental and safety regulations must be considered. A graphical method has been proposed66 to predict the most adequate solvent blends for solvent-borne polychloroprene adhesives. Fillers are not commonly added to solvent-based polychloroprene adhesives. Calcium carbonate or clay can be added to reduce cost. Maximum bond strength is obtained using fillers with low particle size (lower than 5 m) and intermediate oil absorption (30/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently a flame retardant, aluminum trihydrate, zinc borate, antimony trioxide or silane (mercapto or chlorosilanes) treated silicas can be added to further improve this property. Curing agents are generally added to solvent-borne polychloroprene adhesive formulations to increase heat resistance. Thiocarbanilide and polyisocyanates can be used as curing agents. The reaction of an isocyanate with polychloroprene that leads to improved heat resistance property has not been fully explained. There are no active hydrogen atoms in the polychloroprene to allow reaction with an isocyanate group, so it has been proposed65 that the small amount of 1,2-addition in the polymer chain provides allylic chlorine, which can be converted to hydroxyl during the manufacturing of polychloroprene. Although it is not common, additional ingredients can be added to solventborne polychloroprene adhesive formulations to improve specific properties. Plasticisers can decrease the glass transition temperature, influence crystallisation tendency and reduce cost. Highly aromatic mineral oils can be used when a reduction in the crystallisation rate is required. Stearic acid improves processability and reduces mill sticking. Amounts of 0.5 to 1 phr can be added.

18.8 Waterborne polychloroprene adhesives In recent years, the use of solvent-borne polychloroprene adhesives has been seriously restricted and waterborne adhesives have been developed. Polychloroprene latices differ from their solid elastomer counterparts (used in solvent-borne polychloroprene adhesives) in that they are gel polymers (i.e., insoluble in organic solvents). Latex systems derive their strength characteristics from the gel structure rather than crystallinity as in solvent solution systems. Higher gel content leads to the same properties as polymers with higher crystallinity.67 Polymers with higher gel content exhibit higher cohesive strength, modulus and heat resistance but tack, open time and elongation are

Shoe industry

437

reduced. An important difference between gel content and crystallinity is that the effects caused by gel content will not disappear as the polymer is heated. A typical composition of a waterborne polychloroprene adhesive is as follows: latex polymer, 100 phr; surfactant, as required; antifoam, as required; tackifying resin, 50 phr; thickener, as required; zinc oxide, 5 phr; antioxidant, 2 phr. The formulations of polychloroprene latex adhesives contain essentially the same components as the solvent-borne adhesives, except that water-based ingredients have to be used and the compounding is particularly demanding. Several ingredients can be found in most polychloroprene latex adhesive formulations. About a dozen polychloroprene latices have been developed for adhesive applications. The polymer determines the initial tack and open time, the bondstrength development and hot-bond strength, the application properties and adhesive viscosity. There are a few nonionic latices (for example Neoprene latex 115), and most of the polychloroprene latices are anionic. Anionic latices are stabilised with rosin soaps and with polyvinyl alcohol to provide better freezethaw stability than the anionic types.68 Various latices can be mixed to adjust the crystallisation rate, heat stability, contactability and hardness of the adhesive film. Because most latices have low viscosities after compounding, most of the waterborne polychloroprene rubber adhesives are sprayable. Thickeners such as fumed silicas can be added to increase viscosity and thixotropy. To give non-compounded polychloroprene latex good mechanical and storage stability, emulsifiers are added to attain stabilisation or to convert water insoluble chemicals (e.g., antioxidants, plasticisers) into emulsions. They function by strengthening the interfacial film by maintaining or increasing the degree of solvation, or by increasing the charge density on the latex particle. More precisely, surfactants are added to improve storage stability, substrate wetting and attain improved freeze resistance. Incorporation of surfactants has an adverse effect on cohesive properties and should be kept to a minimum. Water resistance and tack may also be affected.69 Excessive stabilisation of the adhesive mixture may negatively affect coagulation (which is desirable in the wet bonding process). Anionic emulsifiers (alkali salts of long-chain fatty acids, and alkyl/aryl sulfonic acids) or non-ionic emulsifiers (condensation products of long-chain alcohols, phenols or fatty acids with ethylene oxide) can be used. Zinc oxide is the most effective metal oxide and should have a low lead content. Zinc oxide has three main functions: (i) to promote cure; (ii) to improve ageing, heat and weather resistance; (iii) as an acid acceptor. In general, 2±5 phr zinc oxide is added in latex formulations. Resins influence the adhesion, open time, tack, contactability and heat resistance of polychloroprene latex adhesives. In general, 30±60 phr are added and attention should be paid to the pH and compatibility with the emulsifier systems. The glass transition temperature, the softening point, polarity and compatibility of the resin with the polymer determines the adhesive properties.

438

Adhesive bonding

Thus, hot-bond performance is generally proportional to the softening point of the resin. t-butyl phenolic resins cannot be used in polychloroprene latex adhesives because of the colloidal incompatibility. Terpene, terpene phenolic, coumarone-indene, and rosin acids and esters resins can be added to polychloroprene latex adhesive formulations. Terpene phenolic resins can also be added to polychloroprene latex without great reduction in hot strength as the resin content is increased, however, contactability is reduced and an adhesion failure occurs even at the 50 phr level. Furthermore, terpene phenolic resins have relatively poor tack and impart the best resistance to elevated temperatures but adhesives based on this class of resin require either heat activation or pressure to achieve adequate bond strength. Rosin ester resin emulsions are effective in latex adhesives. These resin emulsions extend the tack life of the polychloroprene latices, but they do not have the reinforcing characteristics of the terpene phenolic or alkyl phenolic resins. Hence, the cohesive strength and heat resistance are sacrificed to obtain surface tack. Similar antioxidants to those used for solvent-borne polychloroprene adhesives can be used. Addition of antioxidants is important when resins sensitive to oxidation are included in polychloroprene latex formulations; 2 phr is the common amount of antioxidant in latex adhesives. Addition of thickeners increases the viscosity of polychloroprene latex adhesives. Amounts up to 1 wt% of polyacrylates, methyl cellulose, alginates and polyurethane thickeners can be used. Particular attention should be paid to fluctuations in pH when thickener is added to the formulations. For low pH formulations, fumed silica or some silicates can be used. Curing agents have little effect on the performance of latices with the highest gel content but are sometimes used with low-gel polymers to improve hot-bond strength while maintaining good contactability. Suitable curing agents are thiocarbanilide either alone or in combination with diphenylguanidine, zinc dibutyldithiocarbamate, and hexamethylenetetramine. Two-part systems are also being developed using more active materials such as aqueous suspension isocyanates (e.g., Desmodur VP from Bayer) and hexamethoxy melamine. These agents produce a crosslinking reaction at room temperature and give fast bond development, but exhibit a finite pot-life. The most common use of curing agents is with carboxylic latices. Isocyanates and melamines can be used but zinc oxide is the most common curing agent. Zinc oxide crosslinks carboxylated latices and improves bond strength by ionomer formation.69 Carboxylated polychloroprene reacts slowly with zinc oxide in dispersed form, causing a gradual increase in adhesive gel content. This can lead to restricted adhesive shelf life. Resin acid sites compete with the polymer acid sites for ZnII. The more resin acid sites, the more stable the adhesive. Bacterial and fungal attack can be a problem in polychloroprene latex formulations with pH below 10. It is manifested by odour, discolouration and gas evolution; 500±1500 parts per million of a biocide should be added.

Shoe industry

439

18.9 Testing, quality control and durability In order to produce an optimum adhesive bonding of upper to sole, the adhesion of different sole materials to urethane or chloroprene adhesives must be optimised. To achieve adequate adhesion, the following aspects must be considered.

18.9.1 Selection of the upper and sole materials In general, formulators of upper and sole materials do not consider that they have to be bonded. They pay more attention to matching the hardness and mechanical properties, and aesthetics of those materials required by the shoe industry. Furthermore, depending on fashion, different materials that are difficult to bond are used to produce shoes, making it hard to standardise on their bonding. In general, the formulation of the shoe materials must avoid the presence of additives that decrease the adhesion to polyurethane or polychloroprene adhesives (plasticisers, excessive amounts of processing oils, inadequate selection of antiozonants and antioxidants), and especially the use of greasy leathers must be avoided. On the other hand, formulations must be repetitive and first-class raw materials must be used. As a general rule, the use of adequately formulated uppers and soles avoids 90% of the bonding problems in the shoe industry.

18.9.2 Selection of the adhesive The adhesive must be selected considering the performance required for each joint. It must properly wet the upper and sole surfaces, and in this aspect solventborne adhesives are excellent. One of the problems in the use of water-based adhesives is their poor wettability, although an adequate formulation may solve this limitation quite satisfactorily. In the selection of an adhesive, the following aspects should be particularly considered: nature and formulation of the materials to be bonded, stresses produced while the shoe is worn, environment (solvents, acid or alkali media), adhesive application restrictions (viscosity, rheological properties), specific requirements of the adhesive (pot-life, schedule in the shoe factory) and safety regulations.

18.9.3 Design of the joint Fashion determines the geometry of shoes. Sometimes, shoes become difficult to bond because of high heels or the heterogeneous shapes of soles. In general, shoe designers do not pay attention to bonding.

440

Adhesive bonding

18.9.4 Adequate bonding operation Several cases of poor adhesion in shoe bonding arise from a deficient operation. As a summary, the following aspects must be properly considered to ensure an adequate bond. Surface treatments of upper and sole The method used to achieve surface preparation and the instruments used are critical. Furthermore, the correct method of surface preparation for upper and sole must be selected. Adhesive application A thin film of adhesive (about 50 m thick) is applied to the two substrates to be bonded and the solvent removed by natural or forced evaporation. A heavier coat of adhesive is more likely to result in a cohesive failure in the substrate, i.e., the failure is not due to the weakness of the adhesive or its stick to the materials. The application procedure (brush, doctor knife, spraygun, roller, coater) and the amount of adhesive must be carefully controlled. The choice of adhesive application devices depends mainly on the type and size of the materials to be bonded as well as on the rheological properties of the adhesive. Furthermore, the viscosity of the adhesive must be controlled and the operation times (evaporation rate of solvents or water, open time, shelf life) must be strictly followed. Adhesive film drying Controlled drying of shoe sole cements is preferable to natural drying. Removal of excess water or solvent from an adhesive film is not only governed by the process of evaporation but also by the speed of absorption into the substrate. Porous substrates (such as leather) absorb water or solvents without detrimental effect on bond strength, whereas non-porous substrates need complete solvent removal to produce adequate adhesion. Furthermore, force drying may produce skin formation on the surface of the coating (especially if a heavier adhesive coating is applied) leading to poor coalescence of the adhesive.70 A slight trace of solvent in the cement film on the upper when attaching the sole is beneficial in giving complete coalescence. When necessary, although very fast drying can be achieved by radiant heat, IR radiation and hot air are more convenient methods and both provide more uniform heating. Bond formation The components with the dry adhesive film are placed for 10±30 seconds in a `flash heater' ± hot air or IR radiation can also be used ± where a radiant heat

Shoe industry

441

source raises the temperature of the adhesive film above the crystalline melting point of the polyurethane (heat activation or reactivation process). Recommended reactivation temperature for polyurethane adhesives ranges between 45 and 85 ëC, depending on substrates and adhesive characteristics.70 An insufficient reactivation temperature causes non-coalescence of the adhesive and an excessive reactivation temperature decreases creep because the substrate is too hot and the adhesive is still soft when sole and upper are attached.71 In some particular cases, reactivation temperatures of adhesives higher than 100 ëC are recommended to produce good performance.72 While still in their amorphous state the adhesive films are brought together under pressure (typically 35±200 kPa) for 10±30 seconds (stuck-on process). The time interval after the heat activation process during which the films will adhere is known as the `spotting tack'. Both the applied pressure and the pressing time of the upper to the sole must be adequately selected for each kind of material. Crystallisation or curing of the adhesive After bond formation, the adhesive joint needs time to gain sufficient cohesion. Depending on the above issues, this time can be short or longer, although in general a minimum of 24 hours is required for crystallisation, 72 hours being the optimum.

18.9.5 Types of test The T-peel and creep tests are the most commonly used to establish adhesion performance in shoe bonding. The peel test serves to determine the bonding properties of upper to sole in the shoe industry. Peeling rate, material thickness and size of test samples must be optimised. Standard T-peel tests require two rectangular test pieces of 150 mm long, 30 mm width and 3 mm thick be stuck together to cover each other to a length of at least 50 mm (Fig. 18.7). Standard peel rate is 100 mm/min. The joints are stored for 72 hours in standard atmosphere (23 ëC and 50% RH) before carrying out the separation tests. Both the peel resistance and the way in which the separation occurs help to assess the bond. A minimum of five replicates for each joint must be tested and averaged. Bond strength should be expressed as kN/m, although very often N/mm is generally used in the shoe industry. The loci of failure of the joints are generally expressed using capital letters: A: adhesion failure (detachment of the adhesive film from one of the materials). C: cohesive failure in the adhesive (separation within the adhesive film without detachment from the material). N: non-coalescence failure (failure of the two adhesive films to combine without detachment from the material).

442

Adhesive bonding

Figure 18.7 Typical test sample for T-peel strength determination in the shoe industry.

S: surface cohesive failure of the material (breakdown of a substrate of low structural strength at its surface). M: cohesive failure of one of the substrates. Under the influence of force and when heated, the adhesive layers of footwear material bonds suffer plastic flow. The creep test at constant temperature serves to assess the behaviour of shoe material bonds when heated under the influence of a constant peeling force over a fixed time. Weight between 0.5 and 2.5 kg can be fixed on the lower holders outside a heating oven where the test pieces could be heated to 50±70 ëC. The unbonded ends of five test pieces are bent apart carefully and inserted in the holders. Temperature is generally raised to 60 ëC after a warm-up period of one hour. The test pieces are loaded with the chosen weight constantly for ten minutes. The heating oven is then opened and the separations of the bonds are marked. Moisture and temperature are the main agents able to degrade shoe joints. Therefore, adequate tests have been developed to ensure adequate performance. In general, the addition of isocyanate increases durability and retards ageing in most of the upper to sole adhesive joints. Several tests have been proposed to establish the ageing resistance of upper to sole bonds. The most common ageing tests involve immersion in water, exposure of the joints to 50 ëC and 95% relative humidity for one week, freeze-thaw cycles and/or UV light. Generally, T-peel tests are carried out after ageing to determine durability.

18.10 Future trends Although most of the materials used in the shoe industry can currently be bonded using different surface preparation and adhesives, faster bonding and more environmentally friendly technologies are envisaged for the future. The removal of solvents in all bonding operations is the final main current objective

Shoe industry

443

being pursued in the shoe industry, in such a way that shoes can be manufactured without solvents in any operation. The use of solvent adhesives in industrial processing is declining significantly not only for ecological reasons but also because of concern regarding industrial hygiene and safety at work. On the other hand, limitations in the volatile organic compounds (VOCs) emissions will certainly push the shoe industry to use alternative bonding technologies to the current solvent-based adhesives and surface preparations. Several limitations exist to the removal or substitution of solvents in shoe bonding, such as the use of new equipment and machinery, the modification in the procedure to produce bonding, cost increase and some materials will be difficult to bond without the use of solvents. Two strategies are envisaged in the shoe industry to develop solvent-free technologies, solvent-free adhesives and solvent-free surface preparation. Each will be considered in the following sections. It should be kept in mind that the substitution of the current solventbased technology by a solvent-free one will necessarily imply a modification in the global strategy of bonding upper to sole. For example, the use of waterborne adhesives will produce excellent final bonds, but initial strength is too low and therefore in industrial production the bond quality should not be estimated immediately after bond formation but after 24 hours.

18.10.1 Future trends in adhesives Solvents are, in general, volatile, flammable and toxic, to some degree. Further, solvent may react with other airborne contaminants contributing to smog formation and workplace exposure. Although solvent recovery systems and afterburners can be effectively attached to ventilation equipment, many shoe factories are switching to the use of waterborne adhesives, hot melts or 100% solids reactive systems, often at the expense of product performance or labour efficiency. Although hot-melt urethanes could replace solvent-borne adhesives, this should take longer to occur because of the vastly different equipment requirements. Currently, waterborne adhesives are being slowly introduced into the shoe industry. Their performance is quite similar to that of the solvent-borne adhesives, so it can be estimated that for about ten years they will be used in the shoe industry. However, the future seems to be directed towards the use of moisture-curing holt-melt urethane and thermoplastic urethane adhesives. Some recent literature has been published dealing with moisture-reactive hotmelt polyurethane adhesives.54,64,73 Most moisture-curing hot-melt adhesives are obtained by reacting a crystallisable polyol (generally poly(hexamethylene adipate) and monomeric MDI (NCO/OH ratios ˆ 1.5±2.2). A catalyst is also necessary. The polyester polyol plays a key role because over time, viscosity and glass transition temperature can be adequately tailored. A mixture of hydroxyl terminated polyesters having different characteristics allows control of the adhesion and hardening time of the adhesive.

444

Adhesive bonding

A typical formulation for a moisture-curing hot-melt adhesive is given below:54 Poly(hexamethylene adipate) 4,4'-methylene di(phenylisocyanate) (NCO/OH=1.75) Dimorpholinediethyl ether catalyst

100 parts 11.7 parts 0.2 parts

The moisture-curing hot-melt adhesives must be stored in nitrogen-sealed containers and moisture must be fully excluded. These adhesive are crystalline solids at room temperature. For application, heating is necessary (about 125 ëC) to change them to low-viscosity liquids, and they are applied only on one of the substrates to be joined; the second substrate is immediately joined with minimal pressure. Simultaneously with application of the adhesive, water can be sprayed onto the adhesive/materials in the amount necessary for the curing process. Although after cooling, crystallinity is reached, about 24 hours are necessary to develop the strength of a structural adhesive. Initial strength can be improved by adding polyester, thermoplastics or polymerised acrylates.54 The addition of ketone-formaldehyde and terpene phenolic resins also increases adhesive performance. Limitations of these adhesives derived from the special equipment necessary for application and from the substrate nature (not all substrates can be bonded). However, some pre-reacted hot-melt adhesive types are liquids and may be applied by hand, by spraying or even better by nozzles. When the polyurethane hot-melt adhesive comes into contact with moisture, the irreversible curing of the adhesive is initiated. First, water adds to the isocyanate group producing unstable carbamic acid, which in turn is transformed into a primary amine group. These amine groups generate linear urea bridges by reaction with additional isocyanate; a three-dimensional network is produced by crosslinking with more isocyanate groups,73 providing the structural bonding in moisture-cured hot-melt polyurethane adhesives. Thermoplastic polyurethane adhesives are another future alternative for bonding in the shoe industry. These adhesives have good holding strength after crystallisation but their cost is more than the majority of the adhesives used in the shoe industry. Most thermoplastic polyurethane adhesives are based on fast crystallisation polyesters, they are prepared using a NCO/OH ratio near 2 and they have a linear structure.54 They consist of two components (polyol and isocyanate) that have to be mixed at about 60 ëC; after reaction, the resulting polymer is pelletised and packed in nitrogen and moisture-free atmosphere containers. Application temperatures are higher than 170 ëC and to produce bonding the same procedure as for moisture-cured hot-melt urethane is used (except that pressure has to be applied to the upper to sole joint). A typical formulation for a thermoplastic polyurethane adhesive is given below:54

Shoe industry Component 1 Poly(tetraethylene adipate) Mw ˆ 2000 1,4 butanediol Dibutyltin dilaurate Component 2 4,40 -methylene di(phenylisocyanate)

445

100% 4% 5  10ÿ3% 23.6%

18.10.2 Future trends in surface preparation Several environmentally friendly surface preparation procedures for materials in shoe bonding have been developed. Future trends seem to be directed to the use of solvent-free, water-based and radiation-based surface modifications of materials. To be effective these treatments should produce improved wettability, creation of polar moieties and surface roughness in the materials, and the weak layers due to contaminants, mould release agents and anti-adherend moieties have to be effectively removed. Solvent-free treatments Some solvent-free technologies have been proposed for the treatment of leather, PVC and polyurethane soles. Cryoblasting74 consists of the bombardment of the materials with particles of solid carbon dioxide at a pressure of about 0.4 MN/ m2. The impact of the solid particles successfully removes mould release agents (e.g., silicones) from polyurethane soles, improving their adhesion to polyurethane adhesives. Treatment with supercritical fluids is currently under study to remove grease from leathers, plasticisers and stabilisers from PVC, oils and lubricants from TR, and zinc stearate and waxes from vulcanised SBR soles.75 The effectiveness of this treatment is ascribed to the dissolution of anti-adherend moieties by penetration of the fluid into the surfaces of those materials. Water-based surface treatments Water-based surface treatments for rubbers and other sole materials have been successfully proposed,37,74,76±78 as their use does not require noticeable changes with respect to current technology in the shoe industry. A few of them have been introduced in shoe production, but most of them have been used only at laboratory scale. Several water-based chlorination treatments have been proposed.76±78 Bleach has been successfully used to chlorinate rubber soles in the past. Recently acidified sodium hypochlorite solutions (55.6 g/l active chlorine content) containing 1-octyl2-pyrrolidone as wetting agent have been successfully used for the treatment of TR

446

Adhesive bonding

Figure 18.8 Chemical formula of sodium dichloroisocyanurate (DCI).

rubber soles.77 This agent increases the wettability of the TR rubber although treatment in an ultrasonic bath for 30 seconds is mandatory to obtain adequate adhesion to waterborne polyurethane adhesive. The improved adhesion is due to improved wettability and creation of chlorine moieties on the rubber surface. The treatment is restricted to about 1 m depth surface of TR rubber. Aqueous solutions of sodium dichloroisocyanurate (DCI) have recently been used to increase the adhesion of SBR and TR rubber soles.76 The chemical structure of DCI is somewhat similar to that of TCI (Fig. 18.8). Surface treatment with aqueous DCI solutions modified the surface chemistry of TR and SBR rubbers, creating C-Cl moieties and removing the zinc stearate from the SBR rubber surface. The use of a low DCI concentration in water is less effective in modifying the TR rubber, but is sufficient to obtain good peel strength values for SBR rubber/waterborne polyurethane joints (Fig. 18.9). On

Figure 18.9 T-peel strength values of DCI-treated SBR and TR rubber/ waterborne polyurethane adhesive/canvas joints. Influence of the DCI concentration.

Shoe industry

447

the other hand, heterogeneities and cracks are created on the rubber surface (mainly on the SBR surface) that may contribute to enhancing mechanical interlocking with the adhesive. Successful treatment of SBR and TR rubbers has been achieved using aqueous N-chloro-p-toluensulphonamide solutions (obtained by acidifying chloramine T solutions).78±81 The rubber is immersed in the solution at 20± 80 ëC for about one minute. The effectiveness of the treatment is ascribed to the introduction of chlorine and oxygen moieties on the rubber surface. SO2-C6H4NCl-Cl seems to be the actual chlorinated species created by acidifying chloramine T.78 Treatment of rubbers with oxidant inorganic salts (acidified potassium dichromate, potassium permanganate, Fenton's reactive) has recently been shown to be partially successful78 and needs exhaustive washing. Further, chromium or manganese ions are deposited on the rubber surface. Electrochemical treatments have been successfully proposed to improve the adhesion of SBR and TR rubbers.74,78,82 The treatment consists of the immersion of the rubber in an electrochemical cell of silver nitrate in 3.25 M nitric acid. The platinum cathode is placed in a porous pot containing a solution of diluted nitric acid, and the platinum anode is placed directly in the electrochemical cell. A constant current of 2 A is generated by the power supply unit. After treatment, the rubbers have to be washed with nitric acid and then twice with bidistilled deionised water. Treatment with 10% aqueous solutions of sodium hydroxide has been successfully applied to increase the adhesion performance of plasticised PVC soles.74 Treatment is carried out at 60±80 ëC for 10±15 minutes, and after immersion the treated substrates are thoroughly rinsed in cold running tap water and allowed to dry. The effectiveness of the treatment is due to the removal of plasticisers and stabilisers from the PVC sole surface. Radiation-based treatments Several environmentally friendly surface preparations involving the treatment of sole materials with radiation have recently been studied. These treatments are clean (no chemicals or reactions by-products are produced) and fast, and furthermore on-line bonding at the shoe factory can be achieved, so the future trend in surface modification of substrates in the shoe industry will most likely be directed to the industrial application of those treatments. Corona discharge, low-pressure RF gas plasma and UV treatments have been successfully used at laboratory scale to improve the adhesion of several sole materials in the shoe industry. Recently, surface modification of SBR and TR rubbers by UV radiation has been industrially demonstrated in the shoe industry.83 Corona discharge has been successfully used to improve the adhesion of TR rubbers to polyurethane adhesives.84±85 Treatment with corona discharge

448

Adhesive bonding

improves the wettability of TR rubber due to the surface formation of polar moieties, mainly C±O, CˆO and COOÿ groups. Besides, surface cleaning and removal of contaminants (mainly silicon moieties) can be achieved but roughness is not created. The modifications are enhanced when high corona energy is applied. Peel strength values of corona discharge treated TR rubber/ polyurethane adhesive/leather joints only moderately increase due to the absence of surface roughness. On the other hand, the adhesion of EVA copolymers to polychloroprene adhesives36 is also enhanced by treatment with corona discharge. Low-pressure RF gas plasma is effective in enhancing the adhesion of SBR and TR rubbers.86±90 Different gases (oxygen, nitrogen and oxygen-nitrogen mixtures) can be used to generate the RF plasma. RF plasma treatment can be produced at 50 watts and 1 torr residual pressure, and a decrease in contact angle values on SBR and TR rubbers, irrespective of the gas used to generate the RF plasma, was obtained. Treatment with RF plasma produces partial removal of hydrocarbon moieties from the rubber surface and the generation of oxygen moieties (C±O and CˆO moieties). An increase in surface roughness is also produced. The degree of oxidation and the amount of hydrocarbon-rich layer removed from the rubber surfaces are more important by treating with oxygen plasma. The higher the percentage of oxygen in the plasma, the greater the degree of oxidation on the rubber surface, the higher the degree of roughness and the more effective the treatment. The treatment in oxygen plasma for one minute was enough to noticeably increase adhesion of rubbers to polyurethane adhesives. However, an extended treatment (15 minutes) is needed when nitrogen plasma is used. The adhesion of EVA copolymers containing different vinyl acetate contents is also increased by treatment with low pressure RF plasma of non-oxidising (Ar, N2) and oxidising gases (air, a mixture of 4N2:6O2(v/v), O2 and CO2).38,91 The enhancement of the surface chemistry of EVA is more noticeable after treatment with non-oxidising plasmas than with oxidising ones. The surface etching produced with the non-oxidising plasmas, giving rise to a high roughness, depends on the different resistance of vinyl acetate and polyethylene to the non-oxidising plasma particles bombardment. The adhesion properties obtained by using a polyurethane adhesive are improved in the joints produced with EVA treated with oxidising plasmas, and shows adequate resistance to ageing under high relative humidity and temperature. Treatment of TR rubber with UV radiation has been shown to be successful in increasing its adhesion to polyurethane adhesives.84,92 A low pressure mercury vapour lamp (main emission at 254 nm; power ˆ 20 mW/cm2) has been used. The UV treatment of TR rubber improves the wettability, produces the formation of C±O, CˆO and COOÿ moieties, and ablation (removal of a thin rubber layer from the surface) is also produced. The extent of these modifications increased with increasing treatment time. The extended UV

Shoe industry

449

treatment produced greater surface modifications, as well as the incorporation of nitrogen moieties at the surface. Peel strength values increase after UV treatment of TR rubber to a greater extent by increasing the treatment time.

