Mechanical Testing of Advanced Fibre Composites Edited by J M Hodgkinson

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Mechanical Testing of Advanced Fibre Composites
Edited by J M Hodgkinson
Mechanical Testing of Advanced Fibre Composites

Contents
Preface xi
List of contributors xiii
1 Introduction 1
j m hodgkinson
References 3
2 General principles and perspectives 4
s turner
2.1 Mechanical testing in perspective 4
2.2 Formal framework for mechanical test methods 10
2.3 Special features of the mechanical testing of composites 13
2.4 Nature and quality of test data 19
2.5 Mechanical tests for long-fibre composites 24
2.6 Concluding comments 33
References 34
Bibliography 35
3 Specimen preparation 36
f l matthews
3.1 Introduction 36
3.2 Laminate production 36
3.3 Quality checking 39
3.4 Specimen preparation 39
3.5 Strain gauging 41
3.6 Summary 42
References 42
4 Tension 43
e w godwin
4.1 Introduction 43
4.2 Testing equipment 50
4.3 Specimen details 56
4.4 Test procedure 62
4.5 Data reduction 64
4.6 Material and sample preparation 67
4.7 Practical example 70
4.8 Future developments 71
References 73
5 Compression 75
f l matthews
5.1 Introduction 75
5.2 Types of test 76
5.3 Standards 82
5.4 Specimen preparation 83
5.5 Specimen configurations 85
5.6 Execution and problems 87
5.7 Typical results 89
5.8 Conclusions 97
References 97
6 Shear 100
w r broughton
6.1 Introduction 100
6.2 Test methods 101
6.3 Summary of test methods 118
6.4 Comparison of data 118
6.5 Recommendations and concluding remarks 118
Acknowledgements 122
References 122
7 Flexure 124
j m hodgkinson
7.1 Introduction 124
7.2 Three-point and four-point flexure tests 125
7.3 Comparison of recommended test methods 128
7.4 Failure modes 133
7.5 Typical data 133
7.6 Steel versus soft lined rollers 138
7.7 Through-thickness flexure 140
7.8 Conclusions 141
References 141
8 Through-