18.11 Acknowledgements Helpful discussion and advice by the Spanish Footwear Research Institute (INESCOP) is appreciated. The valuable help of MarõÂa Victoria Rico-Rico and Juan Jose HernaÂndez-GonzaÂlez in solving some difficulties in the writing of this chapter is acknowledged. My gratitude to all my co-workers in the Adhesion and Adhesives Laboratory of the University of Alicante who produced the experimental results used in this chapter. The financial support for research in surface modification and adhesive performance in the shoe industry from the Spanish Research Agency (CICYT, MCYT) and the University of Alicante is greatly appreciated. Finally, my deep recognition and acknowledgements to my children from whom I took the time to write this chapter.

18.12 References 1. `Preparation of leather uppers' (May, 1963), SATRA Bulletin, 10(17), 229±230. 2. `Sticking to unroughened uppers. An indecisive experiment' (November, 1965), SATRA Bulletin, 11(23), 369±372. 3. Hall E F (February, 1966), `Sole attachment to man-made uppers by cementing and moulding-on', SATRA Bulletin, 12(2), 32±36. 4. FerraÂndiz-GoÂmez T P, Almela M, Maldonado F, MartõÂn-MartõÂnez J M and OrgileÂsBarcelo A C (1993), `Effect of skin type and direction of applied force on peel strength of skin layers', J. Soc. Leather Technologist & Chemists, 77, 115±122. 5. FerraÂndiz-GoÂmez T P, Almela M, MartõÂn-MartõÂnez J M, Maldonado F and OrgileÂsBarcelo A C (1994), `Effect of surface modification of leather on its joint strength with polyvinyl chloride', J. Adhesion Sci. Technol., 8, 1043±1056. 6. Almela M, GascoÂn M J and Maldonado F (October±December, 1999), `New bonding process for wax-finished leather: 1. General aspects', AQEIC, 50(4), 199± 203. 7. Almela M, GascoÂn M J and Maldonado F (October±December, 1999), `New bonding process of wax-finished leather: 2. Manufacturing trials', AQEIC, 50(4), 205±211. 8. VeÂlez-PageÂs T (February, 2003), `ModificacioÂn de un serraje sin lijar por aplicacioÂn de un agente imprimante monocomponente para mejorar su adhesioÂn a adhesivos de poliuretano', Master Thesis, University of Alicante. 9. `Non-coalescence due to grease in leather' (May, 1963), SATRA Bulletin, 10(17), 228±230. 10. `Non-coalescence failures of cemented joints' (July, 1966), SATRA Bulletin, 12(7), 114±115. 11. `Urethane sole-attaching cements in perspective' (April, 1967), SATRA Bulletin, 12(16), 262±264. 12. `Combination adhesive systems' (March, 1971), SATRA Bulletin, 14(5), 226±232.

450

Adhesive bonding

13. `Plasticiser migration in PVC/resin-rubber joints' (March, 1967), SATRA Bulletin, 12(15), 248±251. 14. `Polyurethane sole-attaching adhesives' (April, 1970), SATRA Bulletin, 14(4), 50± 52. 15. Blackwell F B (October, 1973), `Adhesion of solings', SATRA Bulletin, 15(22), 423± 427. 16. Carter A R (July, 1973), `Adhesion to Nylon', SATRA Bulletin, 15(19), 366±368. 17. Martin D (January, 1971), `Cork sandals. Weaknesses to guard against', SATRA Bulletin, 14(13), 195±197. 18. Pettit D and Carter A R (February, 1964), `Adhesion of translucent rubber soling', SATRA Bulletin, 11(2), 17±21. 19. Pettit D and Carter A R (1964), `Adhesion of translucent rubber: Application of Infra-red Spectrometry to the problem'. SATRA Research Report 165, Kettering. 20. Pastor-Blas M M, SaÂnchez-Adsuar M S and MartõÂn-MartõÂnez, J M (1995), `Weak surface boundary layers in styrene-butadiene rubber', J. Adhesion, 50, 191±210. 21. Pastor-Blas M M and MartõÂn-MartõÂnez, J M (1995), `Mechanisms of formation of weak boundary layers in styrene-butadiene rubber', in Proceedings of The International Adhesion Symposium, H Mizumachi (ed.), Melbourne, Gordon and Breach Science Publishers, 215±233. 22. BernabeÂu-GonzaÂlvez A, Pastor-Blas M M and MartõÂn-MartõÂnez J M (1998), `Modified adhesion of rubber materials by surface migration of wax and zinc stearate', Proc. World Polymer Congress, 37th International Symposium on Macromolecules MACRO 98, Gold Coast, Australia, 705. 23. Romero-SaÂnchez M D, Pastor-Blas M M and MartõÂn-MartõÂnez J M (2001), `Adhesion improvement of SBR rubber by treatment with trichloroisocyanuric acid solutions in different esters', Int. J. Adhesion Adhesives, 21, 325±337. 24. Romero-SaÂnchez M D, Pastor-Blas M M and MartõÂn-MartõÂnez J M (2002), `Improved peel strength in vulcanised SBR rubber roughened before chlorination with trichloroisocyanuric acid', J. Adhesion, 78, 15±38. 25. Symes T E F and Oldfield D (1991), `Technology of bonding elastomers`, in Treatise on Adhesion and Adhesives. J D Minford (ed.), New York, Marcel Dekker, Volume 7, 231±331. 26. Cepeda-JimeÂnez C M, Pastor-Blas M M, FerraÂndiz-GoÂmez T P and MartõÂn-MartõÂnez J M (2000), `Surface characterization of vulcanized rubber treated with sulfuric acid and its adhesion to polyurethane adhesive', J. Adhesion, 73, 135±160. 27. Bascom W D (1977), `SEM study of a cyclized rubber surface', Rubber Chem. Technol., 50(2), 327±332. 28. Cepeda-JimeÂnez C M, Pastor-Blas M M, FerraÂndiz-GoÂmez T P and MartõÂn-MartõÂnez J M (2001), `Influence of the styrene content of thermoplastic styrene-butadiene rubbers in the effectiveness of the treatment with sulfuric acid', Int. J. Adhesion Adhesives, 21, 161±172. 29. Cepeda-JimeÂnez C M, Pastor-Blas M M and MartõÂn-MartõÂnez J M (2001), `Weak boundary layer on vulcanized styrene-butadiene rubber treated with sulfuric acid', J. Adhesion Sci. Technol., 15(11), 1323±1350. 30. Pettit D and Carter A R (1972), UK Patent Specification 1,278,258. 31. Pettit D and Carter A R (1972), UK Patent Specification 1,295,677. 32. Langerwerf J S A (1972), Centrum voor Schoentechniek van het Instituut voor Leder en Schoenen TNO, Report: 137±71S.

Shoe industry

451

33. Langerwerf J S A (1973), `Halogenation. Bonding without roughening', Technicuir, 7(3), 79±86. 34. Carter A R, Pettit D and Langerwerf J S A (1978), US Patent 4,110,495. 35. Romero-SaÂnchez M D, Pastor-Blas M M, FerraÂndiz-GoÂmez T P and MartõÂnMartõÂnez J M (2001), `Durability of the halogenation in synthetic rubber', Int. J. Adhesion Adhesives, 21, 101±106. 36. MartõÂnez-GarcõÂa A, SaÂnchez-Reche A, Gisbert-Soler S, Cepeda-JimeÂnez C M, Torregrosa-Macia R and. MartõÂn-MartõÂnez J M (2003), `Treatment of EVA with corona discharge to improve its adhesion to polychloroprene adhesives', J. Adhesion Sci. Technol., 17(1), 47±65. 37. MartõÂnez-GarcõÂa A, SaÂnchez-Reche A and MartõÂn-MartõÂnez J M (2003), `Surface modifications on EVA treated with sulfuric acid', J. Adhesion, 79(6), 525±548. 38. Cepeda-JimeÂnez C M, Torregrosa-Macia R and MartõÂn-MartõÂnez J M (2003), `Surface modifications of EVA copolymers induced by low pressure RF plasma from different gases related to their adhesion properties', J. Adhesion Sci. Technol., 17(8), 1145±1159. 39. Landete-Ruiz M D (2004), `Improved adhesion of polyolefins by treatment with UV radiation', PhD Thesis, University of Alicante. 40. HernaÂndez-GonzaÂlez J J (2002), Personal communication. 41. Carter A R (November, 1971), `Adhesion to PVC soling compounds', SATRA Bulletin, 14(23), 398±400. 42. Dollhausen M (1985), `Polyurethane adhesives`, in Polyurethane Handbook, G Oertel (ed.), Munich, Hanser, Chapter 11, 548±562. 43. Hepburn C (1992), Polyurethane Elastomers, London, Elsevier. 44. Chen W, Frisch K C and Wong S (1992), `The Effect of Soft Segments on the Morphology of Polyurethane Elastomers', in Advances in Urethane Science and Technology, K C Frisch and D Klempner (eds), Lancaster, Technomic, Volume 11, Chapter 3, 110±137. 45. Dieterich D, Grigat E and Hespe H (1985), `Chemical and physical-chemical principles of polyurethane chemistry', in Polyurethane Handbook, G Oertel (ed.), Munich, Hanser, Chapter 2, 7±41. 46. SaÂnchez-Adsuar M S and MartõÂn-MartõÂnez J M (1997), `Influence of the length of the chain extender on the properties of thermoplastic polyurethanes', J. Adhesion Sci. Technol. 11, 1077±1087. 47. Penczek P and Nachtkamp K (1987), `Resins used in adhesives', in Advances in Urethane Science and Technology. K Frisch and S Reegen (eds), Las Vegas, Technomic, Volume 4, 121±162. 48. AraÂn-AõÂs F, TorroÂ-Palau A M, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (2000), `Synthesis and characterization of new thermoplastic polyurethane adhesives containing rosin as an internal tackifier', J. Adhesion Sci. Technol., 14, 1557±1573. 49. AraÂn-AõÂs F, TorroÂ-Palau A M, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (2002), `Characterization of thermoplastic polyurethane adhesives with different hard/soft segment ratio containing rosin resin as an internal tackifier', J. Adhesion Sci. Technol., 16, 1431±1448. 50. TorroÂ-Palau A, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C, PeÂrez-Lozano V and MartõÂn-MartõÂnez J M (1998), `Attapulgite as a filler for solvent-based polyurethane adhesives', J. Adhesion Sci. Technol., 12, 479±495. 51. MaciaÂ-Agullo T G, MartõÂn-MartõÂnez J M, FernaÂndez-GarcõÂa J C, TorroÂ-Palau A and

452

52.

53. 54.

55. 56. 57.

58.

59. 60.

61. 62. 63. 64. 65. 66.

Adhesive bonding OrgileÂs-Barcelo A C (1995), `Hydrophobic or hydrophilic fumed silica as filler of polyurethane adhesives', J. Adhesion, 50, 265±277. TorroÂ-Palau A, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C, Pastor-Blas M M and MartõÂn-MartõÂnez J M (1997), `Structural modification of sepiolite (natural magnesium silicate) by thermal treatment: Effect on the properties of polyurethane adhesives', Int. J. Adhesion Adhesives, 17, 111±119. Sepulcre-Guilabert J, FerraÂndiz-GoÂmez T P and MartõÂn-MartõÂnez J M (2001), `Use of calcium carbonate-fumed silica mixtures as filler in polyurethane adhesives', Macromol. Symp., 169, 185±190. Frisch K C Jr (2002), `Chemistry and technology of polyurethane adhesives', in Adhesion Science and Engineering. Surface Chemistry and Applications. M Chaudhury and A V Pocius (eds), Amsterdam, Elsevier, Volume 2, Chapter 16, 776±801. MaciaÂ-Agullo T G, FernaÂndez-GarcõÂa J C, Pastor-Sempere N, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (1992), `Addition of silica to polyurethane adhesives', J. Adhesion, 38, 31±53. MartõÂn-MartõÂnez J M, MaciaÂ-Agullo T G, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C and TorroÂ-Palau A (1996), `Properties of solvent based polyurethane adhesives containing fumed silicas', Macrom. Symp., 108, 269±278. JauÂregui-Beloqui B, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C, Mahiques-Bujanda M M and MartõÂn-MartõÂnez J M (1999), `Thermoplastic polyurethane-fumed silica composites: influence of the specific surface area of fumed silica on the viscoelastic and adhesion properties', J. Adhesion Sci. Technol., 13, 695±711. JauÂregui-Beloqui B, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C, Mahiques-Bujanda M M and MartõÂn-MartõÂnez J M (1999), `Rheological properties of thermoplastic polyurethane adhesive solutions containing fumed silicas of different surface areas', Int. J. Adhesion Adhesives, 19, 321±328. TorroÂ-Palau A, FernaÂndez-GarcõÂa J C, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (2001), `Characterization of polyurethanes containing different silicas', Int. J. Adhesion Adhesives, 21, 1±9. PeÂrez-LiminÄana M A, TorroÂ-Palau A M, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (2001), `Rheological properties of polyurethane adhesives containing silica as filler: Influence of the nature and surface chemistry of silica', Macromol. Symp., 169, 191±196. PeÂrez-LiminÄana M A, TorroÂ-Palau A M, OrgileÂs-Barcelo A C and MartõÂn-MartõÂnez J M (2003), `Modification of the rheological properties of polyurethanes by adding fumed silica: Influence of the preparation procedure', Macromol. Symp., 194, 161±167. Isar-Rakoll Chemie GmbH (1971), Polyurethane adhesives modified with carboxylic acids to improve adhesion strength, UK Patent 1,391,722. Isar-Rakoll Chemie GmbH (1991), Klebstoffe auf Basis von Polyesterurethane fuÈr die Verklebung von Kautschuck-materialen und anderen Werkstoffen, German Patent 2,113,631. Abbott S G (October, 1992), `Solvent-free adhesives for sole attaching', SATRA Bulletin, 109±112. Whitehouse R S (1986), `Contact adhesives', in Synthetic Adhesives and Sealants. W C Wake (ed.), Chichester, John Wiley, Chapter 1, 1±29. `Solvent systems for Neoprene-Predicting solvent strength'. DuPont Elastomers Bulletin.

Shoe industry

453

67. Lyons D and Christell L A (August, 1997), `Waterborne polychloroprene adhesives', Adhesives & Sealants Industry, 46±50. 68. Lyons D F and Christell L A (December, 1997/January, 1998), `Formulating with polychloroprene latex adhesives', Adhesives & Sealants Industry, 28±32. 69. Cuervo C R and Maldonado A J (October, 1984), `Solution adhesives based on graft polymers of Neoprene and methyl methacrylate', Du Pont Elastomers Bulletin. 70. Blackwell F B (November, 1968), `A factory adhesion survey', SATRA Bulletin, 13(7), 104±108. 71. `Force drying of cement on shoe bottoms' (May, 1963), SATRA Bulletin, 10(17), 230±231. 72. Tanno T and Shibuya L (Spring 1967), `Special behaviour of para tertiary phenol dialcohol in polychloroprene adhesives', Adhesives and Sealant Council Meeting. 73. Kim J H, Park M H and Choi J S (9±12 October 1994), `Solventless polyurethane adhesives for footwear application', 36th Annual Polyurethane Technical/Marketing Conference, 88±94. 74. Abbott S G, Brewis D M, Manley N E, Mathieson I and Oliver N E (2003), `Solventfree bonding of shoe-soling materials', Int. J. Adhesion Adhesives, 23, 225±230. 75. Garelik-Rosen S, Torregrosa-Macia R and MartõÂn-MartõÂnez J M (2003), unpublished results. 76. Cepeda-JimeÂnez C M, Pastor-Blas M M, MartõÂn-MartõÂnez J M and Gottschalk P (2002), `A new water-based chemical treatment based on sodium dichloroisocyanurate (DCI) for rubber soles in footwear industry', J. Adhesion Sci. Technol., 16(3), 257±284. 77. Cepeda-JimeÂnez C M, Pastor-Blas M M, MartõÂn-MartõÂnez J M and Gottschalk P (2003), `Treatment of thermoplastic rubber with bleach as an alternative halogenation treatment in the footwear industry', J. Adhesion, 79(3), 207±237. 78. Brewis D M and Dahm R H (2003), `Mechanistic studies of pretreatments for elastomers', in Proceedings of Swiss Bonding 2003, Zurich, 69±75. 79. Petravicius A, Rajackas V and Kabaev M M (1980), `Study of chemical modification of the surface of sole rubbers with N-halosulfamides', Izvestiya Vysshikh Uchebnykh Zavedenii, Tekhnologiya Legkoi Promyshlennosti, 76±79. 80. Petravicius A, Jankauskaite V, Rajackas V and Kabaev M M (1988), `Mechanism of the modification of thermoplastic elastomer surfaces by solutions of haloorganic substances', Izvestiya Vysshikh Uchebnykh Zavedenii, Tekhnologiya Legkoi Promyshlennosti, 31(2), 90±95. 81. Navarro-BanÄoÂn M V, Pastor-Blas M M and MartõÂn-MartõÂnez J M (15±18 February 2004), `Water based halogenation for rubber materials', in Proceedings of the 27th Adhesion Society, Wilmington. 82. Brewis D M, Dahm R H and Mathieson I (1997), `A new general method of pretreating polymers', J. Materials Sci. Lett., 16(2), 93±95. 83. AS-3000 treatment system for Shoe industry (2002), CELME brochure, Alicante. 84. Romero-SaÂnchez M D, Pastor-Blas M M, MartõÂn-MartõÂnez J M, Zhdan P A and Watts J M (2001), `Surface modifications in a vulcanized rubber using corona discharge and ultraviolet radiation treatments', J. Materials Sci., 36(24), 5789±5799. 85. Romero-SaÂnchez M D, Pastor-Blas M M and MartõÂn-MartõÂnez J M (2003), `Treatment of a styrene-butadiene-styrene rubber with corona discharge to improve the adhesion to polyurethane adhesive', Int. J. Adhesion Adhesives, 23(1), 49±57. 86. Ortiz-MagaÂn A B, Pastor-Blas M M, FerraÂndiz-GoÂmez T P and MartõÂn-MartõÂnez J M

454

88. 87. 89.

90.

91. 92.

Adhesive bonding (2000), `Treatment of vulcanized SBR rubber with low-pressure gas plasma using oxygen-nitrogen mixtures', in Polymer Surface Modifications: Relevance to Adhesion'. K L Mittal (ed.), Zeist, VSP, Volume 2, 91±119. Pastor-Blas M M, MartõÂn-MartõÂnez J M and Dillard J G (1998), `Surface characterization of synthetic vulcanized rubber treated with oxygen plasma', Surf. Interf. Analysis, 26, 385±399. Pastor-Blas M M, and MartõÂn-MartõÂnez J M (2002), `Different surface modifications produced by oxygen plasma and halogenation treatments on a vulcanised rubber', J. Adhesion Sci. Technol., 16(4), 409±428. Pastor-Blas M M, FerraÂndiz-GoÂmez T P and MartõÂn-MartõÂnez J M (2000), `Assessment of the locus of failure of oxygen plasma treated rubber/polyurethane adhesive joints using XPS and IR-ATR spectroscopy', Surf. Interf. Analysis, 30, 7± 11. Ortiz-MagaÂn A B, Pastor-Blas M M, FerraÂndiz-GoÂmez T P, Morant-ZacareÂs C and MartõÂn-MartõÂnez J M (2001), `Surface modifications produced by N2 and O2 RFplasma treatment on a synthetic vulcanised rubber', Plasmas and Polymers, 6(1,2), 81±105. Cepeda-JimeÂnez C M, Torregrosa-Macia R and MartõÂn-MartõÂnez J M (2003), `Surface modifications of EVA copolymers by using RF oxidizing and nonoxidizing plasmas', Surface and Coatings Technology, 174±175, 94±99. Romero-SaÂnchez M D, Pastor-Blas M M, MartõÂn-MartõÂnez J M and Walzak M J (2003), `UV treatment of synthetic styrene-butadiene-styrene rubber', J. Adhesion Sci. Technol., 17(1), 25±46.

19

Electrical J-A PETIT AND V NASSIET

19.1 Introduction If it is considered that more than 50,000 different electrically conductive or nonconductive adhesives are being processed in the electronics industry worldwide, it appears that the electronics industry is undoubtedly the main industrial field for adhesive bonding. Obviously, from microelectronics to power electronics, which represent the fastest growing branch of the industry, adhesive bonding technology is finding more and more important, extended and diversified applications in electronics packaging. Therefore several research and development networks have been recently constituted to overcome the problems of the use of adhesives in electronics manufacturing. One among the very active, called `Adhesives in Electronics' and funded by the EC, has been working since 1998 (http://www.adhesives.de). To ensure their correct function, electronics systems must have the best electro-thermo-mechanical reliable behaviour. Then adhesive bonding offers a lot of advantages and satisfies most of the requirements: use of economical substrates, automatic ultra-rapid application processes with the utmost precision, increasing trend towards miniaturisation and high component density integration, positioning and securing of components during wavesolder and reflow processes, use when solder joining is not feasible and substitution of soldering for environmental advantages and at least securing cards and components by encapsulation from external damaging stresses and environments. Then almost all the technical adhesive types can be used. But adhesives with special processing, curing and use characteristics must be employed depending on whether isolating or electrically conductive connections are required. Thermal conductivity is always an additional criterion. Large adhesive gaps, together with restricted dot areas and the need for fast application, are the main origins of rather strict demands on rheological properties which must be extended on the whole. Mechanical strength and resistance to thermal fatigue are also needed. As a general rule the economical trend is to avoid surface preparation. Since exceptional cleanliness is needed, only physical methods for surface cleaning

456

Adhesive bonding

and preparation, such as plasma or laser, must be carried out in a sterile room. Thus, most of the filled or unfilled adhesives used achieve a sufficient adherence and mechanical strength. Moreover, since microelectronics products are protected from the external environment, delaminations in aggresive media may be avoided. As soon as more power is required, the circuits, including adhesive bonds, may suffer from thermal dissipation. In the electronics industry, at the moment reliability appears to be more important than durability, even if the effect of ageing is taken account. In the peculiar meaning of the electronics industry, indeed, common failures result from mechanical fracture including fatigue, adhesion and bonding, corrosion and wear and finally from delamination. A number of external factors are responsible, such as shocks, vibration, humidity, temperature, electrical loads, particles, etc. Most of them have an effect on adhesive bonds. Inspection, testing, quality and reliability control of adhesive joints must be understood among those responsible for electronic products in order to reduce early failures and the bottom of the bath curve of reliability. Thermal cycling and power cycling allow the stability of the processes and enable the measurement of a constant increase of the potential life of the products. For adhesive bonds, fracture mechanics tests (DCB, wedge test), shear and peel tests, according to the adhesive type, are performed. Non-destructive tests (US, acoustic microscopy) are also commonly used. The electronics industry therefore provides numerous and various examples of the use of adhesive bonding with conductive and non-conductive adhesives. Those used in surface mount (SMT), printed circuit board (PCB), die-attach, flip chip, encapsulation, smart-cards, display devices, microelectromechanical systems (MEMS) and power electronics packaging are emphasised here.

19.2 Basic needs The fabrication of electronic assemblies and packages requires the joining and sealing of similar as well as dissimilar materials with a wide range of physicochemical properties. Within the electronics industry a number of interfaces are involved, mainly metal-metal, metal-ceramic, metal-polymer, ceramic-polymer and polymer-polymer. Soldering and adhesive bonding are the main processes to ensure joining. From the point of view of adhesion science and technology, only three kinds of interfaces can be encountered: metal-polymer, ceramic-polymer and polymer-polymer, since adhesives are basically polymers. Several levels of packaging can then be distinguished (Puligandla, 2000). The first level concerns the integration of semi-conductor devices into single chip or multichip modules. Adhesive bonding plays a major role at this level, with electrically conductive adhesives (Gilleo, 1999). Being either isotropic conductive (ICA) or anisotropic conductive (ACA) in one direction, e.g., Z ± direction, electrically conductive adhesives are currently used in the micro-electronics

Electrical

457

industry. High electrical and thermal conductivity are expected from ICA, used in the form of pastes or films in the assembly or die-bonding of IC chips from lead frames for housed components. ACA adhesives come in two distinct forms, pastes and films. They are screen-printable, pad-stamping and dispensable. They achieve mechanical strength of a compounded structure and offer weak electrical resistance along the given axis and good insulation resistance inside any plane orthogonal to this axis. In spite of the required heat with pressure used for curing, with newly developed materials screen printing with more than 30 lines per mm with 30 m pitch connections can be obtained, without the risk of short circuiting. They are therefore particularly suitable for fine pitch attachments (flip chip bonding), flexible circuit applications (LCD displays), flip chip interconnections (smart-card, chip on board, etc.), TAB (tape automated bonding) interconnections and for flip chip contacting on glass substrates. Second-level packaging gathers the assemblies that integrate the first-level packages and other components such as connectors, discretes, etc., onto printed circuit boards (PCB). Adhesives, mainly non-conductive (NCA), are typically employed in surface-mounted PCB assemblies to hold without displacement passive, but also sometimes active, components on the underside of the board during wave soldering. Conductive adhesives are also used in hybrid engineering where various chip types, semiconductor chips or passive components must be bonded to substrates in an electrically conductive way. The assemblies of temperature-sensitive components, flexible circuits and difficult-to-bond substrates are mainly concerned. To increase mechanical strength and protection against the environment (thermal cycles, mechanical shocks and moisture), the various devices at the two first levels packaging, encapsulation and adhesive bonding are also required (Bartholomew, 1999). In the case of lead connections at least three types of encapsulation are currently used with the glob-top, the dam and fill and the transfer moulding technologies. They are also applied to fix chips on boards. In flip chip assemblies, underfills are known to secure components and to increase reliability. The underfill process is a part of the electrical process of interconnection using ACA and NCA adhesives. Moreover, a supplementarty underfill process must be performed after electrical interconnection has been achieved, in the case of ICA adhesives and solder bump bonding. Encapsulants are composite materials with an epoxy and/or a silicone matrix thoroughly filled (up to 80%) with silica to obtain the low CTE (12  10ÿ6 Cÿ1 to about 180 ëC), needed to make silicon and metal dilatations thermally compatible, and with a Tg around 170 ëC. For power electronics, silicone materials are preferred since they are able to be used at over 200 ëC. With underfills that involve three kinds of adhesive interfaces inside the package: the solder ball and the underfill, the hip passivation layer (silicon nitride or polyimide) and the underfill, and the substrate surface and the underfill, the main requirement is the very high ionic purity of the liquid adhesive necessary to secure the proper operation of the chips.

458

Adhesive bonding

The final integration of PCB, power modules, cooling systems, cables and peripherals calls for the third level packaging. Adhesive bonding must then achieve the utmost either thermo-mechanical or hygro-thermo-mechanical efficiency, in particular if high power (high voltage-high temperature) integration is expected and water cooling is used. At least to integrate microelectromechanical systems (MEMS) into a complete product, they require to be packaged in some form. Adhesives are used to bond different layers strongly together to seal the device or as conformal encapsulants. Thus, at every level of packaging the reliability of electronic devices assembled using adhesives is a challenging issue. It is often more than a simple combination of material, electrical and mechanical reliability.

19.3 Adhesive characteristics The development of many modern polymeric materials came from the electrical and electronics industries, largely because of the numerous areas for application in electrical and electronics components. The principal reasons for this popularity are that polymers are generally inexpensive, readily shaped dielectric materials with easily controlled physical, mechanical and electrical properties. Polymer adhesives can be employed depending on whether isolating or electrically conductive connections are required. Polymers used in the electronics industry are commonly classified as either thermosets (such as epoxies (Lutz and Cole, 1990), polyimides (Pujol et al., 1989), silicone and acrylic adhesives) or thermoplastics. More than 20 other polymers (such as polyether ether ketone (Shore, 1988), polyethersulfone (Ongley, 1995) and copolymer-based types can be found. Thermoplastics are also referred to as remeltables without altering their intrinsic properties and hot melts. Thermoplastic adhesives are available in film form and in solvent solution pastes. Thermoplastic-based adhesives have good reworkability and offer short bonding processes. The principal advantage of thermoplastic adhesives is the relative ease with which the interconnection can be disassembled for repair. On the other hand their rheological properties are altered through the use of solvent when thermoplastics are used in paste form. They also tend to form weaker adhesive bonds. Moreover, they are somewhat limited in service temperature performance because of low glassy temperatures (and melting point) compared to thermosets. In addition, their great flow under the application of load is thought to be a factor that may cause electrical resistance increases with thermal cycling (Keusseyan et al., 1993). Thermosets are crosslinked polymers. They undergo chemical reactions and form chemical crosslinks between polymer chains that resist deformation even at relatively high temperatures. Curing techniques include heat, UV light and added catalysts. Many thermosets require little or no solvent. These solventless systems are ideal from an environmental point of view. Moreover thermoset

Electrical

459

adhesives typically are cured from low molecular weight liquids that can wet out a surface and create strong bonds that can improve the durability of adhesive joints. On the other hand, they have a limited shelf life, poor reworkability and require long curing times compared to thermoplastic bonded processes. Whether thermoplastic or thermoset, polymers have advantages over other materials (metal and ceramic) in terms of low weight. Indeed density considerations become important in applications where overall weight must be minimised as in transport or mobiles. The fact that polymers are good insulators does not mean that they are inert in an electrical field and the intrinsic properties of a material can usually be related to performance under specific conditions. Indeed dielectric constant, volume and surface resistivities, dissipation, power and loss factors, arc resistance and dielectric strength change with service environment factors such as temperature, frequency, voltage and stress. Polymers may be required to operate as insulators in conditions of varying voltage from a few in communications equipment up to several million volts in power distributions systems. An electrical field involves heat dissipation so polymers must have a certain degree of thermal conductivity linked to a thermal stability (heat resistance). Moreover, polymer choice depends on this thermal expansion coefficient (CTE) value which will match the CTE of the substrate and reduce the thermal stress problem in encapsulated devices or in conductive adhesive bonded multilayer structures subjected to temperature cycling. It has to be noted that intrinsically conducting polymers such as polyaniline, polypyrrole and polythiophen, polyacetylen have been the subjects of more studies and some developments have appeared. As previously mentioned, the adhesive uses depend on whether isolating or electrically conductive connections are required. Because of their polymer structure and their chemical bonding behaviour, adhesives are normally not or only weakly conductive. For a whole range of electronic applications, these non-conductive properties of adhesives are important and particularly for encapsulation processes.

19.3.1 Polymer for encapsulation: materials processes and reliability The functions of encapsulation are to protect, to position and to secure the components during wavesolder and reflow processes and to increase production yield at low cost. Moreover, when properly employed, encapsulants can enhance the reliability of the product while protecting it from insidious and deleterious elements such as ion contaminants. The protection of the passivated integrated circuits (ICs) from the effects of mobile ions is possible with the ultra-highpurity materials of encapsulation (ionic contaminant content < a few ppm). In addition moisture is one of the major sources of corrosion (electrooxidation and metal migration) in ICs. So polymer permeability is a crucial factor to choose the best encapsulants. These latter must also have suitable

460

Adhesive bonding

Figure 19.1 Examples of electronic devices.

mechanical, electrical and physical properties. One can summarise as follows: minimum stress, matching thermal expansion coefficients, ease of dispensing, convenient cure schedule, neutral pH, good adhesion to silicon die surface and board and good electrical properties at device operating frequencies among other things. In addition, the resin materials in common use as die or underfill encapsulants are represented by the epoxy, silicone resins and to a lesser extent, by polyurethane and polyimide families. The performance characteristics are linked to the encapsulant function (Bartholomew, 1999) such as glop-top, underfill encapsulant, dam and fill encapsulation and potting or transfer moulding technologies in IC packaging for flip chip, chip scale package applications and surface mount applications (Fig. 19.1). Conventionally the application of encapsulant has been mostly carried out using manual or automated dispensing technologies which consist of a cartridge filled with adhesive, a mechanism for feeding small doses and a fine capillary or hollow needle for applying small volumes onto the substrate by dot formation. Due to the development of glob top encapsulants with suitable rheologies, stencil printing is now a serious alternative to syringe dispensing. The stencil printing process is detailed in a later section.

19.3.2 Conductive adhesives Electrical conductive adhesives (ECA) play a major role in electronics. More than an alternative to metallurgical solder, polymer-based adhesives are the key to most of our modern electronic products. Basically, there are two types of ECAs, isotropic conductive adhesive (ICA) and anisotropic conductive adhesive (ACA). But non-conductive adhesives (NCAs) are also used to form electrically conductive joints (Fig. 19.2).

Electrical

461

Figure 19.2 Bonding processes using ICA, ACA and NCA.

ECAs provide an environmentally friendly solution for interconnections in electronic applications. ECAs offer several advantages over conventional solder interconnection technology including finer pitch capability (use of anisotropic conductive adhesive (ACA)), lower temperature processing, more flexible, simpler processing (no need to get rid of flux) (Liu et al., 1998; Jagt et al., 1995) and a health and environment benefit by eliminating lead. Moreover the conductive adhesive systems exhibit, because of the properties of this polymer binder, greater flexibility, creep and fatigue resistance and energy damping (Wong et al., 1998), which can reduce the possibility of failure that occurs in lead-free solder interconnections. On the other hand, conductive adhesive technology has some drawbacks. Limited impact resistance, increased contact resistance and weakened mechanical strength in various climatic environmental conditions are several major obstacles (Zwolinski et al., 1996). Fillers For conductive fillers, metallic materials such as gold, silver, copper and nickel or non-metallic materials such as carbon or metal coated polymer particles have found application in ECA technology (Rusanen, 1999). Silver is the most used

462

Adhesive bonding

Table 19.1 Criteria for selecting polymer and filler materials for conductive adhesives (Rusanen, 1999) Criteria for selecting polymer matrix

Criteria for selecting filler material

Suitable adhesion to various surfaces High filling ability Low shrinkage due to cure Low outgassing during cure Low release of ionic contaminants Insensitive to common solvents Sufficient mechanical strength Suitable for production and rework Health and environment friendly

Good adhesion to the polymer matrix Good electrical and thermal conduction Non-forming insulating oxides Corrosion resistant Health and environment friendly The lowest cost function of the technical application

conductive filler for isotropic conductive adhesives because of its high electrical conductivity, chemical stability and low cost compared to gold. Moreover, silver oxides created by exposure to heat and humidity show high conductivity contrary to oxidised copper. Silver can also be precipitated into a wide range of sizes that can be checked and needles pre-treated with organic lubricants (Lu et al., 1999). The incorporation and mixing are easy in the resin matrix involving the proper rheology of the ECAs. Nevertheless problems with electro-migration can occur. Nickel resistant to oxidation is used as a stable conductive filler. But nickel adhesives show both higher filler resistance and contact resistance than silverbased products (Shimada et al., 2000). Moreover, corrosion of nickel surfaces has been found during accelerated ageing tests. Gold offers the best properties but its cost may be prohibitive for large volume applications. The alternative is the use of metal-coated particles. They have been employed to reduce cost, each type of plated particle having been designed for specific characteristics and end uses. Silver, nickel and gold plating on non-metals such as glass and polymer are among the most common types of filler product (Jagt, 1998). Table 19.1 lists the criteria for selecting the polymer and filler materials. Isotropic conductive adhesive (ICA) ICAs (Fig. 19.2) contain conductive filler concentrations between 20 and 35% in volume. This high filler content causes direct contact between the particles and surface without applied pressure. These adhesives are conductive in all directions. Generally, there are two conductive pathways for conductive adhesives. One is conduction caused by particle-particle contact within the polymer matrix. The other is percolation, which involves electron transport brought about by electron tunnelling between particles close enough to allow dielectric breakdown of the matrix. Whatever the pathway, the aim is to

Electrical

463

guarantee the lowest electrical resistance. Conductive bonding by isotropic conductive adhesive is currently the process used most in industry, over 90% of the time. Firstly they are typified by the silver-filled epoxies in chip attachment application as a substitute for solder, either in the semiconductor industry or for general interconnection of temperature-sensitive components to flexible circuits in various consumer product applications. To a lesser extent, ICAs are also used in hybrid engineering where various chip types (semiconductors chips or passive components) are used. For ICAs, the use of epoxies has been the state of the art for a long time because of the many beneficial properties such as excellent chemical and corrosion resistance, good adhesion, thermal insulation, low shrinkage, good electrical properties and low cost. Nevertheless, conductive adhesives do not stick onto wires and form fillets as do solders, so they work poorly with feedthrough components devices. As an example, surface mount technology nowadays provides an excellent form factor for adhesives. ICAs are supplied in the form of pastes or films. The paste adhesives are applied to the bonded surface by screen or stencil printing. The ICAs are screen or stencil printed only on the circuit contact pads and form reasonably reliable electromechanical junctions with components placed in the paste for a low process cost. Screen printing (Fig. 19.3) is a process where the conductive adhesive is forced through a nylon or a steel screen by a squeegee (rubber blade). The underside of the screen is coated and punctured with openings so that the adhesive is applied only to the screen at the points where there is a lack of coating, so the surface to be printed is placed below the screen. The squeegee goes through the screen and forces conductive adhesive through the screen pattern. Stencil printing (Fig. 19.4) is done with a metal stencil with the printing pattern etched into it. The stencil is mounted into a frame and is fixed over the

Figure 19.3 Screen printing principle.

464

Adhesive bonding

Figure 19.4 Stencil printing principle.

substrate. As the squeegee moves, the stencil is pushed down onto the substrate and the adhesive is brought into contact with the substrate via apertures in the stencil. To improve the adhesive's electrical and thermal conductivities, the amount of conductive filler particles may be increased. A volume concentration of 30% for silver particles corresponds to 70±80% by weight. As a consequence, the adhesion of the adhesive is restricted (Rusanen, 1999) associated to the risk of a short-circuit being formed. So for fine pitch applications, this process becomes problematic. A new class of adhesives that are conductive in one direction only have therefore been developed and are referred to as anisotropically conductive adhesives. Anisotropic conductive adhesive (ACA) In ACAs (Fig. 19.2), the volume fractions of conductive fillers are between 5± 15%, far below the percolation threshold of the used adhesive. The most commercially significant ACAs are based on the single particle bridging concept. The electrical conductivity is based on the contact of the conductive filler with substrate and/or bump. So ACAs have an electrical conduction generally built only in the pressurisation direction Z during curing, providing unidirectional conductivity and electrical isolation between the different connectors in the X-Y surface. The dimensions of a connector and the gap between different connectors are two of the most important factors in the choice of filler (nature, size and repartition inside the matrix) and the polymer matrix. They are used in form of paste (ACAPs) or film (ACAFs). Paste materials are either printed (mostly screen or stencil techniques) or dispensed with a syringe. These techniques enable a rapid application of ACAPs over large areas whereas using an ACAF requires cutting, aligning and tacking the adhesive on the substrate. On the other hand, film bonding with ACAFs is advantageous when the substrate is non-planar as in the case of assembled liquid crystal displays (LCDs).

Electrical

465

Figure 19.5 ACAs forms.

The most used ACAs are matrices filled with particles in a monolayer or column form (Fig. 19.5). For this last case, after drilling or etching of a precured polymer film, the fillers are put onto the matrix in column form. The electrical conduction is built in the pressurisation direction during the final curing process between the fillers themselves and between the conductive adhesive and substrate. Mechanical strength and conduction are ensured by this constant pressure. Generally the polymer matrix is silicone because of its great flexibility to minimise the non-coplanarity of the different connectors, its low moisture adsorption and its high temperature stability. Owing to its high strength, high reliability and its good adhesion on most materials, epoxy is also used in electronics assembly. The fillers are commonly silver or silver plated polymer spheres. This kind of distribution prevents fine pitch application with a pitch size below 10 m because of the conductive column volume and the necessary isolating spacing between the different columns. For the miniaturised applications the used ACAs are the conductive monolayer adhesives. In brief, ACA technology is well suited for fine pitch bonding (pitch < 0.5 mm) when one of the jointing partners is a flexible substrate such as display elements (LCD displays). Nevertheless ACAs are already being used for flip chip contacting rigid jointing partners such as glass substrates and chip-on-glass technology. Moreover, bumping is not an absolute requirement. The curing process involves low thermal shock and underfilling is not necessary, contrary to application using ICAs. Non-conductive adhesives (NCA) Electrically conductive adhesive joints can be formed using non-filled adhesives. Conduction is obtained by very thin layers (below 1 m) (Fig. 19.2) of polymer according to different conduction mechanisms like tunnelling, hopping and charge carrier injection (Pope and Swenberg, 1982). Electrical connection is achieved by sealing the two contact surfaces under heat and

466

Adhesive bonding

pressure, with the formation of contact spots that allow the electric current to flow. After the connections are made, shrinkage in the cured adhesive, aided by the difference of thermal expansion coefficients between surfaces and adhesive and the mechanical properties of materials will be responsible for the compressive force needed to maintain the electrical contacts (Cognard and Ganfguillet, 1983; Hieber and Thews, 1987). Conductive joining with non-conductive adhesives provides a number of advantages compared with other adhesive bonding techniques using ICAs and ACAs. NCAs joints avoid short-circuiting and are not limited to a reduction of connector pitches, in terms of particle size or percolation phenomena. Moreover, the lower temperature and shorter time for curing is beneficial as it reduces the thermal expansion of flexible circuits. The mechanical alignment is also simplified by the use of a NCA as opposed to anisotropic conductive film. The principal drawback is the limited electrical conduction of unfilled polymers.

19.4 Surface preparation Prior to bonding conductive adhesives on surface substrate or integrated circuit encapsulation, cleaning is critical to ensure long-term device reliability since a dirty surface or device is a guarantee of circuit failure. It is imperative that all trace amounts of contamination from the component surface be removed prior to the bonding or encapsulation process. Generally, the types of residue found on the surfaces of circuit boards are ionic contaminant from previous electroplating, finger grease, flue residue from a previous SMD attachment operation and general dust and dirt from an industrial environment. The three main cleaning processes are conventional cleaning, reactive oxygen and hydrogen cleaning. Laser ablation is also performed. Cleaning processes applied to the boards (substrates) prior to bonding or dispensing of the encapsulant vary from a dip in water or in a solvent to a more sophisticated treatment in an oxygen/argon plasma or under an excimer laser beam, performed in a sterile environment. Conventional cleaning includes the use of organics, such as detergents and solvents (chlorofluorohydrocarbons, freons, chlorohydrocarbons) to remove organic contaminants. Activated rosin fluxes are used in soldering processes to minimise defects due to poor wetting on oxidised surfaces. These materials contain ionic and acidic components that can cause electrical leakage and corrosion. So a cleaning agent is used to remove these activation agents and the rosin component. Water fulfils this requirement when some form of `saponification' is used to pre-treat the rosin which becomes soluble in water washing. Without saponifier, an alcohol cleaning process (to dissolve the rosin) followed by a de-ionised water washing is applied to remove the soluble activation agents. A cleaning line for alcohol/water cleaning is available at reasonable cost. In addition to conventional cleaning, reactive oxygen cleaning is very effective in removing low-level organic contaminants.

Electrical

467

There are three types of reactive oxygen gas processes used in cleaning: UVozone cleaning, plasma oxygen and microwave discharge. The principle of UV-ozone cleaning is the production of ozone by a UV source. The ozone is continuously created and destroyed, atomic oxygen being formed in the process. This form of oxygen is a powerful oxidising agent able to break up organic molecules chemically bonded on the surface of the board. So this cleaning method is prohibited for polymeric coatings and substrates. It is confined to cleaning ceramic surfaces. Moreover, UV ozone can generate oxide formation. Organic-based photoresists and generally organic residues are removed by oxidation, by placing the silicon wafers into a chamber where an oxygen plasma can be set up. It is the second reactive oxygen gas process. High-voltage a.c. is passed through the evacuated chamber into which a small O2 flow has been introduced. The resultant gases, water vapour, CO and CO2 are removed by a vacuum pump. Plasma oxygen operates at 13.6 MHz. An inert gas such as argon is used to form the plasma. The effect of this is to gently scour mechanically in the chamber by ion bombardment the exposed surface of the components. Combinations of oxygen and argon have also been applied (Evieux et al., 2004). Moreover plasma cleaning increases the adhesion of encapsulants to organic substrates. However, this process, while not as expensive as laser, has a cost higher than that of a solvent cleaning process. In addition the thermal stresses associated with the plasma process may damage some device structures. A powerful device cleaning technique similar to the oxygen plasma process is the microwave discharge cleaning with oxygen but with the microwave frequencies around 2.5 GHz. Hydrogen plasma cleaning has been reported as an alternative cleaning process for high performance IC packages. The plasma process is based on an argon-hydrogen discharge generated between the heated filament (cathode) and the reactor wall (anode). For the discharge the current density covers the range 10±100 amperes with a low voltage of 20±30 volts. The process is simple and environmentally friendly as it reduces organic and inorganic contaminants. Futhermore, the hydrogen cleaning process eliminates oxide formation, such as that generated by UV-ozone. Laser ablation can remove unwanted areas of the surface and surface contaminants (Doyle, 1997). It works by breaking the atom-atom bonds in the material. The wavelength of the laser beam has to be adapted to the type of bond to be broken. Otherwise the material is heated to remove residues by thermal degradation. To conclude on surface preparation before adhesive bonding in electronics, it must be said that physical and thermal treatments are the most suitable, since they can be performed in a sterile environment. Otherwise, adhesive bonding without surface preparation, if possible, is always a main requirement.

468

Adhesive bonding

19.5 Strength and durability: reliability According to the common structure of the chapters on industrial applications in this book, it has been decided to retain the heading `strength and durability' for electronics applications of adhesive bonding. However, it is imperative to associate it with reliability. The biggest challenge, indeed, for electronics is the harsh environment that results in great demands on devices and materials reliability. That, of course, includes the reliability of adhesive bonds. Then in the field of electronics, reliability is the usual term which covers failure physics and analysis, evaluation and prediction, modelling and simulation, methodologies and assurance. All important areas of microelectronic engineering, such as design, fabrication, packaging and testing have to stress reliability as the main requirement. Strength and durability of adhesive joints must be considered within this parameter but the classical concept of mechanical strength of adhesive bonds, which is so important in other industrial fields, is not the main concern of the electronics industry. With some exceptions, taking account of a few high mechanical stresses due to vibration and shocks, the strength levels required of adhesive joints are generally low. Thus, what is expected is the integrity of the adhesive interfaces over time. It is the problem of durability of adhesive joints that is of concern to the adhesion science and engineering community (Petit and Baziard, 1998; Petit, 2002). It is included in the wide scope of reliability according to the electronics industry. Following the standards: · reliability is the ability of an entity to achieve a required function under given conditions during a given period of time, · durability is the ability of an entity to achieve a required function under given conditions of service and maintenance till a limit state is reached. The limit state can be fixed either by the end of the useful life or by economical or technical obsolescence or by other factors. In electronics, two elementary criteria are continually used to characterise a set of packaged components: the MTTF (mean time to fail), mean time of emergence of a breakdown and the MTBF (mean time between failure), mean time between two breakdowns. A third notion is the unit of the rate of breakdown or the FIT (failure in time). Since the components are effectively very reliable, the FIT corresponds to the probability of having a breakdown after an hour. Then, the reliability curves (rate of breakdown versus time in years) which define the ageing of the components all have the same `bath' shape due to three effects (Fig. 19.6). The left part of the curves represents the `youth defects': they can be debugged by vibration and thermal tests. In the constant zone of the curves,

Electrical

469

Figure 19.6 The `bath' curve of the reliability of electronics systems.

corresponding to the useful life time, a second effect is involved, known as `extrinsic random breakdown' because the breakdowns can be the result of external events. Much later appears the third effect concerning breakdowns due to physical wear phenomena. Adhesive bonds, sensitive to the electro-hygrothermo mechanical effects of ageing, are strongly affected during this period of obsolescence of the devices. Indeed, a third of the breakdowns during this period results from packaging defects, assembly defects and mechanical forcing following dismantling carried out without care. An electronic device consists of a large number of components, sometimes several thousands. Generally, the ageing of the components results from physical phenomena that are known and predictable. However, under some environmental conditions, the components can exhibit characteristics of evolution and degradation more unfavourable than expected. Electronic components are particularly sensitive to: · temperature increases. The rate of breakdown follows an exponential law in the function of this parameter. It is a well known phenomenon for anybody working with semiconductors, · humidity which induces very harmful electro-migration phenomena, · ionising radiation which damages the structure and weakens the performance. Ageing has another detrimental consequence since it also affects the mechanical architecture of electronic systems. Therefore, modelling of ageing is obviously important. In fact, two main types of laws govern the mechanisms of ageing. Firstly, the laws describing the occurrence of cracking, which is at the root of most of the problems, are used. The Coffin Manson law and its derivatives are

470

Adhesive bonding

quite suitable to predict the time after which strains and cracks could be observed. When cracks are created, diffusion phenomena occur following Arrhenius and Eyring diffusion laws. These models work rather well, so a good modelling of product reliability can be achieved. Some examples chosen at the different levels of packaging show applications of these reliability fundamentals to adhesive joints in electronics. In the first example underfill/polyimide interfaces are used to illustrate the importance of surface interactions on mechanical adhesive strength (Pearson, 2000). Delamination at such interfaces is a concern in flip chip assemblies. Using the asymmetric DCB test, the critical energy release rate varies from 15 to 80 J/m2 following the chemistry and the rheology of the 70% silica filled epoxy underfill resins. Being firstly adhesive, the failure becomes cohesive in the same conditions. Since the high value of Gc is obtained with a tough underfill resin which gives a lower value of the work of adhesion, the adhesive strength between epoxy underfill resins and polyimide organic passivation layers seems to be controlled both by molecular interactions and deformation mechanisms occurring in the underfill. In the case of weak interfacial interactions, Gc can be improved up to 100±130 J/m2 by UV-ozone and O2 plasma treatment of the PI underfill surface. It is better to avoid this additional process step by selecting a more suitable underfill resin with proper chemistry and rheology to better improve reliability at lower cost. In the second example, the effect of moisture absorption in flip chip packages with underfill is emphasised (Luo et al., 2000). With different epoxy underfill formulations, with the same hardener and various catalysts, there is some variation in moisture absorption after 500 hours into an 85 ëC/85% RH environment among the different catalysts. However, a much more significant difference in moisture absorption comes from the difference in epoxy system. As a general rule, the lower moisture absorption concerns the epoxy system with the higher Tg. Exceptions for highly crosslinking systems, with poor packed chains favouring diffusion of humidity can be mentioned. Die shear strengths of the same epoxy systems of underfills bonded to silica passivated silicon dies exhibit little difference around 50 MPa. Weak adhesion strengths, about 10±15 MPa, come from the worse catalysts. After ageing in a high humidity and high temperature environment, the high die shear strengths are divided by 3±5 according to the epoxy system, following the rule Tg/moisture absorption. The bad catalysed systems for adhesion strength before ageing continue to show worse adhesion than the others after ageing, in spite of high Tg values. It is perhaps catalyst migration to the interface during curing that makes it more sensitive to humidity. To provide a much larger input/output count over a given area of integrated circuit, flip chip technology uses an area array of solder balls. In the first generation of flip chips, the chips were attached to a ceramic substrate. Thus the good CTE match between the die and the substrate prevented thermal stress

Electrical

471

from developing. With the use today of cheaper organic PCBs such as the FR-4, some reliability problems arose. To prevent the failure of the solder joint, an underfill encapsulant adhesive is used. Among all the properties required for an underfill, a low CTE is desirable to minimise the stress in the package together with a high glass transition temperature, Tg, which is the upper-limit temperature for the use of the flip chip packages with underfill. With thermal cycling stresses, delamination between the die and the underfill is a major concern for reliability. This problem is particularly prevalent when the assembly is subjected to thermal cycling in a high humidity environment. Then delamination can occur at the underfill/die or at the underfill/substrate interface, due to low adhesion, contamination and void forming during curing. These three examples, among many others, show, with general results and always some exceptions, how crucial is underfill adhesion to the integrity and reliability of this type of assembly. They also lead to a more generally well known established fact in adhesion science and engineering: adhesion strength, G or Gc is mainly dependent upon the overall adhesive system, whatever the application. For example, the same 50 MPa shear strength value has been obtained for: · silicon die/epoxy underfill assemblies for microelectronics (Luo et al., 2000) · stainless steel/epoxy/composites bonds for leading edges of helicopter blades (Emery, 1996) · silicon carbide/epoxy/silicon carbide assemblies for space telescope mirrors (Levallois et al., 1999). Coincidentally, a lot of other adhesive epoxy bonds are used for aircraft, automotive and miscellaneous applications. What is important for adhesive bonding is not the level of strength or of fracture toughness, but how to manage these in designing assemblies for a long useful life under harsh environments. That is the management of reliability. Several good reviews on reliability of adhesive joints in microelectronics have appeared in the literature. A most interesting and useful one has appeared recently for surface mount applications with isotropic electrically conductive adhesives (Jagt, 1999). An evaluation of reliability investigations made by many academic and industrial institutions worldwide is presented. The data mainly concern surface mountings, but the methodology has been applied to almost all the ECA joints in low power applications, including packaging, which exhibit higher reliability than soldering. Among the gathered data, one can find the main reliability (durability) tests performed for various applications (automotive, telecom, military, industrial) and the means to understand mechanical failure. The overview of reliability investigations worldwide with isotropically conductive adhesives (ICA) must be considered as the most relevant data in this field today. The reliability of anisotropically conductive adhesive (ACA)

472

Adhesive bonding

interconnections depends on the particular formulation, substrate and component combination. Because each system must be evaluated separately it is difficult today to establish overall data (Lyons and Wong, 1999; Lu et al., 1999). It is the same for reliability data on electrically conductive joints using non-conductive adhesive (NCA) (Bauer and Gesang, 1999). In all these examples the expected electrical and mechanical reliability are determined following the approaches and standards of the electronics community. A last example provides reliability and durability information following physico-chemical and mechanical approaches and standards of the adhesion community (Evieux et al., 2002b, 2004). It relates to structural adhesive bonding with epoxies of an aluminium nitride heat sink to a polyetherimide collector. Such assemblies are used in water-cooled power modules with durability in hot/ wet environments a main requirement. In the clean-room, usable physical and environmentally friendly surface treatments (excimer laser, low pressure plasma) are needed to improve adhesion of the substrates. For AlN/epoxy joints, the compressive shear failure load can reach 9.6 kN and the fracture energy 420 J/m2 by an optimised laser treatment of AlN. Using a -GPS silane coupling agent on a laser treated surface enhances the fracture energy to 1000 J/ m2, that is 12 times higher than the value obtained with just degreased samples. For PEI/epoxy joints, with optimised plasma treatment of PEI, a 10.4 kN shear failure load and a 536 J/m2 fracture energy are obtained using silane adhesion promoter. The silane effect on the mechanical properties of joints is not significant. It becomes favourable during hot/wet severe ageing. Moreover, rheometry has underlined an overall plasticising of bulk epoxy adhesives after ageing, with degradation of one of the epoxy/amine networks. The plasticising of the adhesives is increased in the joints as confirmed by nanoindentation. Durability of AlN/epoxy/PEI adhesive joints after ageing in coolant liquid has been evaluated, using a specially designed asymmetric wedge test (Fig. 19.7(a) and (b)). It has been previously validated by finite element analysis and by image stereocorrelation of displacements. It appears that the AlN/epoxy interface is the weak link of the assembly because the structural adhesive bonding of the ceramic to the thermoplastic introduces high residual stresses within the adhesive and at the ceramic/adhesive interface. As a result, cracking of the ceramic is observed. At least some models, based on logarithmic variation of the stress with the crack length and an Arrhenius evolution with temperature of the cracking velocity, have been developed to obtain a reliable and rapid prediction of the long-term behaviour of structural adhesive joints. This example devoted to high power electronics packaging is sustained by the general methodology of investigation to identify the strength and durability characteristics of structural adhesive bonds whatever the application. Much research work involves the same methodology to study and model thin electronics adhesive joints, representative of actual joints and good information can be gathered on the adhesive fracture energy.

Electrical

473

Figure 19.7 Durability of AlN/epoxy/PEI adhesive joints after ageing in coolant media: (a) effect of PEI plasma treatment on PEI/epoxy interface, (b) effect of AlN laser and silane treatments on AlN/epoxy interface.

But taking account of the very small dimensions of real adhesive joints, it is not easy to perform fracture mechanics tests so it is difficult to obtain adherence data. Therefore shear failure loads are often measured, which are geometrysensitive and can be used only to compare different processing parameters.

19.6 Common failures Several types of failure mechanisms of electronic components have been identified which upset mechanical and electrical integrity. Some of them are intrinsic due to the component architecture. The nature of the encapsulation also plays an important role but the weak link in electronics remains the connections, which originate most of the defects. The extrinsic mechanisms result from overvoltages, over-heating and mechanical shocks, which have a more detrimental effect on the interconnection than on the component itself. Some mechanisms depend on manufacturing processes; centring defects, presence of dust.

474

Adhesive bonding

Figure 19.8 Ageing mechanisms of polymer encapsulated electronic components.

Indeed, failure mechanisms of adhesive bonds in electronics mainly come from encapsulation and packaging. They are summarised in Fig. 19.8. Among them, the popcorn degradation is typical of plastic packages (Dudek et al., 2001). They are permeable to gases and vapours. Then moisture can accumulate inside the packages and explode from them during the second reflow, especially for ball grid arrays and chip scale packages. The most frequently observed popcorn degradation is the delamination of the epoxy moulding compound from the lower side of the die pad with bulge formation. It is the type I popcorn effect. The type II popcorn effect consists of failure at the die bonding adhesive/die pad interface. Delamination and cracking of underfill/passivated adhesive interfaces, mainly at corner fillets, are common failures of chip connection to its substrate, causing an electrical open circuit. Acoustic microscopy is a very suitable method, widely used in the electronics area, to observe such failures. In surface mount applications, electrically conductive adhesive joints present several failure mechanisms. Oxidation of Sn-Pb component terminations as well as Sn-Pb pitting corrosion accelerated by Cl- ions coming from epoxy adhesives are responsible for large electrical increases. Cracks at Sn-Pb/adhesive interface and Cu/adhesive interface have been observed after exposure to moisture. Sometimes the shear strength remains high after testing despite the very large resistance increase. Evidence has also been reported of a thin silver-depleted layer between Sn-Pb metallisation and adhesive Ag filled layer (Jagt, 1999). Creep effects in the adhesive layer on durability have also been considered. At least, in the presence of an electrolyte (water) and an electric field (d.c. voltage), silver can undergo migration from adhesive, that can lead to electrical shorts between adjacent conductor patterns. It is a well known phenomenon that can occur with many Ag-filled products and can be avoided by covering them with

Electrical

475

suitable organic coatings (Olsson et al., 2002). Even if all these mechanisms have been identified, understanding the actual effect of each mechanism in joint resistance deterioration still needs to be resolved (Jagt, 1999). Other kinds of adhesive bond failures can be encountered. In printed circuit boards made with several layers of alternating electric layers (rolled copper) and glass fibre reinforced dielectric layers, can be found adherence loss and cracking at copper/epoxy interfaces and between glass and epoxy within the composite. Some failures also occur between the solder mask on a PCB and the underfill for interconnection. Most of these failures are due to large CTE mismatches between different materials used in heterogeneous assemblies to satisfy multifunctional electric and dielectric requirements together with mechanical stress transfer by interfaces. These CTE mismatch effects are amplified by harsh thermal cycling and humidity cycling, that packages must withstand. Thermal management is growing with power integration. Today, several heat sinks are used with water or air cooling. Whatever the cooling mode, the two types of ceramic/adhesive and polymer/adhesive interfaces are involved with epoxy, silicone or high temperature stable adhesives. Our experience in adhesive bonding of ceramic materials (Petit and Baziard, 2000) for many application areas, and particularly in power electronics, integration shows that common failures in power electronics integration packaging are not peculiar to this application. Indeed, ceramic/polymer and ceramic/metal bonds using adhesives always suffer the same adhesive failure under severe hygro-thermo-mechanical environments. To provide a comprehensive report it must be mentioned that the main failure mechanisms in microelectromechanicalsystems (MEMS) (Fig. 19.9) are caused

Figure 19.9 Main failure mechanisms of MEMS: (a) mechanical fracture, (b) adhesive bonding, (c) wear, (d) delamination.

476

Adhesive bonding

by mechanical failure due to the geometry, to presence of flaws and to fatigue. Wear resulting from abrasion and corrosion is also observed. Delamination between layers results from thermo-mechanical stresses. Adhesion (or adhesive bonding) of structures due to Van der Waals or electrostatic interaction is also considered as an important failure mechanism of MEMS. It is shown that because of bending and residual stresses of different layers, variable capacitance which does not meet specifications can be expected (Rigo et al., 2003). As an overall conclusion, in electronics, as well as mechanical fracture which is the usual reason for the failure of adhesive bonds, we must add electrical failures of components and packages.

19.7 Inspection, testing and quality control To prove that a sufficiently high standard of performance of electronic devices can be reached, an electronics manufacturer must often undertake a number of tests since it is not usually possible to obtain performance data from materials suppliers. Each industry has its own methodology, depending on the type of component and available inspection equipment. Tests carried out directly on the materials are often well standardised. On the other hand, tests performed on devices which are each devoted to one kind of use are rather peculiar to each producer, even if they follow specifications. Therefore, it is difficult to establish a general methodology of inspection for all electronic devices. Encapsulant materials for underfill are more than others the subject of several tests (Bartholomew, 1999); capacitance and resistance measurements in wet and dry state, for dielectric and electric properties, dilatometry for CTE, DSC for Tg, shrinkage by curing, viscosimetry for rheology. Moisture presence and content linked with popcorning are determined by gravimetry but also by TGA, DTA, NMR and IRS. The tensile pull test is used for modulus measurement and the wafer bend test for residual stresses. Lap shear tests are widely used for obtaining a measure of the adherence of resin encapsulants to a given substrate. It is the case, for example, for chip-on-board components. Stud-pull tests can also be carried out. It is well known by the adhesion community that these tests are geometry sensitive, and thus suitable only as comparative tests. Actual encapsulated components are inspected as follows: measurement of stress by strain gauge and/or piezo-resistive element, DSC/FTIR for the state of cure, thermal shock and thermal cycling to ensure material compatibility, and accelerated ageing for long-term reliability prediction. Several test standards (JESD 22, 26) have been produced by the Joint Electronic Devices Engineering Council (JEDEC). They fall into two groups: moisture induced stress tests at elevated temperatures and elevated temperature stress (with and without bias in each case). The more used are the moisture resistance test involving thermal and humidity cycling under bias in order to evaluate corrosion resistance of the die metallisation and the cycled temperature±humidity bias (THB) test, typically

Electrical

477

thermal cycling from ÿ30 ëC to 65 ëC in a 90±98% RH environment for 1000 hours, eventually causing delamination of the encapsulant and metallisation corrosion. Obviously the widely used steady THB life test (85 ëC/85% RH/1000 hours) for general reliability purposes is the most involved. The pressure cooker test in autoclave (T ˆ 120 ëC, saturated steam under 110 KPa pressure to favour moisture diffusion until metal corrodes) provides very severe conditions. All these tests include electrical testing performed at various points during the tests. In order to reduce the rather lengthy test times in the standard steady THB life test, highly accelerated stress testing (HAST) is recommended. Elevated pressure and temperature are applied similar to those for unbiased autoclave testing except that the moisture level is maintained between 50% and 100% RH. Actual device life is predicted using modified Arrhenius relationships. In spite of their attraction as brief tests, HAST test and more generally ALTs (accelerated life test) results cannot be compared or correlated with steady THB test results. Moreover, owing to different environmental conditions, it is not possible to compare the results given by each reliability test (Suhir, 2002). More generally, for reliability investigations, whatever the devices and the adhesive joints, either for die attachment or for power electronics packaging, the most demanding climatic tests are temperature shock and thermal cycling. The requirements depend greatly on the application area. Outdoor applications and especially automotive, military and telecom applications have the highest requirements, e.g. ÿ65 ëC to ‡150 ëC for 1000 cycles (or ÿ55 ëC to ‡125 ëC for less severe uses) damp heat at 85 ëC and 85% RH for 1000 h (steady THB) and hot storage at 125 ëC or 150 ëC for 1000 h. Mechanical shock tests such as drop tests and sine and random vibrations tests are also performed. To inspect an apparently failed component, many methods are used. Scanning acoustic microscopy or microfocus X-ray inspection are among the non-destructive methods. Scanning electron microscopy combined with EDX analysis, surface analytical techniques such as Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) for analytical purposes and atomic force microscopy (AFM) for morphology and topography investigations are also carried out on surfaces of a sectioned or failed specimen. Instead of providing an industrial quality of products, all these methods are mainly devoted to a better understanding of failure mechanisms in industry and research laboratories. In research works, all the most up-to-date experimental techniques are used (Vogel et al., 2001). Without coming back to the main methods already quoted, all the fracture mechanics tests (DCB, TDCB, SEN, symmetric and asymmetric wedge tests) under static and periodic stress (even dynamic at high speed) have been performed to understand the behaviour under dry and wet environments of electronic packages and devices, including adhesives joints. These tests, which provide fracture energy values, are the only ones to provide adherence data.

478

Adhesive bonding

Finally, it is worthwhile to mention that if some guidelines exist to inspect or to study adhesive bonds in electronics, taking account of some peculiarities, they are always the same as in other industries. The only difference is that electronic products must firstly fulfil high electrical specifications. Other requirements such as mechanical strength, environmental integrity and durability are secondary. And, of course, hygro-thermo-mechanical service conditions have a great effect on reducing or even failing electric and dielectric properties, together with a loss of adherence of adhesive joints. Therefore adhesive bonding in electronics manufacturing must be considered as a leading technology. The requirements for adhesive bonding worldwide, whatever the applications, are often the same. And the same concepts are used for structural adhesive bonding in aircraft or automotive structures as well as for adhesive bonding in electronic packaging. When today, modelling is more and more based on multi-physics approaches, systematic relationships between the mechanical and electrical properties of the joints should be very powerful for design and inspection. But, unfortunately even for the most typical case of conductive adhesive joints, such relationships are not yet established (Liu and LundstroÈm, 1999).

19.8 Examples of use In electronics, examples of the use of adhesive bonding are numerous and various. In order to give the most accurate image of the application areas some of the most significant are presented. Among the oldest applications of adhesive bonding in electronics are those in printed wiring board (PWB) or printed circuit board (PCB) and in surface mount devices (SMD).

19.8.1 Printed circuit board In PCB, such as the most common FR-4 production, adhesive bonding is involved in the technologies of composite fabrication with fibre to polymer matrix adhesion. The PCB generally consists of several layers of alternating fibre reinforced dielectric layers and circuits formed by copper layers, adhesively bonded together. Interconnection of the underfill of the chip package to the solder mask on the PCB is also a good example of polymer to polymer adhesive bonding.

19.8.2 Surface mount devices In surface mount devices two manufacturing techniques are commonly employed: one is to use a solder cream, the other an adhesive. The second technique, indeed, takes account of the presence on most of the cards of a combination of components for mounting through holes and surface-mount

Electrical

479

components. These last are bonded to the card with an adhesive placed between the pads and then fixed together with components mounted through holes by wave soldering. Acrylic and epoxy adhesives are commonly used in surface mount technologies (SMT). In addition to a good strength of the adhesive in the liquid state to keep in place the components and a good strength after polymerisation of the adhesive joint to bear handling loads during the production cycle, the adhesive joint must be solder resistant, i.e., with standing elevated temperatures (> 230 ëC) for a very short time and thermal shock at the wave entrance. After soldering, the role of the positioning adhesive is ended, because the solder joint has a higher mechanical strength than the adhesive joint. But since the adhesive stays in place, it must be harmless to the operation of the circuit. It must retain good dielectric properties to avoid short-circuits, without creating long-term corrosion problems. Conventional SMT involves large surface mount devices, however, in spite of the ecological interest of ACAs as a replacement for solder, anisotropically conductive adhesive joint geometry is not sufficiently optimised for mechanical reliability in this field. ACAs have, however, been successfully used in highvolume production of chip scale components.

19.8.3 Die attach Attachment of silicon semiconductor chips to substrates and packages is an important step in the fabrication of microelectronic devices. Silver-filled adhesives, often epoxies and polyimides, are competitively used with Pb-Sn-InAg based alloys, Si-Au eutectic and glass filled with silver. Due to a simple process and relatively cheap equipment, conductive adhesives are extensively employed. They have low curing temperatures that allow the use of almost all the materials, particularly polymer materials in the assemblies. Die attachment with ECA, mainly ICA, adhesives to a substrate or a chip carrier is an interesting alternative to soldering. Elimination of the cleaning steps and substitution of Pb solders offer economical and ecological advantages. At least, despite the lower strength and the use of solvent to obtain pastes of suitable rheology, with a risk of void formation, thermoplastic filled adhesives like PEEK and PES, are promising owing to easy rework capability.

19.8.4 Flip chip One of the most important applications of adhesive bonding in microelectronics seems to be for flip chip. Flip chip is considered as a well qualified direct chip level interconnection technology to meet the continuous pursuit of higher interconnection density at low cost and lower weight for mobile phone devices and for smart-cards (Murakami, 2001). Essential requirements are fine-pitch capability, low temperature processing possibility, elimination of underfills and

480

Adhesive bonding

simple, flexible, cheap processing. Anisotropically conductive adhesives (ACA) are the preferred application field in flip chips attached to rigid substrates (Liu, 2000). Since the pitch for assembly of bare chips is very small, less than 100 m, ACA flip chip joining presents technical and cost advantages compared with soldering (Takeichi and Nagashima, 2001). ACA bonding is also performed on flexible circuits. Joining with solder-filled or low-melting alloy-filled adhesives provides a technically feasible interconnection method for bare chips and flexible circuits and substrates, such as disk drives and driver chips for liquid crystal display (LCD) applications. Moreover, these adhesives have a better selfalignment capability than isotropically conductive silver filled adhesives, that is important for flip chip interconnection. ACA film interconnection is also potentially useful for high-frequency applications, such as high-performance digital IC packages and microwave device packages. On the other hand, adhesive flip chip technologies are becoming more and more important for mass production of low-cost products. Smart labels, combining ACA or NCA adhesives with low-cost substrate materials like PET are a good example (Kriebel and Seidowski, 2001). As radio frequency identification labels, they satisfy a lot of applications; in logistics during production, in sales, for inventory management and for general identification purposes. All the described flip chip applications using adhesives are recent, but flip chips using solder or other materials are still produced. These last obviously need underfilling which represents another main involvement of adhesive bonding beyond the protection offered by moulding.

19.8.5 Underfill The adhesive bonding aspects of underfilling and encapsulation in electronics can be endlessly discussed. Facing so huge a subject, it has been decided to emphasise the use of the new generation of no-flow underfills. Due to the interference (Shi and Wong, 1999) of silica fillers with solder joint formation, the no-flow underfills are not filled with silica fillers, having a high coefficient of thermal expansion (CTE) as a consequence. In a novel patented process, a double-layer no-flow underfill is used to incorporate silica fillers into no-flow underfill in a design using quartz chips (Zhang and Wong, 2001; Zhang et al., 2001). In this process the high viscosity bottom layer underfill is not filled with silica fillers (Fig. 19.10). With this process the fillers can be prevented from entering the gap between the solder bumps and contact pads. The reliability of the double-layer underfill is checked and process optimisation is obtained. Therefore the thickness and the viscosity of underfill layers, the size of silica fillers and their weight percentages is varied to quantify their effects on solder bumps wetting (Zhang et al., 2002). Another interesting capability of no-flow underfills is the thermal reworking of a defective chip.

Electrical

481

Figure 19.10 Double layer no-flow underfill process.

19.8.6 Smart-cards As already discussed smart-cards provide a wide area of flip chip applications. Dual interface smart-cards are a combination of contact and contactless cards. Usually, a single chip is connected to an antenna structure, made of a copper or aluminium foil laminated to a card body and then etched or wired. But wirebonding and polymer flip chip assemblies require encapsulation and other limitations appear. Therefore isotropic and anisotropic conductive adhesives are used to attach chip modules to embedded antennas. This attachment is critical for electrical performance and reliability so the selection of the appropriate adhesive for each application is essential. For mechanical interconnection of components to the card body hot melt and cyanoacrylate adhesives are mainly used. Another aspect of adhesive bonding in the smart-card industry concerns the assembly of the multilayer composite structure of the cards. Laminated cards are obtained by hot pressing. An overlay, which protects the printed faces of the cards, is adhesively bonded onto the thermoplastic (PVC then PET, since ABS is quite expensive) printed bodies of the cards. Generally a symmetrical assembly, in order to avoid strains due to the shrinkage of the different materials during pressing, forms the laminated cards. An adhesive film operating by force interdigitation under temperature and pressure gives its cohesion to the combined materials.

482

Adhesive bonding

19.8.7 Display applications With the progress of the information society, the need for display devices has grown extensively. Liquid crystal devices (LCDs) are now the most important of these devices which are gaining a greater market share against cathode ray tubes technologies. The electrical interconnection between the flat panel LCD and the LCD driver circuit is a challenge for performance and reliability. Adhesive bonding is among the bonding techniques to take up this challenge (Kristiansen and Liu, 1999). Initially, elastomer connectors (alternating segments of electrically conducting and isolating silicone rubber) or heat seals were employed to electrically connect the driver electronics and the glass. Since the electrical conductivity is low, these technologies are now used only for small LCDs in watches, calculators and office equipment. IC packaging has called for polyester film heat seal connectors patterned with conductive polymer thick film traces which are bonded to the glass traces and to PCBs by a thermoplastic adhesive. Chip on flex tape automated bonding (TAB) packages have been directly connected to LCD panels using mainly ACAs but also NCAs adhesives (Murakami, 2001). Another process based on flip chip technology uses chip on glass (COG) mounting with a flexible polyimide circuit bonded to the glass substrate by conductive adhesives. This technology is restricted to high pixel density products. Each manufacturer has developed its own COG technology, always using adhesive bonding. In most cases Au bumped bond pads are plated onto the chip, but conventional ball bonding is also performed in stud-bump technology. When ACAs and NCAs are used, underfilling is a part of the interconnection. With ICAs and solders, an additional underfilling is needed after electrical interconnection. Rework capability is an important requirement in LCD applications that conductive adhesive bonding can provide better than metallurgical bonding.

19.8.8 Microsystems As shown in a recent review (Sarvar et al., 2002), adhesive bonding is also used in microsystems packaging. Microelectromechanical (MEMS) or micro-optoelectromechanical (MOEMS) systems are fabricated using micromachining processes. Their integration into a product requires packaging. Adhesives are finding many applications in structural bonding such as wafer to wafer or wafer to substrate, as an encapsulant or to mount fibre optics (Fig. 19.11). In a flip chip configuration, ECA adhesives have also been used and new applications are emerging such as for wafer transfer or for microgasketing in microfluidic systems.

19.8.9 Power electronics Power integration can be simply summarised as `more power less space'. Conductive adhesives can be used in die attachment for semiconductor

Electrical

483

Figure 19.11 Usefulness of adhesives in microsystems.

components dissipating less than 35 W/cm2. Beyond this, for power devices, die bonding is often carried out with high lead content Pb95Sn5 solder. When the power is greatly increased (>200 W/cm2), IGBT (insulated gate bipolar transistor) packs and power diode interconnections with metallised substrates are made by wire bonding or by direct bumping technology. Soldering of the whole uses either PbSbSn or SnAg solder. In high power electronics adhesive die bonding cannot be used because of electrical strength and reliability limitations. But adhesive bonding is involved firstly in the encapsulation of the power modules mainly by silicone polymers and secondly at the third-level packaging to bond the heat-dissipating ceramic Al2O3 or AlN metallised substrate, die bonded previously by soldering, to the cooling device. As soon as power is required the circuits may suffer from thermal dissipation so the increase in density of electronic components, which results from this integration, leads to cooling problems and to the definition of a new assembly process (Evieux et al., 2004) (Fig. 19.12). It involves structural non-conductive adhesive bonding for assembling a polyetherimide substrate used as a water collector with an AlN ceramic support for electrical packaging which dissipate the heat energy very well. Thus the adhesive joints must fulfil the utmost hygrothermo-mechanical behaviour. Therefore reliability of the joints is improved by the optimisation of surface treatments (Evieux et al., 2001, 2002a, 2002b). The adhesive's mechanical and chemical resistance against aggressive mediums are checked and the durability of the joints is studied, using an asymmetric wedgetest to measure the crack propagation rate and to observe interfacial degradation. As concluding remarks, adhesive bonding is finding many extended applications from microelectronics to power electronics. Some of them, which are the more important, are peculiar to this industrial field for first- and secondlevel packaging where electrical properties are needed. Adhesive bonds in third-

484

Adhesive bonding

Figure 19.12 Diagram of the cross section of a power convector cooling device (courtesy PEARL Laboratory).

level packaging often involve the same requirements as in other industries: mechanical strength and toughness, environmental resistance, hermeticity, reliability and durability. All industries and end-users of electronic products are subject to adhesive bonding: automotive, aerospace, military, telecom, computers and information retrieval, manufacturing and consumer goods, including smart-cards and smart labels.

19.9 Conclusion The key requirements of electronics industries are more integration with optimised performance and reliability, whatever the application. Place gains and weight gains are simultaneous needs and consequences of increased integration. Health and durable development are also of concern. Adhesive bonding can broadly satisfy these expectations and it is no surprise that adhesives play a major role in electronics. Since the electronics industry has become so dependent on polymer-based adhesives, electronics production would be impossible without them. This chapter has pointed out both strengths and weaknesses of the various adhesive joints used in electronic packaging. Some comparisons have been made to highlight what is peculiar to electronic adhesive bonding and what is common to adhesive bonding whatever the application. For example, conductive adhesive bonding, underfilling and encapsulation are typically electronics industries own processes. And the trend to avoid cleaning and to ban lead soldering will expand the use of conductive adhesives in the future. In particular, anisotropic conductive adhesives are very promising for fine-pitch

Electrical

485

capability enhancement without underfilling for flip chip joining which could be simply, flexibly and cheaply processed at low temperature. Adhesive bonding is also volume and weight saving compared to soldering and wire bonding, which contributes to better performance and environmental protection. In all cases, reliability is the main requirement since components, circuits and adhesive packages must have an actual 20 to 30 years' useful life before becoming obsolete. Thus, to keep the same levels of strength and toughness during its lifetime, even if these levels are average, is better than to have short-term higher levels. As soon as more power integration is required, such as for railway traction drives and more and more for cars, buses and trucks, adhesive bonding cannot be reliably used for die bonding and interconnections. Nevertheless research work on metal-filled thermostable adhesives offers some prospects. Heat dissipation of power modules is performed by air or water cooling devices. Adhesive joints used to bond the die-attached ceramic substrates to the devices must have the best hygro-thermo-mechanical reliable behaviour. Ultimately, adhesive bonding could advantageously replace metallurgical soldering for numerous electronic packages. With excellent processing capability at low cost, reworkability at low temperature of thermoplastic adhesives and good recycling capability, electronic adhesives seem to have a great future.

19.10 References Bartholomew M (1999), An Engineer's Handbook of Encapsulation and Underfill Technology, Isle of Man, Electrochemical Publications. Bauer A and Gesang T (1999), `Electrically conductive joints using non-conductive adhesives (NCAS) in surface mount applications', in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 313±341. Cognard J and Ganfguillet C (1983), `Process for connecting two conductors', Patent Application, No. 83 10015, France, 1506. Doyle R (1997), `Technology for plastic encapsulated devices', National Engineering Laboratory Focus Group Workshop, Coventry, November 1997. Dudek R, Walter H and Michel B (2001), `Studies on parameters for popcorn cracking', 1st Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Postdam, IEEE, 140±148. Emery M (1996), `Applications of adhesive bonding to the leading edges of helicopter blades', 11th SURFAIR Conference, Toulouse, 1±8. Evieux J, Petit S, Nassiet V, Baziard Y and Petit J A (2001), `Durabilite d'assemblages colleÂs structuraux aÁ substrats en nitrure d'aluminium (AlN)', 11th Adhesion Conference JADH 2001, LeÁge-Cap Ferret, SFV, 80±84. Evieux J, Mistou S, Dalverny O, Petitbon A, Nassiet V, Baziard Y and Petit J A (2002a), `Study of an asymmetric wedge-test: application to packaging in power electronics', 2nd World Cong. Adhesion and Related Phenomena, Orlando, The Adhesion Society, 24±26. Evieux J, Montois P, Nassiet V, Baziard Y and Petit J (2002b), `Durability of

486

Adhesive bonding

polyetherimide structural adhesive joints in a hot/wet environment', 6th European Adhesion Conference EURADH 2002, Glasgow, IOM Ed, 330±334. Evieux J, Montois P, Nassiet V, DedryveÁre R, Baziard Y and Petit J A (2004), `Study of bonded plasma treated polyetherimide power integration components: durability in hot/wet environment', Journal of Adhesion, 80, 263±290. Gilleo K (1999), `Introduction to conductive adhesive joining technology', in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 1±16. Hieber H and Thews W (1987), `Process for Making an Electrically Conductive Adhesive Connection', European Patent: EP 237114/B& 920708/Application: A287ë916. http://www.adhesives.de Jagt J C (1998), `Reliability of electrically conductive adhesive joints for surface mount applications: a summary of the state of the art', IEEE Transactions on Components, Packaging and Manufacturing Technology, Part A, 21(2), 215±225. Jagt J C (1999), `Reliability of electrically conductive adhesive joints in surface mount applications' in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 272±312. Jagt J C, Beris P J M and Lijten G F C M (1995), `Electrically conductive adhesive: a prospective alternative for SMD soldering?', IEEE Transactions on Components, Packaging and Manufacturing Technology, Part B, 18(2), 292±298. Keusseyan R L and Dildaj J. L (1993), `Electric contact phenomena in conductive adhesive interconnections', International Symposium on Microelectronics, Isle of Man, IEEE, 44±49. Kriebel F and Seidowski T (2001), `Smart labels ± high volume applications using adhesive flip chip technologies', 1st Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Postdam, IEEE, 304±308. Kristiansen H and Liu J (1999), `Overview of conductive adhesive interconnection technologies for display applications', in Liu J, Conductive adhesives for electronics packaging, Isle of Man, Electrochemical Publications, 376±399. Levallois F, Helt S, Baziard Y and Petit J A (1999), `Structural adhesive bonding of sintered silicon carbide (SSIC) subjected to thermal treatment in air atmosphere', J. Adhes. Sci. Technol., 13, 273±287. Liu J (2000), `ACA bonding technology for low cost electronics packaging applications ± current status and remaining challenges', in Adhesives in Electronics 2000, Helsinki, IEEE, 1±15. Liu J and LundstroÈm P (1999), `Manufacturability, reliability and failure mechanisms in conductive adhesive joining for flip chip and surface mount applications', in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 212±255. Liu J, Lai Z, Kristianen H and Khoo C (1998), `Overview of conductive adhesive joining technology in electronics packaging applications', 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Binghamton, NY, IEEE, 1±18. Lu D, Tong Q K and Wong C P (1999), `A study of lubricants on silver flakes for microelectronics conductive adhesives', IEEE Transactions on Components and Packaging Technologies, 22(3), 365±371. Luo S, Yamashita T and Wong C P (2000), `Adhesion performance and thermomechanical property of epoxy-based underfill', in Adhesives in Electronics 2000,

Electrical

487

Helsinki, IEEE, 70±76. Lutz M A and Cole R L (1990), `High performance electrically conductive silicone adhesives', Hybrid Circuits, 23, 27±30. Lyons A and Wong C P (1999), `Recent advances and evaluation of anisotropically conductive adhesives for microelectronics assembly', in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 183±211. Murakami G (2001), `Semiconductor packaging technology for mobile phones in Japan', 1st Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Postdam, IEEE, 9±19. Olsson K, Johansson D, Li S, Ovesen K and Liu J (2002), `Isotropically conductive adhesives for high power electronics applications', 2nd Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Budapest, IEEE, 29±37. Ongley D E (1995), `New innovations in thermoplastic die attach adhesives for microelectronic packaging', International Seminar: Latest Achievements in Conductive Adhesive Joining in Electronics Packaging, Eindhoven, Philips, 69±85. Pearson R A (2000), `Adhesion studies for flip chip assemblies' in Adhesives in Electronics 2000, Helsinki, IEEE, 35±40. Petit J A (2002), `Lifetime engineering and models of the evolution of systems: application to structural adhesive bonded joints', 16th Int. Symp. Structural Design and Engineering around Adhesive Bonding ± Swiss Bonding 2002, Rapperswil, Swibotech, 337±340. Petit J A and Baziard Y (1998), `Loss of adherence and durability of adhesive joints', 1st World Cong on Adhesion and Related Phenomena, Garmish-Partenkirchen, Dechema, 203±205. Petit J A and Baziard Y (2000), `Adhesive bonding of ceramic materials: a review', 5th European Adhesion Conf. Euradh '2000, Lyon, SFV, 204±209. Pope M and Swenberg C E (1982), Electronic Processes in Organic Crystals, New York, Oxford University Press, 273±336. Pujol J M, Prud'homme C, Quenneson M E and Cassat R (1989), `Electroconductive adhesives: comparison of three different polymer matrices, epoxy, polyimide and silicone', Journal of Adhesion, 27(4), 213±229. Puligandla V (2000), `Role of adhesion and its reliability implications in electronics assemblies', in Adhesives in Electronics 2000, Helsinki, IEEE, 28±34. Rigo S, Goudeau P, Desmarres J M, Masri T, Petit J A and Schmitt P (2003), `Correlation between X-ray micro-diffraction and a developed analytical model to measure the residual stresses in suspended structures in MEMS', Microelectronics Reliability, 43, 1963±1968. Rusanen O (1999), `Replacing solder with isotropically conductive adhesives in die bonding of power semiconductors' in Liu J, Conductive Adhesives for Electronics Packaging, Isle of Man, Electrochemical Publications, 359±375. Sarvar F, Hutt D A and Whalley D C (2002), `Application of adhesives in MEMS and MOEMS assembly: a review', 2nd Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Budapest, IEEE, 22±28. Shi S H and Wong C P (1999), `Recent advances in the development of no-flow underfill encapsulants ± A practical approach towards the actual manufacturing application', 49th Electronic Components and Technology Conference, Maryland, IEEE, 770± 776. Shimada Y, Lu D and Wong C P (2000), `Electrical characterizations and considerations

488

Adhesive bonding

of electrically conductive adhesives (ECAs)', International Symposium on Advanced Packaging Materials, IMAPS, 336±342. Shore A A (1988), `Adhesive bonding hybrid microcircuit substrates with a thermoplastic film', SAMPE Quarterly, 49±53. Suhir E (2002), `Accelerated life testing in microelectronics and photonics, its role, attributes, challenges, pit falls, and its interaction with qualification tests', 2nd Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Budapest, IEEE, 44±48. Takeichi M and Nagashima M (2001), `Trend of solder-less joint in flip chip bonding', 1st Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Postdam, IEEE, 168±172. Vogel D, Gollhardt A, Walter H, Dudek R, KuÈhnert R and Michel B (2001), `m-Test ± a new approach to measure material properties for microscopic specimens', 1st Int. IEEE Conf. Polymers and Adhesives in Microelectronics and Photonics, Postdam, IEEE, 366±374. Wong C P, Lu D and Tong Q K (1998), `Lubricants of silver fillers for conductive adhesives applications', 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Binghamton, N Y, IEEE, 184± 192. Zhang Z and Wong C P (2001), `A novel approach for incorporating silica fillers into noflow underfill', 51th Electronic Components and Technology Conference, Orlando, IEEE, 310±316. Zhang Z, Lu J and Wong C P (2001), `A Novel Process Approach to Incorporate Silica Filler into No-flow Underfill', Provisional Patent 60/288, 246. Zhang Z, Lu J and Wong C P (2002), `Double-layer no-flow underfill materials and process', 2nd Int. IEEE Conference on Polymers and Adhesives in Microelectronics and Photonics, Budapest, IEEE, 84±91. Zwolinski M, Hickman J, Rubin H, Zacks Y and McCarthy S (1996), `Electrically conductive adhesives for surface mount solder replacement', IEEE Transactions on Components, Packaging and Manufacturing Technology, Part C, 19, 241±250.

20

Aerospace L J HART-SMITH

20.1 Basic needs There are two basic classes of adhesive bonding in aerospace structures. One is structural bonding, with epoxy, phenolic, or acrylic adhesives, that transfers loads between members. The other is sealants, to protect against corrosion at interfaces. The stiffnesses of these classes of polymers differ greatly, but the two basic needs are remarkably similar. The first is that the adhesive or sealant will stay stuck for the life of the structure, in all service and storage environments, while the second is that the adhesive will not fail even when the surrounding structure has been broken. Given that the mechanical properties of these polymers are orders of magnitude lower than those of the adherends they bond together, this second requirement may seem difficult to achieve. Nevertheless, it has been possible, by distributing the load transfer as a shear load over a sufficiently large area. The key is to protect the adhesive from direct or induced peel loads while using the shear capability of the adhesive to transfer the loads. This requires efficient joint designs, which are easily established for thin structures, but which become progressively more complicated for thicker structures. The details of the joints are described here and in other chapters in the volume. Ensuring that the adhesive or sealant is stuck properly in the first place, and that it continues to adhere for the life of the structure, is actually the more challenging of the two needs. The science of adhesion is vital to the success of bonded joints, but its importance is largely unrecognized today.* The greatest *

This chapter is presented to encourage improvements to be made throughout the industry and regulatory authorities. Without these, it is unlikely that the full benefits of this technology will ever be attained, because lapses in quality can result in dramatic failures. Even so, with the exception of the Aloha 737 incident in 1988, the kind of surface preparation and processing problems described here have resulted only in additional costs, which have inhibited the application of this technology, rather than created circumstances that made safety a concern. Rather than present information in the form of a handbook, the contents of this chapter describe the key issues that need to be understood to create reliable bonded structures, such as why it is imperative to ensure that the bonded joints will never be the weak link in the structure, rather than design to nominal loads.

490

Adhesive bonding

difficulty in this regard is that success is completely dependent on correct surface preparation and processing, which cannot be verified by any of the standard non-destructive post-inspections. Worse, while the correct processing techniques are well known, incorrect process specifications continue to be used because they do not always result in instantaneous separations between the components. The visible indication of a problem is often delayed. This doubt is reinforced by inspection methods that detect only physical gaps between the members, not whether or not they are actually bonded together. The procurement system makes it difficult for military services to allocate costs that would be incurred to update repair manuals for products that are long out of production, but still in service. There is also a concern that any change in specifications would cast doubt on the integrity of every part made under the older procedures, regardless of whether or not such concerns could be justified. Sadly, the consequence of not implementing improvements promptly has been a reluctance to apply bonding technology as widely as it ought to be. Good bonding processes are actually remarkably tolerant of generous deviations from nominal requirements, but when the specifications are inappropriate, or not followed, the consequences are widespread. Worse, defects in mechanically fastened structures tend to be local and 100 percent of the design strength can be restored by drilling out and replacing defective fasteners or by replacing a few discrete parts. But processing errors in bonded and composite structures create situations in which no repair short of complete disassembly and remanufacture can restore 100 percent of the design strength and durability. It is not possible to inspect quality into bonded and composite structures afterwards; they need to be made correctly in the first place. The cultural changes needed to ensure this are actually the greatest of all the obstacles to more widespread use of bonded structures. Conversely, the durability of those bonded structures that were made appropriately is legendary, in regard to both fatigue and corrosion. The incentives for widespread use of bonded aerospace structures remain tremendous.

20.2 Adhesive characteristics required for design and analysis At the macro level, at which most bonded joint analyses are made, the mechanical data needed are complete stress-strain curves in shear for the operating temperature range in the appropriate environment. Typical structural adhesives are relatively stronger and more brittle at low temperatures and weaker and more ductile at high temperatures than at room temperature, as shown in Fig. 20.1. Despite these great differences in bulk individual mechanical properties, the strength of structural joints (in which the adhesive strains are far from uniform) is far less sensitive to the test temperature than that of short-overlap test coupons

Aerospace

491

Figure 20.1 Adhesive stress-strain curves in shear as a function of temperature.

(in which the adhesive strains are close to uniform, which is why they are used to generate the stress-strain curves). The reason for this is that the strength of structural joints can be expressed in terms of the adhesive strain energy in shear, which is far more consistent with the test data than the initial moduli or shear strengths. This is explained in more detail below, in terms of elastic-plastic adhesive models.1 At the micro level, initial damage in bonded joints is defined by the first strain invariant of the adhesive, regardless of the specific stress components. This model was first identified by Gosse.2 The knee in the stress-strain curve in shear is not really the start of plastic behaviour, as the standard adhesive models would suggest. Rather, it is the onset of the formation of hackles formed at roughly 45ë from the adherends being bonded together, as shown in Fig. 20.2. (Exactly the same model applies for in-plane-shear failures in the matrix between parallel fibres in composite laminates.) Once hackles start to form, the adhesive layer is no longer a continuum, but a series of parallel ligaments being bent elastically. There is no precise model for progressive damage to adhesive layers as more and more shear strain is applied, but a constant-stress model has served as a very useful model. This is explained in Fig. 20.3, based on the notion that damage starts at the knee in the stress-strain curve, and that there should be some limit on how much can be tolerated, like the 2-percent offset definition of design ultimate load for mechanically fastened joints in ductile members. The tail end of the stress-strain curve is reserved for load redistribution around bond flaws and damage, as discussed later. It is recommended here that the knee in the stress-strain curve be defined to be the design limit load for each environment, but it should be acknowledged that the

492

Adhesive bonding

Figure 20.2 Formation of hackles in adhesively bonded joints.

bonded structures and composite repair facility for the Royal Australian Air Force, in Amberley, Queensland, has successfully set the design limit strain at twice that value. What is now clear is that the original notion1 of designing to ultimate design load at the very end of the stress-strain curve is no longer appropriate. In the event that the adhesive is so brittle, in a particular environment, that there is insufficient `non-linear' behaviour to satisfy the conditions described in Fig. 20.3, design ultimate strength is established at the end of the short stress-

Figure 20.3 Relations between knee in stress-strain curve and design limit and ultimate loads.

Aerospace

493

strain curve in shear, and the design limit load is then set at two-thirds of that load. These conditions are more frequently encountered in deep space than in atmospheric flight conditions. Brittle adhesives are used on aircraft primarily in high-temperature applications such as the vicinity of engines. In such environments, even brittle adhesives are quite ductile. The invariant polymer failure model permits separate analyses for shear and induced peel loads to be interacted. The need for this is mixed since, even without it, it had already been established that the only effect of significant induced peel loads, caused by bending of the adherends in both double-lap and single-lap joints, is to detract from the shear capability of bonded joints. Therefore, good design practice always called for gentle tapering of any thick ends of bonded adherends, to reduce the peel stresses to insignificance. However, the new model puts this technique on a secure scientific foundation and also enables account to be taken of any applied transverse shear loads. The value of the first strain invariant, the sum of three orthogonal strain components, J1 ˆ 1 ‡ 2 ‡ 3 ;

20:1

cannot be measured on any pure (neat) adhesive test coupon, because failure by distortion according to the other (von Mises shear strain) invariant, r i 1h 0

crit ˆ …1 ÿ 02 †2 ‡ …02 ÿ 03 †2 ‡ …03 ÿ 01 †2 ; 20:2 2 (where the prime denotes principal values) occurs before the dilatational limit has been reached. Failure of polymers by dilatation (increase in volume) occurs only in a constrained environment, as between two circular rods bonded at their ends and pulled apart, as in Fig. 20.4. Transverse constraints, perpendicular to the applied load, are needed to prevent the natural Poisson contractions. The stress-strain curves of adhesive layers under a close-to-uniform state of deformation, as in Fig. 20.1, are customarily measured on a thick-adherend test coupon using very sensitive displacement extensometers, developed by Krieger,3 as shown in Fig. 20.5. Each aluminium adherend is typically 9.5 mm (0.375 inch) or 12.7 mm (0.5 inch) thick, to enforce a close-to-uniform shear strain in the adhesive. Most adhesives are still characterized by the average shear stress measured on standard lap-shear test coupons (ASTM D-1002), in a variety of environments. This can be misleading in the context of joint design, since this so-called adhesive shear strength varies with the adherend material, their thickness, usually 1.3 mm (0.063 inch), and with the length of the overlap.4 Such coupons are better suited to quality-control tests, for which their complex mixture of variable shear and peel stresses make them very useful. Some researchers have attempted to characterize fracture-mechanics properties under crack-opening and shear modes, assuming that these properties could form the basis of bonded joint design. However, no such value can be

494

Adhesive bonding

Figure 20.4 Butt-jointed test coupon to measure J1 strain invariant for adhesives.

related to the long overlap needed to ensure that the minimum shear strain in the middle of the overlap, where the adhesive is not critical, is low enough to prevent the accumulation of creep damage to the bond line. If the bonded joint is critical under peel stress (Mode I) loads at the ends of the overlap, it should be redesigned to make the adherends thinner at the ends, so that the crack-opening mode is no

Figure 20.5 Thick-adherend adhesive test coupon and instrumentation.

Aerospace

495

longer critical. It makes little sense accurately to establish the low strength of a badly designed bonded joint when it is so simple to modify the design to achieve a far higher strength, for which the adhesive is not critical in peel ± one in which the merits of the adhesive's shear strength are allowed to dominate.

20.3 Surface preparation Proper preparation of surfaces to be bonded is the most critical step in creating durable bonded joints. This was demonstrated in the late 1970s and early 1980s for bonded aluminum structures, by the USAF-funded Primary Adhesively Bonded Structures Technology (PABST) bonded fuselage made by the former Douglas Aircraft Company in Long Beach, California, in response to almost ten years of widespread in-service disbonds associated with the first generation of 250 ëF (180 ëC) cured epoxy adhesives in combination with etched, rather than anodized surfaces. All earlier bonded aircraft structures (done mainly by de Havilland in England and Fokker in Holland), had been trouble free ± because they used phenolic adhesives over chromic-acid anodized surfaces. In the 1940s and 1950s, the pioneers in this field, such as deBruyne5 and Schlieklemann6 had studied potential durability issues very thoroughly before they undertook production bonding. Their diligence, however, created the illusion that all bonded structures were durable. When others later changed materials and processes to simplify the manufacturing of bonded structures, they did not conduct thorough durability testing and relied instead on short-term static tests alone. The consequences were economically disastrous. The entire fleet of aircraft built to U.S. bonding specifications during the late 1960s and early 1970s had all their bonded structures remanufactured. Local repairs to only those portions of the structure that had already separated were unsuccessful. Nearly half of all bonded repairs were to the same structures that had already been repaired for the same global processing errors.7 The disbonds would occur only after water was absorbed into the adhesive layer, which took time, but was inevitable. Local repairs are useful for impact damage to a properly bonded structure, in which none of the surrounding structure is likely to disbond, but they are unsuitable for global processing errors, as shown in Fig. 20.6. All of this should have come to an end in the middle 1970s, when phosphoricacid anodizing and optimized etch processes were established, particularly since Bethune at Boeing developed the simple wedge-crack test,* ASTM D-3762,8 *

It should be noted that, while the test procedure is very reliable, it is rendered ineffective if the `permissible' extent of interfacial disbond growth during the one-hour-long test is set so high that it fails to reject unacceptable surface treatments. The tests in ref. 8 show that the limit should have been set at no greater than 0.125 inch (3.2 mm), which is consistent with the very large number of PABST tests that never showed more than 0.063 inch (1.6 mm) of growth. Yet the limit set for the ASTM standard was 0.75 inch (19 mm) which, according to the tests reported8 would result in more than 95 percent of all of their bonded structures having been `acceptable', no matter how bad they actually were.

496

Adhesive bonding

Figure 20.6 The futility of local bonded repairs to global interfacial failures. Exterior and internal views of the same bonded honeycomb panel, after a second failure (large dark grey area), showing innumerable small holes drilled to inject resin after the first failure, to make it impossible to detect those disbonds, and the shiny resin-free inner skin surface to which the injected adhesive (two irregular light grey areas) had clearly never bonded.

that made it easy to distinguish between properly and improperly prepared bonding surfaces. The properly prepared surfaces had a stable oxide coating, with many pores that the primer could penetrate, as shown in Fig. 20.7. One key element of the successful PABST bonding processes was the use of a phenolic-based primer, BR-127. And it is particularly significant that the first generation of 250 ëF (180 ëC)-cured epoxy adhesives that absorbed water so fast and were associated with the many earlier in-service bond failures, were later found to perform just as reliably on the improved surface treatments as the second generation of such adhesives that were far more resistant to water absorption. Sadly, even 20 years later, the author was to see local repairs being made to large engine cowls using the same obsolete surface preparations that had caused

Figure 20.7 Representation of pores in anodized aluminium surface prepared for bonding (source: ref. 9).

Aerospace

497

the failures that were being repaired. When he pointed out the inevitability of further failures in the not too distant future to the rest of the panel, it was explained to him that the use of the better newer materials and processes in the repairs would violate the aircraft's type certificate. The repair organization had no option but to conform with the bonding processes and materials with which the original structure was built! This attitude was not confined to commercial aircraft. Repairs to helicopter blades continued to be made using paste adhesives over sand-paper abraded aluminium surfaces. In this case, the explanation provided was that the model concerned had been out of production for over 20 years, so there was no mechanism to update the Technical Orders governing its maintenance and repair. In these cases, and many others, the futility of the obsolete repairs was well understood, even by those performing them. Is it not time to put an end to such practices? Surely the interests of aviation are better served by reliable surface treatments for bonding than by perpetuating other techniques that are known to be unreliable. Interestingly, the same organization that provided the service data cited7 has since found that they suffered not a single failure on 3,000 bonded composite patches they applied over grit-blasted/ silane aluminum surfaces. The financial incentives to mandating the use of only reliable surface treatments are well established. Other metal alloys, such as steel and titanium, also need appropriate surface treatments for bonded applications. The surface treatments for various metals are described in other chapters in this volume. For bonded and co-bonded fibrous composite structures, the need for proper surface treatment is far less known. The absence of reliable after-the-fact inspection techniques that hinders the more widespread use of metal bonding is exacerbated by the failure to include any durability tests as part of the quality assurance program for the production of bonded composite structures. Just as with the adhesive bonding of metallic structures, the lap-shear test coupon alone has been found to be insufficient to ensure the durability of bonded composite joints. At best it can ensure only that the adhesive was cured properly (adequately cross-linked) ± not that the adhesive is actually stuck. The need for the surfaces being bonded to have a higher energy level (be more active) than the surface of the adhesive is just as great for bonding polymers as for bonding metal alloys. Most of the surface treatments for composite surfaces to be bonded ensure precisely the opposite ± that the surface will be totally inert. Some peel plies have even been coated with a silicone release agent to ensure that the peel plies can be stripped off without the slightest damage to the underlying substrate.10 This is not to imply that the problems with bonded composite aerospace structures are as widespread as they were with bonded metallic structures. Rather, the problem is that we just do not know and have no way of finding out before the parts separate in service. If there were a mandated durability test for bonded composite structures equivalent to the wedge-crack test at the time of manufacture, we could produce bonded

498

Adhesive bonding

composite structures with complete confidence that they will not fall apart at some later date. Of all the specified surface treatments for bonded composite joints, lowpressure grit blasting has the best service record. In addition, it (or some other form of mechanical abrasion) is the only method that can possibly be used for bonded repairs, whether they are made at the time of initial manufacture or some time later. Therefore it should be considered a standard process that all manufacturers and repairers of composite structures need to master. Another critical issue with bonding of composite structures is pre-bond moisture, which can exist in a multiplicity of forms, each with its own distinct adverse consequences. Moisture absorbed in cured laminates and driven to the surface by the heat of the bond cycle prevents any adhesion from occurring, without creating detectable symptoms. Such water lowers the energy level of the substrate,11 and can lead to the condition illustrated in Fig. 20.8, in which a complete area failed to bond but was not detected. Figure 20.8 shows a perfect replica of the original peel ply, known to be free of silicone, on both sides of the adhesive layer. There was no sign of adhesion at all. Yet this and many similar panels had passed all quality-control lap-shear test coupons and 100 percent ultrasonic inspection at the time of manufacture. It is known that these parts had been cured many months before they were bonded in another part of the country; they were not dried in an oven before bonding. Clearly, they should have been. Moisture absorbed by the adhesive before it is cured has the effect of creating a weak porous bond. Moisture condensed on the surface of uncured adhesive films taken out of the freezer and unwrapped before they have thawed out also collects at the interface and prevents the glue from sticking. The entire supply chain needs to dry laminates properly before bonding. Better yet, the manufacturing plan should be such as to require that the bonding be completed so quickly after the parts are made that there is no time for any pre-bond moisture to be absorbed. The benefits from doing so are described elsewhere.12 Peel plies continue to be the most frequently specified surface preparations for bonding fiber-polymer composites. But they are not always reliable. Worse, there have been cases in which silicone was transferred from bagging film used in consolidation cycles. There is a tendency to under-rate the importance of the so-called `consumable products', like bagging film, breather, peel plies, etc., that are stripped off cured or staged parts and discarded after the cure. Some specifications even leave the selection of these materials entirely up to suppliers, with no restrictions. That is wrong! All surfaces that come in contact with the final composite part in both its uncured and cured states need to be controlled just as tightly as the pre-preg itself. After all, most of them are cured together in intimate contact. It is standard practice to isolate tool-preparation rooms in which release agents are applied from any bonding or composite clean room. Workers there wear clean white gloves. Every article that comes in contact with uncured adhesives or composite materials, or surfaces to be bonded, needs to be

Aerospace

499

Figure 20.8 Peel-ply imprint left by failure of adhesive to bond to a composite surface (5). Note clear imprint at foot of other peel-ply in skin underneath adhesive layer to which the adhesive also failed to bond.

treated as a potential source of contamination or a non-bond, no matter how insignificant the article seems to be. Not all procedures are as strict as this, but the loopholes in the paperwork need to be closed. There is one specific polyester peel ply that several organizations within Boeing prefer that is made on dedicated machines to eliminate contamination. This is the only peel ply permitted to be used for the 777 horizontal and vertical tails. Even so, the most stringent of tests are required to ensure the absence of silicone on each roll before it is allowed into the factory. Their prior experience and current research sponsored by the FAA13 both confirm the need for such care. The author would still prefer to see the universal acceptance of something equivalent to the wedge-crack test for metal bonding ± a sustained peel load in a

500

Adhesive bonding

hostile (hot/wet) environment to confirm the absence of all other mechanisms for creating inert bonding surfaces, as well. Any such lapse in quality will affect the entire structure. It is not difficult to detect the conditions that lead to global processing errors before they occur, provided that one looks, and nothing can be done about them if they are not detected until after the part is built.

20.4 Design of adhesively bonded joints* The design of double-lap and single-lap bonded joints between nominally uniformly thick adherends is straightforward. The design of the 100 percent full-load transfer bonded joints with no fail-safe rivets for the pressurized PABST bonded fuselage,9 was reduced to a single table of overlap versus skin thickness, supplemented by a requirement to gently taper the ends of the overlaps locally for the thicker skins, to prevent premature induced peel failures. It is crucial to note that the overlaps are universal in the sense that they are independent of the magnitude of any applied loads. This enables the design of the bonded joints to be completed before the internal loads in a structure have been established. The key to this design method is explained in Fig. 20.9, for double-lap joints, in which the overlap is established as the sum of the elastic trough needed to ensure that the minimum adhesive shear stress is so low as to prevent creep from occurring there under even sustained loads and the `plastic' load transfer zones needed to transmit the full strength of the adherends outside the joint. As first presented, this method is conservative in the sense that no credit is taken for the small increment of load transferred through the elastic trough. When designing patches in confined areas, the width of the plastic zones could be reduced by a length 2/ to compensate for this effect. All of the tests during the PABST program to validate the method were at the slightly longer overlaps. Figure 20.10 presents the actual sizes used for the PABST fuselage, as a function of basic skin thickness. The splice plates should be made half as thick as the skins, for maximum theoretical efficiency, to make each end of the bonded overlap equally efficient. However, early tests of such joints showed a propensity for fatigue failures in the splice plate, where the skins butted together, rather than in the skins, which *

The design and analysis procedures cited in this chapter were developed by the author under three government-sponsored R&D contracts over a period of years. The first was for NASA Langley during the period 1970 to 197314 in which the elastic-plastic adhesive model was introduced and the first of the A4E. . series of Fortran computer codes was derived. The second increment of work was for the USAF at WPAFB, during and following the PABST bonded fuselage contract, from 1976 through 1983. Three new computer codes, A4EI, A4EJ, and A4EK were produced and the effects of flaws and variable bond layers were covered.15 There were many publications from the first two contracts. The third, also for the USAF, concerned bonded composite patches over cracks and corrosion damage in metallic structures, from 1997 to 2003.16 Not all of the analyses from the most recent contract have yet appeared in print.

Aerospace

501

Figure 20.9 Design procedure for double-lap bonded joints.

had a nominally equal stress. Consequently, the splice plates were made one gauge thicker. The overlaps in this table can be approximated by a simple design rule, that the overlap be 30 times the thickness of the central adherend (skin). If there is a substantial stiffness imbalance between the adherends, the load transfer through the bond will be intensified at one end, with respect to the other, resulting in decreased shear strengths.

Figure 20.10 Standard design overlaps for double-lap bonded joints between aluminium adherends.

502

Adhesive bonding

Figure 20.10 includes a comparison between the theoretical bond strengths if the adhesive were strained to the very end of the stress-strain curve in shear, and the strength of the 2024-T3 skins. For the thinnest skins, the bond has three times the strength of the skins while, for the thickest skins in the table, the ratio has fallen to about 1.5:1. This is why, for still thicker skins, it would be necessary to resort to splice designs with one or more steps in the overlap, to restore the margin in the bond strength. The need for this excess strength is described in a later section of load redistribution around flaws and defects. The information in Fig. 20.10 needed to be supplemented only by the design modifications in Fig. 20.11, whereby the ends of the overlap of the thicker adherends were tapered down to a maximum tip thickness of 0.030 inch (0.75 mm), to minimize induced peel stresses in the adhesive. The corresponding tip thickness for fibre-polymer composite adherends is only 0.020 inch (0.50 mm), because of interlaminar weaknesses. If this local tapering were omitted for the thicker adherends, the adhesive would fail under induced peel stresses long before its shear strength could be attained. The design of single-lap bonded joints is even simpler, because the most critical location for any long-overlap bonded joint is in the adherends, at one or both ends of the adherends, as the result of combined membrane and bending stresses. These are minimized by increasing the overlap.17 Analyses for the PABST bonded fuselage established a design overlap-to-thickness ratio of 80:1. (This can be reduced to 60:1 for single lap joints that are stabilized against bending of the adherends, such as by being part of a sandwich panel.) Tapering of the ends of the overlap, to alleviate induced peel stresses is still necessary, using the proportions shown in Fig. 20.11. The bending moment at one end of

Figure 20.11 Tapering at ends of bonded overlap, to restrict induced peel stresses.

Aerospace

503

the overlap is intensified greatly whenever one adherend is stiffer (thicker) than the other. It is not possible to make bonded single-lap or single-strap (flush) bonded joints with a strength greater than the adherends outside the bonded overlap, so they tend to be used only for thinner more lightly loaded structures. The analyses referred to above are essentially exact, but can be applied only to bonded joints between adherends of uniform thickness. A new approximate method was developed during the Composite Repair of Aircraft Structures (CRAS) R&D contract that covers tapered adherends as well. The method relies on the knowledge that the adhesive stresses will be extremely low everywhere except in the immediate vicinity of changes to or interruptions in the thickness of the adherends.18 It starts with a closed-form analysis in which it is assumed that all the members are fused rigidly together over the entire overlap. The load sharing between the overlapped members can also account for residual stresses caused by thermal dissimilarities between adherends, as between composite patches and cracked metallic structures and between bonded titanium stepped plates at the ends and sides and the composite skins to which they are bonded. This level of analysis will predict local spikes of instantaneous load transfer at every discontinuity in load path. The second step in the analysis, also by closed form, distributes those spikes as plastic load-transfer zones of finite width, assuming locally uniform adherend thicknesses that match the real local thicknesses. If the plastic zone needed to transfer each load spike is less than the characteristic length 1/ defining the elastic load-transfer length, where  is the exponent of the elastic stress distribution (see Fig. 20.9), the load spike is replaced by an elastic load distribution with an integral matching the calculated spike. This simple method was shown to be exactly equivalent to the precise solutions in the event that the adherends really were of uniform thickness. Bonded joints between thicker adherends than those covered by Fig. 20.10 need stepped-lap joint designs, as in Fig. 20.12, for the higher loads associated with thicker members. Instead of an increment of load transferred at each end of the bonded overlap, there is an increment of load transferred at each end of each step in such a joint. The addition of more steps requires an increase in the bonded area, but it is important to note that an increase in bond area without any increase in the number of steps is ineffective, as explained in Fig. 20.13. For bonded composite joints, the strength is predicted to keep increasing all the way to a single ply per step. Each step in a stepped-lap bonded joint is characterized by exactly the same governing differential equations as govern simple overlaps between uniformly thick adherends. There have, for decades, now, been reliable analytical tools available for the design and analysis of adhesively bonded joints. Even so, there is still far too much reliance on the oversimplified model whereby the bond strength is assumed to be the product of some fictitious uniform adhesive `allowable' shear stress and the bond area. If more joint strength were needed, all one had to do,

504

Adhesive bonding

Figure 20.12 Stepped-lap bonded joints for thicker adherends.

according to this procedure, was to increase the bonded area. Bonded joints do not obey such rules. Such a formula is valid today only in the context of shortoverlap test coupons in which the goal is to create as closely as possible a uniform state of stress and strain in the adhesive. In the context of structural

Figure 20.13 Insensitivity of joint strength to bond area without an increase in the number of steps.

Aerospace

505

bonded joints, such a model ceased to be applicable by the end of the First World War, when airframes stopped being made from wood and fabric. That was the last time that the glue was stronger than the materials being bonded together, so that almost any design would work if the scarf angle were low enough. The most reliable of the mechanics-based bonded joint models are of closed form, because the locally very high stress and strain gradients, and the need for iteration to cover material nonlinearities make finite-element solutions difficult. The finite-element models need to be converged with respect to grid size, because accuracy of such analyses is not guaranteed. This problem has been exacerbated in the realm of fibre-polymer composites by the unjustifiable simplifying assumption that the fibre and resin constituents can be homogenized into a single `equivalent' anisotropic solid. A by-product of this error is the myth of singularities at the edges of composite panels, at every change in fibre direction, which has spawned a whole field of study. The singularities vanish the instant that the notion of zero-thickness interfaces is discarded. These singularities are created mathematically by conditions that do not exist in physical reality. One needs to be very careful in interpreting finite-element analyses of bonded joints. Conversely, it was only through the use of properly converged finite-element models of discrete fibres in a block of resin that Gosse was able finally to validate his concept of dilatation as the primary failure mechanism for constrained polymers in bonded joints and composite laminates. There were no singularities, and the correct answer was unchanged when the mesh size was doubled or halved.

20.5 Design features ensuring durability of bonded joints Durability in bonded joints requires both that the bonded interfaces are stable (the glue stays stuck) and that the adhesive is not failed by the combination of mechanical loads and residual thermal stresses caused by dissimilar adherends. The first issue has nothing to do with the geometry of any bonded joint, although joints fail faster under peel-dominated loads than under shear loads. There are two limits associated with durability that are influenced by the geometry of the joint. The first of these is the peak shear and peel stresses at the ends of each overlap. This is obvious and well understood. The other is to limit the strain level near the middle of the overlap, even in the most severe environment. This is the little understood requirement represented by setting the minimum stress level at or less than 10 percent of the maximum, which defines the overlap needed to prevent any creep from accumulating. The need for such a requirement was exposed by some of the early fatigue tests on the PABST program. Quite misleading conclusions, both positive and negative, could be drawn from durability tests on short-overlap coupons.9 The key to the success of

506

Adhesive bonding

Figure 20.14 Differences between short-overlap test coupons and longoverlap bonded joints.

these designs was the acknowledgement that the adhesives shear stresses were, and should be, highly non-uniform. There are enormous differences between the way adhesives behave, under what appear to be the same external loads, in shortoverlap test-coupons and long-overlap structural joints, as explained in Fig. 20.14. Tests on short-overlap coupons cannot be relied upon to differentiate between adhesives (and surface treatments) that will endure in service and those that will (or have) not. At best, they can make comparisons between slightly different adhesives in the same class. It is almost impossible to prove in the short term that a bonded joint will last 30 years or more in service. Any test under representative load conditions would have to last at least 30 years to indicate a satisfactory result. If the load intensities were increased, or the test environment aggravated to ensure a test result in a short time, there would be no way of knowing what the corresponding service life would be under realistic conditions. And the best adhesive systems would last for 30 years under realistic loads even when tested in artificially severe environments, unless the structural elements wore out first. The only saving grace is that the surface treatments associated with premature in-service interfacial failures could be accelerated and made to fail rapidly under adverse conditions. In other words, the inferior systems could be identified rapidly, but the best systems could be identified only by not appearing on the list of unacceptable systems.

Aerospace

507

The reason why properly designed bonded joints do not suffer from mechanical fatigue failures is that the most critical conditions are not developed whenever the adhesive is protected by the adherends. This can be understood by characterizing the minimum and maximum adhesive shear strains as a function first of bonded overlap and secondly as a function of adherend thickness, accounting for the effect of the environment in each case. This is illustrated in Fig. 20.15, for room temperature. The reason why short overlaps should not be used for structural joints is that even the smallest loads can result in critical conditions being developed at the ends of the overlap, under sustained load or by an accumulation of incremental loads, because there is no restraint on the minimum shear strain developed in the middle of the overlap. Once the overlap has exceeded a critical value, proportional to the thickness of the adherends, there is such a constraint on the minimum shear strain, at the middle of the overlap, that simultaneously imposes a limit on the peak shear strain at the ends of the overlap, through compatibility of deformations. No matter how long the loads are applied, the peak shear strain at the ends cannot grow indefinitely. Testing during the PABST program showed that creep accumulated steadily at the ends of the overlap as long as the load remained. However, it recovered during periods of unloading, every time, with exactly the same shear strain at the end of eight hours into the fifth load

Figure 20.15 Effects of adherend overlap and thickness on maximum and minimum shear strains at room temperature.

508

Adhesive bonding

cycle as the 14th, for example. There is no counterpart to this behaviour possible for short overlap test coupons, in which such creep occurs, but accumulates cycle by cycle instead of recovering. For still longer overlaps, the minimum adhesive shear strain decreases asymptotically towards zero, but the peak values remain constant, since the load that can be applied to the bond is limited by the strength of the adherends. So, provided that the design overlap is long enough to extend to the far side of the transition between short- and long-overlap behaviour, for the most critical of the environments, any further increases are of no benefit (apart from the provision of a reasonable assembly tolerance). The influence of the environments on the design overlaps is shown in Figs 20.16 and 20.17, for the hot/wet and cold environments. It is usually found that the upper service temperature limit sets the design overlap because then the adhesive is at its softest and weakest, while the lowest temperature establishes

Figure 20.16 Effects of adherend overlap and thickness on maximum and minimum shear strains at maximum service temperature.

Aerospace

509

Figure 20.17 Effects of adherend overlap and thickness on maximum and minimum shear strains at minimum service temperature.

the limiting joint strength, because that is the condition of least strain energy, but not by much, as noted in Fig. 20.1. Figures 20.15 to 20.17 also show that, for the thinnest adherends, there is a deep precipice on the peak shear strain when crossing the critical overlap, and that the precipice decreases with increasing adherend thickness. There was absolutely no precipice left for 0.25 inch (6.35 mm) thick adherends, indicating that the whole concept of preventing bond failures by limiting the peak adhesive shear strain would no longer prevail, even though the minimum adhesive shear strain could be made low enough. The key to success is that both limits on adhesive strain have to be present. This is why stepped-lap joints are necessary for thicker adherends. But with that design feature to limit the strains in the adhesive, the joints have proved to be just as durable as for the simpler joints used for thinner adherends. The stepped-lap titanium-to-carbon/epoxy bonded joints at the wing roots on the F/A-18 aircraft are a testament to this, at load intensities of almost 30,000 pounds per inch (535.7 kg/mm).

20.6 Load redistribution around flaws and porosity One of the most remarkable characteristics of well designed bonded joints, as defined earlier, is their tremendous tolerance for quite large local defects.

510

Adhesive bonding

Provided that the surface treatment and processing were correct, the damage would not spread. (On the other hand, quite the opposite was true in the case of global processing errors, for which it was only a matter of time for absorbed moisture to attack the interfaces on poorly bonded metallic structures. The mechanism for spreading initial failures on improperly bonded composite structures is not clear, but the result was just as inevitable.) The same state of non-uniform stress and strain that ensures the durability of properly designed bonded joints is responsible for the ability to tolerate local flaws with no loss of overall strength. It is self evident that, if the adhesive layer ever were uniformly critically strained, there could be no tolerance to the slightest flaw. Figure 20.18 shows the calculated adhesive shear stress distribution, at room temperature, for a bonded splice on the side of the pressurized PABST fuselage. What is remarkable is that, even at the 1.3-P proof pressure condition, for which the load in the skin is 1,000 pounds per inch (17.86 kg/mm), the adhesive was not even strained beyond its elastic capability at the ends of the overlap. (The lightly loaded elastic trough appears to be unnecessarily long, but its size is actually determined by the hot/wet environment, not by any room-temperature event.) If we now suppose that there is a half-inch disbond one quarter of an inch from the edge of the two-inch overlap per side, the adhesive stresses would be modified slightly, as shown in Fig. 20.19. The increment of load that used to be transferred through the now defective area is now shared by extra loads in the immediate vicinity of each side of the defect ± without affecting the peak stress at the ends of the overlap. This is indicative of the robust capacity of properly designed bonded joints to tolerate large local defects, provided that one can rely on the remainder of the bonded

Figure 20.18 Adhesive shear stress distribution for bonded joint with no defects.

Aerospace

511

Figure 20.19 Load redistribution due to local bond flaw near the edge of the overlap.

area remaining stuck. (This capacity is lost whenever the adjacent bonded areas are also on the point of failing.) If we further suppose that the same defect, or damage, had occurred right at the edge of the overlap, the redistributed adhesive stresses would be as shown in Fig. 20.20. Again, remarkably, the value of the peak adhesive stress would not be affected. It would simply be moved to the edge of the defect, where load

Figure 20.20 Load redistribution due to local bond flaw at the edge of the overlap.

512

Adhesive bonding

transfer again becomes possible. This kind of defect would need to be sealed to prevent the intrusion of water, which would freeze and expand at high altitude, thereby spreading the initial damage or defect, under what is known as the freeze/thaw cycle. However, the same size defect in Fig. 20.19 would best be recorded but otherwise left alone. Any attempt to repair it would break the environmental protection, by cutting through the primer and exposing bare untreated metal on the edge of any hole that might be drilled to enable resin to be injected to fill the gap and make the discrepancy undetectable in future. All that would be accomplished is to decrease the remaining life, without any increase in joint strength. If the surfaces have been prepared properly, most local damage will not spread. If they were not prepared properly, local repairs are pointless, since the adjacent bonded areas will soon need to be repaired themselves, as noted earlier. If the defect were created in the form of a trapped bubble in the middle of the overlap, as in Fig. 20.21, when the edges of the overlap were pinched off and there were too few small vent holes in the splice plates (this omission is really usually only a problem with large area doublers), there would be a large area of porosity where the gap between the adherends was too great for the adhesive layer to fill. The natural occurrence of porosity has been discussed elsewhere.19 Obviously, since there was no load being transferred there anyway, the presence of such occasional areas of porosity may be considered unimportant, unless it were to cause a misfit with adjacent stiffeners (which is a problem with largerarea bonded doublers with no vent holes). It should also be noted that it is impractical to fill every little bubble in the area of porosity and that, even if this

Figure 20.21 Load redistribution due to local porosity in a bonded overlap.

Aerospace

513

were possible, the gap between adherends could not be reduced and the locally thick bond layer could never pick up its designated increment of load anyway. The thicker bond layer necessarily associated with porosity, unless it occurred everywhere as the result of pre-bond moisture in the uncured adhesive film, ensures that the porous area will not fail, even if it occurred in an area of high nominal shear stress. What most porosity does is to transfer extra load to any adjacent thinner regions in the adhesive bond layer. Figures 20.18 to 20.21 are typical of local flaws in and damage to thin bonded structures. They are usually innocuous and should not be repaired, except when it is necessary to seal exposed edges to prevent the ingress of moisture. This is because of the large excess of strength of the bond over the adherends. For thicker structures, this excess strength is diminished and flaws and damage can become more significant, as has been explained elsewhere.19 The preceding examples refer to one-dimensional situations. When bond flaws are assessed in two dimensions, the need for the bond always to be stronger than the adherends becomes abundantly clear. If the adherends are stronger than a properly processed bonded joint with no defects, any large defect or damage acquires the characteristics of a through crack in metallic skins, as explained in Fig. 20.22. Provided that the bond outside the defective or damaged area is stronger than the adherends, the initial damage cannot possibly spread.

Figure 20.22 Two-dimensional load redistribution around a large flaw in a bonded overlap: (a) adhesive stronger than the adherends; (b) adhesive weaker than the adherends (fasteners needed to provide fail safety and to prevent catastrophic unzipping of bond).

514

Adhesive bonding

Instead, the diverted load will either initiate fatigue cracks in the skin, just outside the damage, or delaminations in composite laminates, at the same locations. In either event, there will be a long interval in which the damage can be detected before it becomes critical. Without such protection from what appears to be merely excess strength in a one-dimensional assessment, large bond flaws would behave like cracks in metallic structures, even if all of the surface treatment and processing had been the best in the world. This is why it is always necessary to design bonded joints so that they can never become the weak link in the load path.

20.7 Effects of thermal mismatch between adherends on strength of bonded joints When thermally dissimilar materials are bonded together, residual thermal stresses are developed that usually reduce the remaining strength available for transmitting mechanical loads. These phenomena occur whenever titanium edge members are bonded around the edges of composite panels to permit the use of mechanical fasteners during final assembly of the structure or to permit disassembly in service for inspection and repairs. These thermal stresses are roughly proportional to the difference in temperature between the curing and operational temperatures. Their analysis is explained20 and illustrated in Fig. 20.23.

Figure 20.23 Effects of adherend thermal mismatch on adhesively bonded joints.

Aerospace

515

The key issue is that, whereas the shear stresses and strains in the adhesive that are caused by mechanical loads have the same sign at each end of the overlap, the shear stresses and strains caused by adherend thermal mismatch have opposite signs from end to end. Consequently, the thermal effects weaken bonded structures below the strength that would be expected if such considerations had been omitted from analyses. Also, the critical end of the joint can change between tensile and compressive lap-shear loads. The issue is complicated by the fact that some of the thermal strains can creep out of short-overlap test coupons, but they cannot be ignored in long-overlap joints. These effects are more pronounced on thicker structures than on thinner adherends and, in extreme cases, can cause bonded joints between thermally dissimilar materials to actually self-destruct during cool-down after curing at elevated temperatures. The problems are significant for aircraft structures and can be the critical load cases for most space structures. For this reason, acrylic adhesives are used more on space structures than on aircraft structures, for which epoxy adhesives dominate.

20.8 Inspection, testing, and quality control Inspection of bonded and composite structures is one of the most contentious issues associated with these two technologies. Ultrasonic inspections are standard at both the time of manufacture and at periodic intervals during service. They incur a disproportionate amount of the total life cycle costs, far more than the cost of materials and fabrication. Yet they have missed all of the major problems, such as the totally unbonded stringers cited by Hart-Smith and Strindberg.21 The world's experts could not find these kissing bonds, even when it was known exactly where they were, not even with the most sophisticated test instruments available. (They seem to work best when there is a change between good and bad structure, to produce a change in the signal.) The only hard-to-find problems of any structural significance have been global weaknesses associated with the use of inappropriate processing at the time of initial manufacture. Local damage, usually from impacts, can be found reliably by traditional NDI techniques, but most local damage is not potentially immediately catastrophic, because it takes far more force to break strong bonds cohesively than to spread an existing plane of weakness interfacially. The basic problem with ultrasonic inspections is that they cannot guarantee the absence of widespread structural weaknesses, either when the parts are made or when they are in service. This has been a great impediment to the more widespread use of bonded and composite structures. Nevertheless, the service record of these structures is far better than this image suggests, because the processes are so easy to follow when they are implemented correctly. One post-manufacture inspection method can reliably assess whether or not the glue is still stuck at any time during the service life of the structure. These

516

Adhesive bonding

Figure 20.24 Use of bonded tabs to assess bond strength at any stage in the life of bonded structures.

are the bonded pull-tabs described by Hart-Smith22 and illustrated in Fig. 20.24. If the surface has deteriorated, the tab will easily be pulled off the surface if a drop of water is applied in conjunction with the peel load. This is known to work on metal-bond structures, per the tests run during the PABST program. It has yet to be tried on composite structures, possibly out of concern about causing damage to the underlying structure if the bond were not defective. Instead of belabouring the inherent inadequacies of some of the expensive standard inspections for bonded and composite structures, the author will now describe what the best of the inspections can do reliably, and how to get the best value out of the inspection dollar. It is first necessary to differentiate between mechanically induced damage, which is relatively easy to detect, and weaknesses created by inappropriate processing, which can be found reliably at the time of manufacture only by a combination of shear and peel test coupons ± and only on test coupons, not the actual structures. This latter condition cannot be detected by standard ultrasonic inspections because, initially, there is no gap to be found. Once the detail parts have separated, it is necessary to examine the fracture surface to confirm that the failure was interfacial (for a processing error) and, therefore, that it could potentially extend over the entire bonded area. Impact damage to properly bonded surfaces is characterized by a rough fractured adhesive surface on both metal adherends, or by interlaminar fractures in composite structures away from the adhesive layer. Discrepancies found by ultrasonic inspections at the time of initial manufacture fall into a different category. These are misfits that cause voids and porosity in bonded joints. That is the one valuable contribution that these expensive and time-consuming inspections can make. Such local defects are usually structurally insignificant, as

Aerospace

517

explained earlier but, once detected, cause inordinate inspection costs during service to prove that they have not grown, even though extensive service history indicates that they will not if the processing has been reliable. However, if such discrepancies are repetitive, considerable future costs can be eliminated by modifying the bonding tool or the individual parts so that such misfits are eliminated from future production, as explained by Hart-Smith.23 While it may seem contrary to intuitive thinking, defect-free bonded and composite structures really are the least expensive to make. The absence of defects is the path to even more reductions in inspection costs since, after ten consecutive defect-free assemblies have been made, it is permissible to switch to a sampling inspection plan instead of having to inspect 100 percent of every assembly. In the same vein, there are two positions on whether or not it is necessary to attach traveller coupons to every single part being anodized, or if it is sufficient to validate the temperatures, chemical concentrations, voltages and the like, in the tank farm only at the beginning and end of each shift. The only additional information that traveller coupons on every detail part can provide is assurance that the electrical connections needed for anodizing have been attached correctly. But that detail can also be verified by visual inspection, and is sometimes nullified by stringing all coupons in a single patch together to save time in testing them, instead of attaching them to parts one at a time in such a way that they could be anodized only by current that first flowed through the part it was validating. The issue is clouded by the need to measure both voltage and current flow to ensure complete anodizing and not merely the use of polarized light inspections to detect anodizing. This method has failed repeatedly to detect underanodizing, as was first noted during the PABST program. This condition was created by undetected corrosion in the electrical circuit that reduced current flow, even though the correct voltage was maintained, and by too low a temperature in the tank farm. In other words, all that the use of 100 percent traveller coupons ensures, beyond what validating the tank farm twice a shift can do, could have been ensured by diligent visual inspections during the processing. It would seem that if these 100 percent inspections were detecting discrepancies not found by the twice-a-shift inspections then the processing specifications were not being followed carefully enough. The author would suggest that the extra tests should be superfluous if there is a stable fully trained workforce in the bond shop, but that they serve as a useful insurance policy if there is so much labour turnover that additional training ought to be occurring. These shear and peel coupon tests are neither expensive nor time consuming. However, it should always be remembered that increments of the inspection budget once dissipated on unnecessary tests will not be available at some future date to resolve some unanticipated real problem. Inspection dollars are most valuable when they are solving problems or confirming their absence, rather than buying off discrepant parts as-is and without causing the discrepancies to be eliminated from subsequent parts.

518

Adhesive bonding

Once reliable process specifications have been established, their application for metal bonding is customarily validated by two tests on coupons referred to as traveller coupons that are processed with the part. One of these tests is the lap-shear coupon (ASTM D-1002), tested at room temperature, and the other some form of peel test in a hot-wet (hostile) environment. Common peel tests are the wedge-crack test (ASTM D-3762 with a far more stringent requirement on the absence of interfacial failures than in this specification) and the climbing-drum peel test (ASTM D-1781). As noted earlier in the chapter, the first test ensures only that the resin in the adhesive has been exposed to the correct thermal profile, while the second ensures durability in service. Both tests are needed. The parts processed with the coupons are primed within the strict limits on exposure after the etching, anodizing, and rinsing has been completed. (In the auto industry, the priming is often an electro-dip process at the end of the other surface treatments, but the lower volume of production and greater size of the parts has not favoured this approach for aerospace. It would help if it had, since the need for such critical control of the primer thickness being sprayed on might then be avoided. But that appears to be a development for the future.) The bare surfaces will deteriorate with time if not primed promptly, but can safely be stored after priming. They are then left on hold until the traveller coupon tests have been completed, typically in an hour, and not released to the assembly area until the coupon tests have been satisfied. The surfaces of the actual parts have to be re-processed if the coupons fail the test. This delay is avoided when the tank farm is validated only twice per shift, rather than for each tankload of parts. But the saving must be weighed against the risk of far greater recovery costs if parts have been bonded together before some discrepancy is discovered. In the case of bonded composite parts, the standard process inspections are incomplete, in the sense that only lap-shear coupons are mandated. There is rarely any use of a peel test to ensure that the adhesive is stuck properly. In the author's opinion, based on years of observation of manufacturing and in-service experience, there ought to be, because there is no other way of discontinuing the use of surface treatments that have been found to be inadequate. This issue is discussed by Hart-Smith.24 It is vital to understand that each kind of test evaluates only one factor. The shear test cannot assess durability, and the peel test cannot ensure complete cross-linking of the resin. Perhaps the worst violation of this principle is the use of ultrasonic inspections to over-rule failure to pass the coupon tests for parts that are deemed too expensive to scrap. There are never any written specifications allowing this, but there are also no written instructions prohibiting material review boards from making such decisions. Fortunately, since the introduction of phosphoric-acid bonding and phenolic-based adhesive primers, the processes have become more robust than the testing techniques and such decisions have not created a safety problem.

Aerospace

519

There is a similar confusion with bonded composite structures, related to a widespread failure to understand that process-verification coupons cannot do so if the requirements are set so low that even badly processed parts can exceed the requirements. It is therefore often necessary that the coupons not have the same fibre pattern as any individual part. Only an all-0ë lap-shear coupon can impart enough load to fail a properly processed adhesive layer. Failure outside the bonded overlap in a far weaker quasi-isotropic laminate tells nothing whatever about the strength of the adhesive layer or whether or not it has been fully cured. So the use of such fibre patterns as bonded-joint test coupons is always inappropriate, no matter what the design of the structure is. Similarly, all-0ë peeltest coupons are needed to evaluate durability, because it is so easy to divert any interlaminar crack through any layer of 90ë fibres. Sadly, not only is there not yet any agreement on the need for durability tests of bonded composite joints, there is not even a universal recognition of the need for the composite coupons that are used to be strong enough to pass only when a full-strength bond has been created. There are some underutilized very reliable and far less expensive visual inspections that can ensure proper fit ± and reveal a lot about the processing too! This is explained in Fig. 20.25. The fillet cannot achieve the preferred shape if the adhesive has not been heated up correctly, to make it flow; it cannot wet a contaminated surface and it cannot even form if the parts are too far apart. The shape of the fillet is an invaluable reliable indicator of a good bonded joint, and such an inspection is both rapid and inexpensive. The total absence of any fillet indicates a gap just as reliably as any ultrasonic inspection. Indeed, it is necessary to seal any edges where there is no fillet before attempting to use ultrasonic inspections because liquid that could ingress through an open edge could mask the extent of any such gap from any ultrasonic inspection, which relies on gaps to create signals of discrepancies. A porous spew for an epoxy adhesive would indicate the presence of pre-bond moisture, but would be expected for a phenolic adhesive. (Despite their excellent service record,

Figure 20.25 Importance of visual inspections of bonded structures.

520

Adhesive bonding

phenolic adhesives tend not to be used much in aerospace structures today because of the higher pressures needed than for curing epoxy adhesives.) To summarize the salient points about inspections and quality control for bonded structures, it is vital that appropriate processes be specified ± and followed. Verifying this requires both shear and peel tests in a hot/wet environment for metal bonding and bonded composite structures. Ultrasonic inspections can be relied upon to find only in-service impact damage, not progressive degradation. However, ultrasonic inspections at the time of manufacture can identify gaps and misfits and point the way to major cost savings by eliminating such discrepancies from future parts, even if they were structurally insignificant. Any defect, once detected, incurs enormous subsequent inspection costs to prove that it has not grown. It is far less expensive to make parts with no defects than to buy them off as-is. Although it is usually not mentioned in any specifications, visual inspections can be incredibly valuable, even if they do not eliminate the need for all other tests. The most important issue about inspecting bonded and composite structures is that it is too late to detect a problem once the cure is over. At that stage, it is not possible to restore the structure to 100 percent of its intended strength or durability. This is why the emphasis for this kind of structure must be on process control to not make mistakes in the first place!

20.9 Bonded repairs Bonded repairs should be looked upon as joints that are made at some time after initial manufacture. They need to be made under the same rules and procedures, but it is obviously not possible to repeat the very same surface treatments for bonded metal structures, or even for composite structures if a peel ply had been used. In both cases, low-pressure grit blasting has proved to be the most reliable surface treatment for repairs. This is often followed by the application of a silane coupling agent when composite patches are bonded over cracks in metallic structures. Such patches have been found to be both very reliable and effective;25 they have most often been used when there is no other alternative but to scrap the part because riveted repairs are sometimes impossible or ineffective (not stiff enough to restrain a crack from any further growth). As noted earlier, local bonded repairs to bonded metallic structures that were made according to inappropriate specifications are an exercise in futility, since the remainder of the structure is also about to disbond. Such structures need to be totally remanufactured in accordance with better processes, even when the specific repair manual states otherwise. The most critical issue about the repair of damaged or disbonded composite structures is the difficulty of thoroughly drying the laminates before the repair is executed. Absorbed water takes a very long time to remove, from both face sheets and honeycomb cores. This causes such parts to be out of service for a long time.

Aerospace

521

20.10 Other industry-specific factors A little-publicized fact about adhesive layers is that they act as electrical insulators. Today's jet transports protect the passengers and crew because they are contained in a Faraday cage. There are a number of adverse consequences if that cage is interrupted by insulators between the individual metallic skin panels. The most obvious is that lightning strikes will cause far more damage if there is no continuous conductive path between the strike zone and exit point. Indeed, simulated lightning-strike tests during the PABST program showed extensive local burning of the adhesive layers at the bonded joint between adjacent skin panels. This is why composite aircraft need special conductive coatings to compensate for the poor conductivity of even carbon fibres and for the periodic total interruptions through bonded joints. A lesser-known issue is that the establishment of even small potential differences between the panels on aluminium aircraft caused by the over-zealous application of sealant to prevent corrosion can interfere with some of the small voltages involved in transistorized communication systems. It is concern about possible interference with these systems that is responsible for the powering off of electronic devices during takeoff and landing. It is therefore important that the individual panels that make up the exterior skin of aircraft, and presumably rockets and missiles also, are grounded together with sufficient connections. The notion of an all-bonded structure is really a myth. Some minimum number of tight-fitting dry rivets are needed to provide electrical connections. These are easily located in the many low-stress areas in bonded joints without causing any fatigue problems. A rivet hole in the middle of a bonded overlap, where the stress level is only half that of the surrounding skins, has a fatigue life some 20 times longer than in a riveted joint between the skins. Once this need for some holes is acknowledged, it makes sense to use such holes for determinate assembly, to minimize the need for most of the traditional costly assembly tools, and to simplify bagging by eliminating the need for holding fixtures. This concept was used on the bonding tools to both simplify and improve the manufacture of the bonded stiffened wing skins used on the SAAB 340, as discussed by Hart-Smith and Strindberg.21 There needs to be a balance between coating every fastener and every faying surface with sealant to prevent corrosion and the need for electrical continuity. Perhaps the cost savings from determinate assembly may provide the encouragement to include the necessary connections to create a true Faraday cage on aerospace structures. Another issue about bonded aircraft structures is that the surfaces created by anodizing and etching to enhance the adhesion of adhesive primers is more prone to corrosion than normal rolled or machined aluminium surfaces. It is therefore necessary to be very careful, particularly in bilge areas and around galleys, to create and maintain reliable corrosion protection. The polymers used in adhesives tend not to degrade with time and environmental exposure, but

522

Adhesive bonding

some do absorb water and other chemicals. Even so, the primary concern has been for the durability of the interfaces.

20.11 Examples of use of adhesive bonding in aircraft structures Some aircraft manufacturers have made far more extensive use of adhesive bonding than others. de Havilland (now absorbed into BAe), and Fokker were the pioneers in using adhesive bonding in primary aircraft structures. SAAB and Cessna used primary structural bonding widely after the PABST program had validated the second successful generation of materials and processes. However, other major airframers have restricted the use of adhesive bonding to mainly secondary structures, primarily because of the failures created by the prePABST bonding processes and surface treatments developed in the U.S. The most significant difference between the two levels of application would appear to be the stability of the labour force, for a variety of reasons. It takes relatively little skill and training properly to follow a correctly specified and implemented set of procedures. On the other hand, it takes a lot of experience to resist the occasional pressures to take short cuts to stay on schedule or to reduce cost. Only with an understanding of why certain things must be done and why others must not be has it been possible to maintain high-quality production. This takes time and experience to accumulate, and making an occasional mistake and correcting it is one of the most powerful techniques for ensuring that it never happens again. Further, improved processing is most frequently the consequence of correcting some earlier mistake, something that would have required understanding rather than blindly continuing to adhere to erroneous processes that were in need of correction. Constant turnover in either the labour force or the engineering staff nullifies all such hard-earned wisdom. Figure 20.26 shows the extensive application of adhesive bonding to the aluminium airframe of the SAAB 340 aircraft. This aircraft has a superior structural efficiency and durability, which could not have been achieved with conventional riveted structures. Cessna made an even more extensive application of bonding to the fuselage of the Citation III jet aircraft, and used the same technology to make wings on other aircraft with far fewer fuel leaks than on conventional riveted wing boxes. Figure 20.27 shows a typical frame/longeron intersection point on the Citation III fuselage skin, showing how not only the waffle doubler and longerons are bonded to the skin, but the outer half of the frame is, too. What is significant about this design is that both stiffener flanges are continuous where they contact the skin; there is no weakness associated with the traditional mouse-hole in the frames to allow longerons to pass through. The secondary structures, control surfaces and fixed panels, on the Boeing 747 made extensive use of metal bonding, mainly with honeycomb. Many of

Aerospace

523

Figure 20.26 Application of adhesive bonding to SAAB 340 fuselage, wings and tail.

these components have been replaced by composite structures on later models, but most of these components should still be classified as bonded structures. For example, the 777 composite tails (see Fig. 20.28) are made by co-bonding precured stiffeners to green skins. These are classified as primary structures, surpassing in both size and load intensities the earlier NASA-funded flight demonstration programs on the 737 and DC-10. All current-production Airbus aircraft have primary composite structures in their horizontal and vertical tails. Today, virtually all modern transport aircraft have composite control surfaces, wing-root fillets, and various other secondary structures.

Figure 20.27 Cessna Citation III bonded frame/longeron intersection.

Figure 20.28 Co-bonded composite primary structure on Boeing 777 tail.

Aerospace

525

Military aircraft have made much more use of primary composite structures than commercial transport aircraft, but most new general aviation aircraft since the resurgence of this industry a decade ago have made even more extensive use of composites, co-cured and bonded, in their primary structures. Their smaller size favours this method of manufacture. Adhesive bonding has a tremendous advantage over co-curing of composites for high production rates. Co-cured structures require the use of the largest tools for a greater time than bonding together of simple details, many of which can be cured in a single autoclave cycle. This was recognized by the team that developed the Lear Fan all-composite executive transport aircraft over 20 years ago. It was planned for a production rate of one aircraft a day. The tooling costs would have been unaffordable if the design had been based on large integrally stiffened assemblies. This issue is still important today. The minimum-cost prototype development program is often a co-cured design because the effects of production rate are not considered. Once the structural tests have been completed, the option of less expensive more dispersed production is lost, and high production costs are often locked in place. It would make more sense not to minimize the cost of the prototypes in isolation, but to build prototypes of the lowest-production-cost design instead. The author concludes with the following suggestion to encourage the more widespread use of bonded aerospace structures. Waiting until someone devises a reliable after-the-fact inspection method for bonded structures before accepting the advantages of bonded structures, where they make sense, is going to take an extremely long time, if it is ever accomplished. Some aircraft manufacturers have already demonstrated that carefully following appropriate processes is both feasible and reliable. The service record of properly bonded aircraft structures is exemplary. There is no need to wait for a safety net to justify less diligence during the manufacturing processes before committing this proven technology to production.

20.12 References 1. Hart-Smith L J, Analysis and Design of Advanced Composite Bonded Joints, NASA Langley Contract Report NASA CR-2218, January 1973; reprinted, complete, August 1974. 2. Gosse J H and Christensen S, `Strain Invariant Failure Criteria for Polymers in Composite Materials', AIAA paper AIAA-2001-1184, presented to 42nd AIAA/ ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference Seattle, Washington 16±19 April, 2001. 3. Krieger Jr, R B, `Stress Analysis Concepts for Adhesive Bonding of Aircraft Primary Structure', Adhesively Bonded Joints: Testing, Analysis and Design, ASTM STP 981, W S Johnson, ed., American Society for Testing and Materials, Philadelphia, 1988, pp. 264±275. 4. Hart-Smith L J, `The Bonded Lap-Shear Test Coupon ± Useful for Quality

526

5. 6.

7.

8. 9. 10.

11. 12. 13. 14.

15.

16.

Adhesive bonding Assurance, but Dangerously Misleading for Design Data', McDonnell Douglas Paper MDC 92K0922, presented to 38th International SAMPE Symposium & Exhibition, Anaheim, California, May 10-13, 1993; in Proceedings, pp. 239±246. de Bruyne N A, `Fundamentals of Adhesion', in Bonded Aircraft Structures, a collection of papers given in 1957 at a conference in Cambridge, England, Bonded Structures, Ltd., Duxford, England, pp. 1±9. Schliekelmann R J, `Adhesive Bonding and Composites', in Progress in Science and Engineering of Composites, Vol 1, T Hayashi, K Kawata, and S Umekawa, eds, Fourth International Conference on Composite Materials, North-Holland, 1983, pp. 63±78. Hart-Smith L J and Davis M J, `An Object Lesson in False Economies ± The Consequences of Not Updating Repair Procedures for Older Adhesively Bonded Panels', McDonnell Douglas Paper MDC 95K0074, presented to 41st International SAMPE Symposium and Exhibition, Anaheim, March 25±28, 1996; in Proceedings, pp. 279±290. Marceau J A, Moji Y and McMillan J C, `A Wedge Test for Evaluating Adhesive Bonded Surface Durability', Adhesives Age, 1977, pp. 28±34. Thrall Jr, E W and Shannon R W, eds, Adhesive Bonding of Aluminum Alloys, Marcel Dekker, New York, 1985, pp. 241±321. Hart-Smith L J, Redmond G and Davis M J, `The Curse of the Nylon Peel Ply', McDonnell Douglas Paper MDC 95K0072, presented to 41st International SAMPE Symposium and Exhibition, Anaheim, March 25±28, 1996; in Proceedings, pp. 303± 317. Mahoney C L, `Fundamental Factors Influencing the Performance of Structural Adhesives', Internal Report, Dexter Adhesives & Structural Materials Division, The Dexter Corporation, 1988. Myhre S H, Labor J D and Aker S C, `Moisture Problems in Advanced Composite Structural Repair', Composites, 13, 3, July 1982, pp. 289±297. Bardis J, `Effects of Surface Preparation on Long-term Durability of Composite Adhesive Bonds", Proc. MIL-HDBK-17 Meeting, Santa Barbara, California, October 16, 2001. Hart-Smith L J, `Adhesive-Bonded Double-Lap Joints', NASA Langley Contract Report NASA CR-112235, January 1973; `Adhesive-Bonded Single-Lap Joints', NASA Langley Contract Report NASA CR-112236, January 1973; `AdhesiveBonded Scarf and Stepped-Lap Joints', NASA Langley Contract Report NASA CR112237, January 1973; and `Non-Classical Adhesive-Bonded Joints in Practical Aerospace Construction', NASA Langley Contract Report NASA CR-112238, January 1973. Hart-Smith L J, `Design Methodology for Bonded-Bolted Composite Joints', USAF Contract Report AFWAL-TR-81-3154, 2 vols, February 1982. See also `BondedBolted Composite Joints', Douglas Paper 7398, presented to AIAA/ASME/ASCE/ AHS 25th Structures, Structural Dynamics and Materials Conference, Palm Springs, California, May 14±16, 1984; published in Jnl. Aircraft, 22, 1985, 993±1000. Hart-Smith L J, `A Demonstration of The Versatility of Rose's Closed-Form Analyses for Bonded Crack Patching', Boeing Paper MDC 00K0104, presented to 46th International SAMPE Symposium and Exhibition, Long Beach, California. May 6±10, 2001; in Proceedings, 2002: A Materials and Processes Odyssey, pp. 1118±1134.

Aerospace

527

17. Hart-Smith L J, `The Goland And Reissner Bonded Lap Joint Analysis Revisited Yet Again ± But This Time Essentially Validated', Boeing Paper MDC 00K0036, to be published. 18. Hart-Smith L J, `Explanation of Delamination of Bonded Patches Under Compressive Loads, Using New Simple Bonded Joint Analyses', presented to 3rd Quarterly CRAS Review, in conjunction with Fifth Joint DoD/FAA/NASA Conference on Aging Aircraft, Kissimmee, Florida, September 10±13, 2001. 19. Hart-Smith L J, `Adhesive Layer Thickness and Porosity Criteria for Bonded Joints', USAF Contract Report AFWAL-TR-82-4172, December 1982. 20. Hart-Smith L J, `Adhesive-Bonded Joints for Composites Phenomenological Considerations', Douglas Paper 6707, presented to Technology Conferences Associates Conference on Advanced Composites Technology, El Segundo, California, March 14±16, 1978; in Proceedings, pp. 163±180; reprinted as `Designing Adhesive Bonds', in Adhesives Age 21, October 1978, pp. 32±37. 21. Hart-Smith L J and Strindberg G, `Developments in Adhesively Bonding the Wings of the SAAB 340 and 2000 Aircraft', McDonnell Douglas Paper MDC 94K0098, presented to 2nd PICAST & 6th Australian Aeronautical Conference, Melbourne, Australia, March 20±23, 1995; abridged version in Proceedings, Vol. 2, pp. 545± 550; full paper published in Proc. Instn. Mech. Engrs, Part G, Journal of Aerospace Engineering, 211, 1997, pp. 133±156. 22. Hart-Smith L J, `Reliable Nondestructive Inspection of Adhesively Bonded Metallic Structures Without Using Any Instruments', McDonnell Douglas Paper MDC 94K0091, presented to 40th International SAMPE Symposium and Exhibition, Anaheim, May 8±11, 1995; in Proceedings, pp. 1124±1133. 23. Hart-Smith L J, `Interface Control ± The Secret to Making DFMAÕ Succeed', McDonnell Douglas Paper No. MDC 96K0132, presented at SAE Aerospace Manufacturing Technology Conference & Exposition, Seattle, June 2±5, 1997, and published in Proceedings, pp. 1±10, SAE Paper No. 972191. 24. Hart-Smith L J, `Is It Really More Important that Paint Stays Stuck on the Outside of an Aircraft than that Glue Stays Stuck on the Inside?', Boeing Paper PWMD020209, presented to 26th Annual Meeting of the Adhesion Society, Myrtle Beach, South Carolina, February 23±26, 2003. 25. Baker A A and Jones R, eds, Bonded Repairs of Aircraft Structures, Martinus Nijhoff Publishers, 1987, pp. 77±106.

Index

A-scans 149, 150, 151 abrasion 269 `absorbed' impact energy 165±6, 168 accelerated ageing 129±30, 318 accidental loads 210 acid anodised aluminium 54, 77, 81 phosphoric acid 54, 57, 77, 129, 495±6 acid damage 346 acoustic impedance 145 acrylic adhesives 17, 308 pressure-sensitive adhesives 34, 35 structural 29±30 activated rosin fluxes 466 active vibration control 252 adherend thickness 507±9 adhesion 23±4, 35±49 by chemical bonding 41±4, 138±40 electrostatic theory of 45 by interdiffusion 45±7 mechanical interlocking 45 by physical adsorption 23±4, 35±41, 137±8 pressure-sensitive 47±9 weak boundary layers 47, 75, 118 adhesion promoters 266 see also coupling agents adhesive application 397, 440 adhesive characteristics aerospace 490±5 automobiles 371±4 boats and marine 393±7 construction using steel and aluminium 306±9 construction using timber 333±7 electronics 458±66 shoe industry 424 adhesive selection 265±8, 439 chemical effects 265±6 design needs 267±8

physical effects 266±7 aerospace 489±527 adhesive characteristics 490±5 basic needs 489±90 bonded repairs 520 design features ensuring durability 505±9 effects of thermal mismatch 514±15 examples of use 522±5 inspection, testing and quality control 515±20 joint design 500±9 load redistribution around flaws and porosity 509±14 other industry-specific factors 521±2 surface preparation 495±500 aesthetics 336 ageing, modelling of 469±70 ageing tests accelerated ageing testing for construction 318 accelerated and natural ageing 129±30, 132, 133 automobiles 379±80 boats and marine 409 shoe industry 442 alcohol cleaning 466 alignment, joint 270 aliphatic amines 27±8 alloy type 132±3, 134 aluminium 415 aerospace see aerospace aluminium-FRP joints in boats and marine 399, 407±15 building and construction 305±27 interlayer 160 pretreatments 76±7, 309±11, 375 surface treatment and water resistance of joints 127±9

Index use in automobiles 358±9, 375 aluminium alloys 209 environmental effects 132±3, 134 joint design and impact load 185±6 aluminium nitride/epoxy/polyetherimide (AlN/epoxy/PEI) adhesive joints 472±3 aluminium oxide 311, 496 aminoplastic adhesives 347 3-aminopropyl triethoxysilane (APES) 139 ammonia-accelerated systems 352 amorphous poly--olefins 35 anaerobic adhesives 17, 31, 308 animal glues 6, 7, 11, 336 Cooper standards 14±15 anisotropic conductive adhesives (ACAs) 456±7, 460±1, 464±5, 471±2, 480 anisotropy 264±5, 280 annual growth rings 331 anodisation 54, 77, 81, 310 phosphoric acid 54, 57, 77, 129, 495±6 underanodising 517 anti-flutter adhesives 362±3, 379 antifreeze 140 antioxidants 26, 123±4, 435±6, 438 anti-plane mode 191 application of adhesive 397, 440 Arcan fixture 287, 288 aromatic amines 27±8 Arrhenius diffusion law 136, 470 artists, medieval 7, 8 assembly aids 270±5 combination joining 270, 274±5 external agents 270, 273±4 internal agents 270, 271±3 ASTM block impact test 164, 165±73 atomic force microscopy (AFM) 55±6, 61±4 Auger electron spectroscopy (AES) 67, 68±70, 76 automobiles 164, 269, 357±85 adhesive characteristics 371±4 basic needs 358±71 body shop 359, 360±6, 372 common failures 380 durability 379±80 examples of use 382 inspection, testing and quality control 381 joint design considering impact load 185±7

529

loads and exposure to detrimental effects 370±1 materials used 358±9 other industry-specific factors 381±2 power train 360, 369±70 process chain 359±60 repair and recycling 381 strength 377±9 surface preparation 375±7 trim assembly 359±60, 367±9 back-face strain method 214, 215 Bacon, Francis 9 ballotini 271 balsa wood 295 bath curve 321, 324 reliability of electronics systems 468±9 beams formula 199±200 Bell, William 10 bending moment 94, 99, 101 Bible 5±6 bi-material singularities 96±7, 108±9, 116±17 birch pitch, processed 4, 5 bitumen 4, 5 block copolymers 34±5 block impact test 164, 165±73 boats and marine 386±416 adhesive characteristics 393±7 basic needs 386±93 common failures 404 designing for strength 401±3 examples of use 405±15 future trends 415±16 hull-deck joint in small FRP boats 406±7 inspection, testing and quality control 404±5 joint between aluminium hull and FRPsandwich deckhouse 407±15 load characteristics 390±3 metal/composite hull/superstructure connections 292, 293, 407±15 repair 405 strength and durability 399±403 surface preparation 397±9 types of connections 387±90 typical materials 387 body shop 359, 360±6, 372 Boeing aircraft 522±3, 524 bolted joints 346 compared with adhesive bonding for boats and marine 389±90, 414±15

530

Index

bond formation 338±9 bond-line thickness (BLT) 260, 270±1 influence in bonding composites 280, 291, 292 bond testers 152±4 sonic 153±4 ultrasonic 152, 153 bonded-in rod connections 351 bonded pull-tabs 515±16 bonded shrink fits 369±70 brittle adhesives 320 aerospace 492±3 bonding of composites 283±5 brittle point 34, 35 brushing 311, 337 building and construction 305±56 adhesive characteristics 306±9, 333±7 basic needs 305±6, 328±31 common failures 319±20, 345±6 examples of use 348±51 future trends 351±4 inspection, testing and quality control 311±12, 320±4, 346±8 repair 324±5, 348 steel and aluminium 305±27 strength and durability 311±19, 339±44 surface preparation 309±11, 337±8 timber 328±56 butadiene based rubber modifiers 18 butt joints 109 butyl rubber 85 butylated hydroxytoluene (BHT) 26, 123±4 C-scans 151±2 canvas 420±1 carrier materials 272 cars see automobiles casein glues 7, 8, 336 Castan, Pierre 17 cataplasma test 379 cellulose 330, 421 Cennini, C. 8 ceramic screen-printed border 395 Cessna Citation III aircraft 522, 523 chain extension 430 charcoal layer 335 chemical bonding 41±4, 138±40 chemical compatibility 266 chemical damage 346 chemical interactions 265±6 chemical reaction, hardening by 27±33 chemical structure 262 chemical surface treatments 75, 338

chip boards 329 chip on flex tape automated bonding (TAB) packages 482 chip on glass (COG) technology 482 chlorination 423 N-chloro-p-toluensulphonamide 447 chromic acid 77, 81 chromium compounds 276 clamped Hopkinson bar test 177±9, 180 clamps 273 cleaning 466±7 climbing-drum peel test 289, 290, 518 clinch-bonding 274±5 cling films 45 closed form stress analyses 97±107 coatings 377, 393 co-cured composites 525 coefficient of thermal expansion (CTE) 95±6, 459 CTE mismatches in electronics 260±1, 475 joint design 258±61 Coffin Manson law 469±70 cohesive properties measurement 159±60 poor cohesive strength 144 cohesive zone modelling 114±15, 203, 204 coil passivation layer 375 coin-tap test 154 cold setting adhesive systems 352 combination joining see hybrid joining combined high impact rate loading tests 177±81 Comeld process 276 common failures automobiles 380±1 boats and marine 404 electronics 456, 473±6 steel and aluminium construction 319±20 timber construction 345±6 Company of Carpenters 9 Company of Joiners 9 compatibility, chemical 266 complex geometric configurations 108±11 complex material responses 111±15 composite beams 330 composite columns 330 composite pipework 291±2, 293 composites 279±304 aerospace composite structures 497±500

Index design of bonded composite assemblies 280±5 durability and long-term performance 296, 297 examples of bonded composite structures 291±6 influence of bond-line thickness 280, 291, 292 recent developments 296±300 specific nature of 279±80 stress analysis of bonded composite repair techniques 110±11 surface preparation 285±7 testing 280, 287±90 timber 330 vibration damping in joints between composite sandwich panels and metals 249±51 compressed air 311 compression wood 332 conductive adhesives 456±7, 460±6, 474±5 conformal polymer coatings 269 conservation repairs 348 constant life diagram 217±18 construction see building and construction contact, zone of 36 contact adhesives 24, 308 contact AFM 61±2 contact angles 37±9, 64, 66 contact time 47, 48 continuous fibre composites 265 cooling, hardening by 26±7 cooling device 483, 484 Cooper Grades 14±15 core/facing interface bonding 289±90 cork 421 corona discharge 63±4, 81±3, 447±8 corrosion 264 aerospace 521±2 protection for automobiles 357, 372±3 resistance for boats and marine 393 Coulomb damping 241 coupled multi-physics problems 115±16 coupling agents 266, 269 silane 79±80, 139±40 covalent bonds 41, 42±3, 139±40 crack initiation phase 212, 214±15 crack propagation phase 212 crack jumps 232 cracks 96 see also fracture mechanics crash-resistant adhesives 364±6, 373±4 creep-fatigue 218, 231±2

531

creep material models 113, 114 creep test 442 critical energy release rate (fracture energy) 194, 196±7, 198, 199±201 critical stress intensity factor (fracture toughness) 169±70, 201 critical water concentration 126±7 crosslinkers 432±3 crosslinking 23, 263 cryoblasting 445 crystallisation rate 427 cure shrinkage 266 curing see hardening curing agents 436, 438 curing time 397 cyanoacrylate adhesives 17, 19, 31±2, 308±9 Daedalus legend 3±4 dam and fill encapsulation 460 damage mechanics approach 281 damage modelling 114±15 damping see vibration damping damping materials 243 debond rate 201±2 debonding 196, 276 degradation mechanism 317±18 delta alpha problem 371 design aerospace 500±9 bonded composite assemblies 280±5 and damping 251±2 design needs and adhesive selection 267±8 designed-in bonding aids 272±3 joints see joint design steel and aluminium construction 305±6 for strength in boats and marine 401±3 using fracture mechanics 202±3 design codes 306, 312±13, 325±6, 347 design errors 319±20 design limit load 491±3 dicyandiamide 28 die attachment 479 dielectric measurements 160 diffusion stress-diffusion coupling 115±16 theory of adhesion 45±7 water diffusion into adhesive bondlines 133±6 diglycidylether of bisphenol-A (DGEBA) 27 direct stresses 92

532

Index

disbonds 144±59 bond testers 152±4 rapid scanning methods 154±9 ultrasonic testing 145±52 dismantling 393 display devices 482 DOGMA thematic network 283±5 double cantilever beam (DCB) 199±200, 227±8 double-lap joints aerospace 500±2 fatigue 219±20, 221, 222, 223 stress analysis 98, 99±101 double through transmission ultrasonic technique 147, 148, 149±50, 151, 152 drop-weight tester 173±4 dry lubrications 359, 375 drying 440 ductile adhesives 283±5 ductility 339 durability aerospace 505±9 automobiles 379±80 boats and marine 399±403 bonding of composites 296, 297 construction with steel and aluminium 316±19 construction with timber 339±44 electronics 468±73 fracture mechanics 201±2 shoe industry 439±42 see also environmental effects duration of load (DOL) effects 334, 340±1 duromers 376 dynamic loads 371 stress impact of high-speed (impact) loading 117±18 see also impact behaviour dynamic mechanical thermal analysis (DMTA) 246, 247, 248 dynamic SIMS 70 dyne test markers 65 Egypt, ancient 5 elastic mismatch parameter 193 elastic modulus 426 elastomers 262 polyurethanes in shoe industry 424±8 pretreatments for 84±6 elasto-plasticity 111±12 electrical conductive adhesives (ECAs) 456±7, 460±6, 474±5

electrical connections, aerospace 521 electrochemical treatments 447 electronics 260±1, 276, 455±88 adhesive characteristics 458±66 basic needs 456±8 common failures 456, 473±6 conductive adhesives 456±7, 460±6, 474±5 encapsulation 457, 459±60, 474, 476±7 examples of use 478±84 inspection, testing and quality control 476±8 strength and durability 468±73 surface preparation 455±6, 466±7 electrophoresis 360 electrostatic theory of adhesion 45 emissions 334±5 emulsified polymer isocyanate (EPI) 347±8 emulsifiers 431, 437 internal 429±30 emulsion polymerisation 434 encapsulation 457, 459±60, 474 encapsulant materials 476 encapsulated components 476±7 endurance limit 213±14 energy absorbed impact energy 165±6, 168 criterion for failure 190 dissipation 240±1 energy release rate approach 194±6 engineered wood products (EWPs) 329±30, 348±51 environmental effects 123±42, 371 additives to reduce photo±oxidative degradation 123±4 aerospace 505±9 bonding of composites 296, 297 construction with aluminium and steel 316±19 construction with timber 140, 334, 340±1 fatigue 218±23, 232±4 future trends 140±1 integration into closed±form stress analysis 103 monitoring environmental degradation 160±1 other fluids 140 structural joints to metals in wet surroundings 125±33 water and adhesive interfaces 137±40 water and adhesives 133±7 see also durability

Index environmental protection 334±5, 352 epoxy adhesives 17±18, 27±8, 309, 337 with high energy absorption at high velocities 364±6 ICAs 463 ESCA (X-ray photoelectron spectroscopy) 67, 68±70, 76 etching 310, 495 ethanol 138 ethyl cyanoacrylate 31±2 ethylene-propylene elastomers 85 ethylene-vinyl acetate (EVA) hot melts 26 shoe industry 423, 448 Eurocodes for construction 306, 312, 325±6, 347 European Adhesive Bonder/Specialist/ Engineer 326 expansion, differences in 320 experimental compliance method 200 external agents 270, 273±4 extractives 331, 332, 338 extruded components 272±3 Eyring diffusion law 470 facing/core interface bonding 289±90 failure criteria 234, 235, 314 bonding of composites 281±5 failure load predictions 283±5 wood-adhesive joints 341±4 failure surfaces 53, 56 failures, common see common failures Faraday cage 521 fasteners 274±5, 348 fatigue 209±39, 319 constant amplitude sinusoidal waveform 210±12 environmental effects 218±23, 232±4 fatigue loading effects 216±18, 228±32 FCG approach 212, 226±36 future trends 235±6 sources of fatigue loading 210 stress-life approach 212, 213±26, 235±6 testing 215±16, 227±8, 412, 413±14 fatigue crack growth (FCG) approach 212, 226±36 effect of environment 232±4 fatigue life prediction 234±5 fatigue loading effects 228±32 testing 227±8 fatigue crack growth law 234 fatigue limit 213±14 fatigue threshold 226 fibre-reinforced composite materials 252

533

fibre-reinforced plastics (FRP) 387, 415 FRP substructure-metal hull connnections 389±90 hull-deck joint in small FRP boats 406±7 joint between aluminium hull and FRPsandwich deckhouse 407±15 surface treatment on joints between FRP and metal 398±9, 400 `weldable' FRP sandwich panels 389±90, 394 fillers 261, 271, 428, 436 conductive 461±2 fillets 108, 270, 273, 295 visual inspections 519±20 finger-jointed structural lumber 348, 349±50 finger-joints 330, 349 finite element method (FEM) 96, 97, 107±18, 203 aerospace 505 boats and marine 403 building and construction with timber 342±4 commercial packages 187 complex geometric configurations 108±11 complex material responses 111±15 coupled multi±physics problems 115±16 high-speed dynamic loading 117±18 local bi-material stress singularities 116±17 stress distribution and variation in joints subject to impact load 181±5 vibration damping 243±4 fire 326, 335 FIT (failure in time) 468 flame treatment 81, 82, 338 flanges 361±2, 363, 372±3 flaws initial and designing with fracture mechanics 202±3 load redistribution around 509±14 flexural tests 407±8, 410 flip chips 260±1, 457, 470±1, 479±80 formaldehyde adhesives 336, 352 condensate adhesives for wood 31 emission from particle boards 334±5 forward shear mode 191 fracture energy 194, 196±7, 198 experimental evaluation of 199±201

534

Index

fracture mechanics 166, 189±208, 285 current research areas 203±5 designing with 202±3 durability 201±2 effect of mode mixity 197±9 energy criterion for failure 190 energy release rate approach 194±6 stress intensity factor approach 191±4 thermodynamic, intrinsic and practical adhesion energy 196±7, 198 fracture toughness 169±70, 201 French glue industry 10 frequency effect in ultrasonic testing 148±51 and fatigue 218, 229±30 furniture 9 Galileo 9 galvanic corrosion 264 galvanising 269 gel polymers 434, 436±7 generalised Volkersen theory 342 generic model for quality assurance 321, 322±3 German glue industry 10 glass 78±80 glass beads 271 glass-fibre reinforced polyester coatings 330 glass transition temperature 23, 262 glassy adhesives 23 glazing 367±9, 379, 395 see also windscreen bonding glob top 460 global loads boats and marine 390±2 global closed form stress analysis of adhesive joints 97±107 glue and screw 274 glued-in rods 330, 343±4, 345, 351 glued laminated timber (glulam) 329, 348, 349 GLUEMAKER 107 gold 461±2 grain 339±40 greasy leather 420 green gluing (wet gluing) 337, 352 green strength 433 grit blasting 286, 310, 311, 498 hackle formation 491±2 hardening/curing 23, 24±33, 262±3, 441 by chemical reaction 27±33 by cooling 26±7

curing time 397 by loss of solvent 24 by loss of water 24±6 promotion or inhibition of cure 265 heartwood 332 heat activation 431, 440±1 heat insulation 393 hem flanges 361±2, 363, 372±3 hemicellulose 330 Hertz equation 36 high-boiling solvents 427 high cycle fatigue (HCF) 210, 212 high cycle fatigue load 314 high-density fibreboard (HDF) 329, 350 high-temperature adhesives 30±1 hindered phenols 123±4 history of adhesives 3±22 from early days to 18th century 3±10 industrialisation of glue making 10±15 synthetic polymers 15±19 honeycomb/composite interface 295 Hopkinson bar clamped 177±9, 180 split 174±7 hot-melts 26±7, 308, 443±4 hull-superstructure joints 292, 293 adhesive joint compared with bolted joint 414±15 aluminium hull and FRP-sandwich deckhouse 407±15 small FRP boats 406±7 hull girder loads 390±2 humidity 125±7, 128, 379 construction with wood 334, 340±1 hybrid joining 270, 274±5 automobiles 359, 360, 361, 362 hybrid side-impact beam 185±7 hydrogen bonds 41, 42, 44 hydrogen plasma cleaning 467 hydrolysis 137 2-hydroxybenzophenones 124 hyperelasticity 112±13 hysteretic damping 241 hysteretic heating 218 I-joists 330 immersion test 379 impact behaviour 164±88 drop-weight tester 173±4 experimental method for impact test 165±81 finite element analysis 117±18 future trends 187

Index impact force for a boat colliding with a quay 406±7 impact loads in automobiles 371 joint design considering impact load 185±7 other special methods 177±81 pendulum test 165±73 split Hopkinson bar 174±7 stress analysis of high±speed dynamic loading 117±18 stress distribution and variation subject to impact load 181±5 impact hammer test 245, 246 impact wedge-peel (IWP) test 170±3, 187 induction heating 372, 373 industrialisation of glue making 10±15 inertia wheel impact test 179±81 infra-red thermography 405 initiation phase 212, 214±15 inorganic materials, pretreatments for 78±80 inspection aerospace 515±20 automobiles 381 boats and marine 404±5 construction in aluminium and steel 320±4 construction in timber 346±8 electronics 476±8 insulation electrical 521 heat 393 integrated circuits (ICs) 456±7, 459±60 interdiffusion 45±7 interfacial cracks 192±4 interfacial failure 53, 56, 319 interfacial fibre orientation 286±7 interfacial properties interface problem and monitoring environmental degradation 160±1 poor adhesive-adherend interfacial properties 144 interlayer 160 internal agents 270, 271±3 internal emulsifiers 429±30 intrinsic adhesion 197, 198 invariant polymer failure model 491±3 ionic bonds 41, 42, 43, 138±9 irreversible processes 136±7 isocyanates 335, 352 crosslinkers 432±3 isotropic conductive adhesives (ICAs) 456±7, 460±1, 462±4, 471

535

J-integral 196, 226, 232 jet-fuel 140 jigging 270±5 JKR equation 36±7 joining similar and dissimilar materials 257±78 adhesive selection 265±8 advantages of adhesives 157 assembly issues 270±4 future trends 275±7 hybrid joining 274±5 joint design 258±65 pretreatments 268±9 joint alignment 270 joint design 258±65 aerospace 500±9 anisotropy 264±5 coefficient of thermal expansion 258±61 considering impact load 185±7 corrosion 264±5 design for strength in boats and marine 401±3 ensuring durability 505±9 factors affecting adhesive properties 262±3 shoes 439 joint thickness 395±6 juvenile wood 332 Kolsky bar 174±7 Lafayette frigates 292, 293 laminated strand lumber (LSL) 329, 351 laminated veneer lumber (LVL) 329, 350±1 laminating 388±9, 415 lap joints impact behaviour 168±9, 170, 181±5 stress analysis 98±103 see also double lap joints; single lap joints lap-shear tests 288±9, 493, 518 lap-strap joints 219±20, 221, 222, 223±4 laser ablation 338, 467 laser ultrasound 155 latex adhesives 25±6 leather 419±20, 421 lengthwise splicing 330 Lewis acid-base interactions 42, 44 lifetime prediction 223±4, 234±5 lightning strikes 521 lignin 330 limit states 312±13

536

Index

linear elastic fracture mechanics (LEFM) see fracture mechanics liquid crystal devices (LCDs) 482 lithium hydroxide 338 load-displacement curve 166±8 local defects 509±14, 516±17 local loads 390, 392±3 local singular adhesive stresses 96±7, 108±9, 116±17 locus of failure 199, 204 long-overlap bonded joints 506±8 long-term static load 313±14 loss modulus 426 Lotus Elise car 273 low-boiling solvents 427 low cycle fatigue (LCF) 210, 212 low cycle fatigue load 314 low-pressure plasma treatment 83±4, 448 magnesium 358±9, 376 magnesium oxide 132±3, 435 marine see boats and marine masking tape 18 material complexity 111±15 material handling 353±4 mechanical fasteners 274±5, 348 mechanical impedance method 154 mechanical interlocking 45 mechanical loads 92±5, 210 mechanically weak boundary layers (MWBL) 337 medium-density fibreboard (MDF) 329 melamine-urea-formaldehyde (MUF) adhesives 336 metal coated particles 461±2 metal coatings 269 metal oxides 132±3, 311, 435, 437, 496 metals behaviour of structural joints to metals in wet surroundings 125±33, 134 joints between metal hulls and composite superstructures 292, 293, 407±15 phenolic adhesives for 28±9 pretreatments 76±8, 268±9, 497 structural joints with composite sandwich panels 249±51 surface treatment on joints between FRP and metal 398±9, 400 methyl ethyl ketone (MEK) wiping 420 methylmethacrylate (MMA) 29±30 micro-electromechanical systems (MEMS) 475±6, 482 micro-hairs 275

microsystems 475±6, 482, 483 microtechnology 276 microwave discharge cleaning 467 Miner's sum 224±6 mixed mode fracture envelopes 288, 289 mobile phones 164 modal strain energy (MSE) approach 243 modal testing 245, 246, 247 mode mixity 192, 194, 197±9 modified silane (MS) polymer adhesive 309, 367±8 moisture see water moisture content (MC) 337, 339 moisture-reactive hot-melt adhesives 443±4 monolithic materials, cracks in 192 MTBF (mean time between failure) 468 MTTF (mean time to fail) 468 multi-material design 358±9 multi-material structures 252 Nahal Hemar cave 4 nanomaterials 276 natural ageing 129±30, 132, 133 NBS-VTT extended rule 392±3 neoprene adhesives see polychloroprene adhesives nickel 461±2 no-flow underfills 480±1 non-conductive adhesives (NCAs) 460±1, 465±6, 472 non-contact AFM 61±2 non-destructive testing 143±63, 202 bond testers 152±4 cohesive property measurement 159±60 conventional ultrasonics 145±52 interface problem and monitoring environmental degradation 160±1 rapid scanning methods 154±9 types of defect 144 see also testing nylon 80, 420±1, 424 obsolescence 469±70 oil-absorbent single part heat curing adhesives 269 Old Bailey cases 9±10 opening mode 191 optoelectronics 276 oriented strand board (OSB) 329, 350 out-of-plane failure criteria 281 out-of-plane mode 191 overlaps 500±3 design and durability 505±9

Index Owens-Wendt plots 37±9 oxidant inorganic salts 447 oxides metal 132±3, 311, 435, 437, 496 stability of oxide layers 137 oxygen 123±4 packaging, levels of 456±8 paint-masking tape 18 paint shop 359±60 painted panels 377 painted sheets 368 painted substructures, integration of 369 Palmgren-Miner (P-M) rule 224±6 parallel strand lumber (PSL) 329, 351 Paris crack growth law 226±7, 229±30 partial safety factors 313 peel compliance factor 106 peel force 48±9 peel joints 94 peel plies 286, 498±500 peel stresses 93±4, 401, 433, 502 peel tests 104, 441±2, 518 pendulum testing 164, 165±73 penetration of adhesive 333 phenol 334±5 phenol-formaldehydes 16, 17 phenolic adhesives 347 for metals 28±9 phenolic-based primer 496 phenolic resins 435, 437±8 philosophy of reliability 312±13, 321 phosphoric acid anodisation 54, 57, 77, 129, 495±6 photo-oxidative degradation 123±4 `Phylon'-type soles 423 physical adsorption 23±4, 35±41, 137±8 contact angles 37±9 contact mechanics 36±7 thermodynamic work of adhesion 39±41 physical treatments 75 pinning 298±300 pipework, composite 291±2, 293 plane stress and strain 109±10 plasma oxygen cleaning 467 plasticity 111±12 plastics see polymers plates 274 Pliny 3, 6 plywood 329, 350 polar group-containing elastomers 85 poly-n-alkylacrylates 34, 35 polyamide hot melts 26±7 polychloroprene adhesives 24, 424, 433±8

537

waterborne 436±8 polyester 421, 424 polyethylene (PE) 80±1 polyfluoro-carbons 267 polyimides 30±1 polymerisation 434 polymers 34±5, 209 advent of synthetic polymers 15±19 conformal polymer coatings 269 electronics 458±60 encapsulation 459±60 factors affecting adhesive properties 262±3 pretreatments for 80±4, 269 used in cars 358±9, 376±7 polymethylmethacrylate 29 polyolefins 267 corona treatment of polyolefin film 63±4 polypropylene (PP) 80±1 polysulphides 33 polytetrafluoroethylene (PTFE) 80, 84 polyurethane adhesives 17, 32 construction with aluminium and steel 309 construction with timber 336, 337, 340±1, 347±8 shoe industry 420, 424±33 solvent-borne 424±8 waterborne 428±33 polyurethane soles 424 polyvinyl acetate (PVA) 17, 25 polyvinyl chloride (PVC) 17 soles for shoes 423±4, 447 uppers for shoes 420±1 popcorn degradation 474 poromerics 420±1 porosity 144 bond testers 152±4 load redistribution around 512±14 rapid scanning methods 154±9 ultrasonic testing 145±52 Post-it Notes 19 potting 460 power electronics 472±3, 482±4 power train 360, 369±70 practical work of adhesion 196±7, 198 see also fracture energy precoated panels 377 prediction 353 failure load predictions 283±5, 341±4 fatigue life prediction 223±4, 234±5 prediction methods for vibration damping 242±4

538

Index

prediction models 314±15 prefabricated joint elements 389±90 prepolymer method 429±30 press shop 359 presses 274 pressure 270 pressure-sensitive adhesives (PSAs) 23, 34±5 adhesion mechanisms 47±9 pretreatments 52, 75±88, 268±9 aerospace 495±500 automobile industry 375±7 boats and marine 397±9 bonding of composites 285±7 construction with steel and aluminium 309±11 construction with timber 337±8 corrosion resistance 264 for elastomers 84±6 electronics 455±6, 466±7 future trends 86±7 for inorganic materials 78±80 for metals 76±8, 268±9, 497 for plastics 80±4, 269 and resistance of joints to water 127±9 shoe industry 418±19, 423, 440, 445±9 Primary Adhesively Bonded Structures Technology (PABST) fuselage 495±6, 500±3 primers 76, 269, 311 reactive 419±20 printed circuit boards (PCBs) 457, 475, 478 processing errors 490, 498±500, 516 production errors 319, 345 propagation phase 212 pulse-echo ultrasonic technique 147±8, 152 quality assurance systems 321, 322±3, 346±7 quality control aerospace 515±20 automobiles 381 boats and marine 404±5 construction with steel and aluminium 320±4 construction with timber 346±8 electronics 476±8 glue manufacture 11±12 shoe industry 439±42 quick fix glazing adhesives 368 R-ratios 211, 216±17, 228±9

radiation-based surface treatments 447±9 radiography 154±5 rainflow cycle counting 224 random failures 469 rapid scanning methods 154±9 shearography 155, 157±9 transient thermography 155±7 reaction wood 332 reactivation temperature 441 reactive hot-melts 308 polyurethane hot-melts 443±4 reactive oxygen cleaning 466±7 reactive primers 419±20 recycling 381 reflection coefficients 159, 160±1 relative permittivity 138±9 reliability construction with aluminium and steel 317±19 electronics 456, 468±73 philosophy of 312±13, 321 reliability index 313 repair aerospace 495, 496±7, 520 automobiles 381 boats and marine 405 construction with steel and aluminium 324±5, 326 construction with timber 348 residual stresses 210 resins, phenolic 435, 437±8 resoles 28±9 reversible processes 136±7 RF gas plasma treatment 448 ribs 272±3 ridges 272±3 riv-bonding 274 rivet-bonded joints 109, 274 RMS roughness 56±8 rosin acid 35 rosin ester resin emulsions 438 rosin fluxes, activated 466 roughness, surface 45, 46, 47, 48, 267 roughening 267, 419 roughness average 56±9 rounding 108±9 rubber 34 developments in surface treatment 445±9 rubber toughening of structural adhesives 30 soles for shoes 421±3, 445±9 rubbery adhesives 23

Index S-N curve 213±14, 216±17 see also fatigue SAAB 340 aircraft 522, 523 SAAS 107 safe-life design philosophy 212 safety shoes 417 salt spray 130, 379 sanding 337 sandwich facing/core interface bonding 289±90 sandwich structures boats and marine 387, 407±15 design code 325±6 stiffened sandwich panels 292±6 vibration damping in structural joints between metals and composite sandwich panels 249±51 `weldable' FRP sandwich panels 389±90, 394 sapwood 332 scanning electron microscopy (SEM) 53±6, 57, 62 scanning probe microscopy (SPM) 59±64 scanning tunnelling microscopy (STM) 59±60 scarf joints 104, 105±6 scraping 311 screen bonding 367±9, 379 screen printing 395, 463 screws 274 secondary ion mass spectrometry (SIMS) 67, 70±3 self-adhesive cellophane tape 18±19 sensory assessment of adhesive quality 12±13 Shakespeare, William 8±9 shape memory alloys 252 sharp substrate corners 96±7, 108±9, 116±17 shear compliance factor 106 shear strain 92 aerospace 507±9 shear stresses 92, 93, 94 shear test (mode II test) for composites 289±90 shear thinning materials 371 shearography 155, 157±9 shims 272, 273 shoe industry 417±54 adequate bonding operation 440±1 adhesive selection 439 adhesives used 418 developments in adhesives 443±5

539

developments in surface preparation 445±9 future trends 442±9 joint design 439 polychloroprene adhesives 433±6 selection of upper and sole materials 439 sole materials 421±4, 439 solvent-borne polyurethane adhesives 424±8 testing 441±2 types of adhesive 424 upper materials 419±21, 439 waterborne polychloroprene adhesives 436±8 waterborne polyurethane adhesives 428±33 short-overlap test coupons 505±8 short-term static load 313±14 shrinkage cure shrinkage 266 wood 345 silanes 77, 78 coupling agents 79±80, 139±40 silane modified polymers 309, 367±8 silicon materials 457 silicones 33, 370 avoiding contamination of surfaces in aerospace industry 498±9 silver 461±2 single cantilever beam test 289±90 single lap joints aerospace 502±3 experimental data on vibration damping 244±51 fatigue tests 221±3 stress analysis 94, 98±103 singularities 281 aerospace joint design 505 local singular adhesive stresses 96±7, 108±9, 116±17 at tips of cracks 204±5 slamming pressure (on bottom of boat) 392±3 smart cards 479, 481 smart labels 480 sodium dichloroisocyanurate (DCI) 446±7 sodium hydroxide 338, 447 sodium hypochlorite 445±6 software packages for stress analysis 106±7 sol polymers 434 solder joints 456, 470±1 sole materials (shoes) 421±4, 439

540

Index

solvent 266 hardening by loss of 24 uptake by adhesives 263 solvent-borne polyurethane adhesives 424±8 solvent-free surface treatments 445 solvent-free technologies 443±9 solvent wipes 286, 311 sonic bond testers 153±4 speckle shearing interferometry 157±9 spew fillet see fillets splice plates 500±1 split Hopkinson bar 174±7 spot welding 358 spotting tack 441 spreading of liquids on a surface 64 stainless steel 399, 400 standards (glue making) 14±15 starved glue line 339 static loads 210, 371 static SIMS (SSIMS) 70±1, 76 statistical analysis 318±19 steel 209 building and construction 305±27 effect of salt spray 130 pretreatments 77±8, 309±11, 375 stainless steel 399, 400 use in cars 358±9, 375 stencil printing 460, 463±4 stepped joints 104, 105±6 stepped-lap joints in aerospace 503, 504, 509 stick-slip behaviour 204 stiffened sandwich panels 292±6 stiffness 373, 397 structural stiffness 363±4 stitching 298 storage modulus 426 strain energy release rate 226±35 strain gauges 214, 215 strain-life approach 212, 214 strains 92, 189 strength automobiles 377±9 boats and marine 397, 399±403, 410±13 construction with steel and aluminium 313±15 construction with timber 339±44 designing for in boats and marine 401±3 electronics 468±73 fall in joint strength on ageing 139 strength approaches to design 189

strengthening aluminium and steel structures 324±5, 326 stress analysis and predicting joint strength 100±1 thermal mismatch in aerospace 514±15 vibration damping, joint strength and adhesive strength 245±6, 247, 249 and water uptake 133±6 strength wearout model 226 stress analysis 91±122 closed form, global stress analysis 97±107 design stress analyses 106±7 designing bonding of composites 280±1 finite element method see finite element method future developments 118±19 lap joints 98±103 local singular adhesive stresses 96±7 other joint configurations 104±6 qualitative description of adhesive joint stresses 91±7 sources of adhesive joint stresses 92±6 stress-diffusion coupling 115±16 stress intensity factor approach 191±4 stress-life approach 212, 213±26, 235±6 effect of test geometry and lifetime prediction 223±4 effects of test environment 218±23 fatigue loading effects 216±18 S-N curve 213±14 test methods 215±16 variable amplitude fatigue 224±6 stress ratio (R-ratio) 211, 216±17, 228±9 stress-strain curves 490±3 stress wave 164±5 see also impact behaviour stressed skin panels 330 stresses 189 distribution subject to impact load 165, 181±5 environmental effects 130±2, 133 sources of fatigue loading 210 and strains 92 thermal see thermal stresses through-thickness stresses of composites 281±3 see also stress analysis structural adhesive joints 91 cars 377±8 to metals in wet surroundings 125±33 vibration damping 249±51 see also stress analysis

Index structural adhesives acrylic 29±30 rubber toughening 30 timber 336±7 structural damping see vibration damping structural failures 189 structural stiffness 363±4 stylus profilometry 56±9 styrene butadiene rubber (SBR) 34 in shoes 421±3, 446±7, 448 sub-modelling 110, 119 substrate corners, sharp 96±7, 108±9, 116±17 supercritical fluids 445 `Superglue' 19 surface chemical analysis 67±73 ToF-SIMS 67, 70±3 XPS and AES 67, 68±70 surface free energy 66±7, 266±7 surface mount devices (SMDs) 478±9 surface mount technologies (SMT) 478±9 surface preparation see pretreatments surface roughness see roughness, surface surface thermodynamics 64±7 surface topography 53±64, 75, 76 SEM 53±6, 57 SPM 59±64 stylus profilometry 56±9 swelling stresses 96 synthetic polymers see polymers T-peel test 441±2 tackifiers 35, 428 Tacoma dome 328 taper 108, 109 aerospace joints 502, 503 tapered double cantilever beam (TDCB) 227±8 tapes, adhesive 272 tapping mode AFM (TM-AFM) 62 tearing mode 191 temperature aerospace 490±1, 508±9 construction with timber 334, 340±1 curing temperature 260 fatigue and 219±23, 232±3 reactivation temperature 441 resistance in boats and marine 394, 410, 411 tensile tests adhesive 245±6, 247 boats and marine 407±8, 410 composites 281±3, 287 joint 245, 247

541

terpene phenolic resins 438 testing aerospace 515±20 automobiles 381 boats and marine 403, 404±5 bonding of composites 280, 287±90 construction with steel and aluminium 311±12, 320±4 construction with timber 346±8 electronics 476±8 fatigue 215±16, 227±8, 412, 413±14 industrialisation of glue making 12±15 non-destructive see non-destructive testing shoes 439±42 Theophilus 8 thermal imaging 157 thermal mismatch 514±15 thermal stresses 95±6 thermal-stress problems 115 thermodynamic work of adhesion 36±7, 39±41, 137±8, 196±7, 198 thermography 155±7, 381, 405 thermoplastic adhesives 262, 377, 444±5, 458±9 thermoplastic (TR) rubber 421, 422, 423, 445±9 thermoset adhesives 262, 458±9 thick adherend shear test (TAST) 287, 493, 494 thickeners 431, 438 thickness adherend 507±9 joint 395±6 through-thickness strength 281±3, 296±300 three-dimensional analyses 110 through-thickness strength 281±3, 296±300 through-transmission ultrasonic technique 147, 150, 152 tilted sandwich debond (TSD) mixed mode testing 290 tilting 346 timber see wood time-dependent behaviour 113±14 time-dependent fracture mechanics parameters 226, 231±2 time-of-flight secondary ion mass spectrometry (ToF-SIMS) 67, 70±3 tip shape 401±2 titanium alloys 77, 78 tooling 270, 273

542

Index

topography see surface topography traction separation laws 203, 204 Trade Guilds of London 9 transient thermography 155±7 traveller coupons 517, 518 trees, adhesives from 6 trichloroisocyanuric acid (TCI) 423 trim assembly 359±60, 367±9

generalised Volkersen theory 342 voids 144 bond testers 152±4 rapid scanning methods 154±9 ultrasonic testing 145±52 volatiles 263 von Mises yield criterion 189 vulcanisation 422

Uccello, Paulo 8 ultimate design load 491±3 ultrasonic testing 145±52, 381 aerospace 515, 516, 520 basis of the technique 145±6 boats and marine 404, 405 bond testers 152, 153 cohesive property measurement 159±60 data presentation 151±2 interface problem 160±1 test configurations 157±8 transducers 148±51 underanodising 517 underfills 457 encapsulant 460, 470±1 interfaces with polyimide 470 no-flow 480±1 unified visco-plastic models 113±14 United States glue industry 10, 11 unsaturated elastomers 85±6 upper materials for shoes 419±21, 439 urea-formaldehyde adhesives 16, 17 UV (ultraviolet) radiation 123 protection of glazing adhesives 368 resistance in boats and marine 394±5 surface treatments in shoe industry 447, 448±9 UV-ozone cleaning 467 UV-stabilisers 124

water 123, 125±40, 316 and adhesive interfaces 137±40 and adhesives 133±7 behaviour of structural joints to metals in wet surroundings 125±33, 134 cleaning with water in electronics industry 466 diffusion into adhesive bond-lines 133±6 fatigue 219±23, 233±4 hardening by loss of 24±6 humidity see humidity hydrolysis 137 moisture absorption in aerospace industry 498 moisture absorption in electronics 470 moisture content for gluing timber 337, 339 moisture resistance in boats and marine 393±4 reversible and irreversible processes 136±7 stress analysis and moisture distribution 115±16 surface treatment and moisture resistance 127±9 unique properties 43 wet ageing and bonding of composites 296, 297 water-based surface treatments 445±7 water break test 65 water jet transducer (squirter) 146 water pressure 392 water solutions/pastes 25 waterborne adhesives 443 polychloroprene adhesives 436±8 polyurethane adhesives 428±33 weak boundary layers (WBLs) 47, 75, 418 weathering tests see ageing tests wedge-crack test 495±6, 518 Weibull statistics 204±5 `weldable' FRP sandwich panels 389±90, 394 weldbonding 274±5

Van der Waals forces 23±4, 35±6, 41, 42 variable amplitude fatigue 224±6 vibration damping 240±53 damping in joints 241±2 experimental data for adhesively bonded joints 244±51 future trends 251±2 prediction methods 242±4 vibratory loads 210 video extensometry 290 viscosity 397, 427 viscous damping 241 viscous modulus 426 visual inspections 519±20 Volkersen's shear lag model 181, 182±3

Index welding 264, 358 wet (green) gluing 337, 352 wetting 64 Wilhelmy plate method 66 windows see glazing; windscreen bonding windscreen bonding 367±9 bonding repair 381 wires 272 wood building and construction with 328±56 characteristics 331±2 durability of wood joints 140 formaldehyde condensate adhesives for 31 improved control of raw material 353±4 influence on strength and durability 339±40

543

properties and adhesives 333±4 surface preparation 337±8 variability 329 woodworking industry 333±4 work of adhesion 36±7, 39±41, 137±8, 196±7, 198 working loads 210 X-cor 300 X-ray photoelectron spectroscopy (XPS) (also known as ESCA) 67, 68±70, 76 youth failures 468±9 Young equation 64 Z-pinning 298±300 zinc oxide 435, 437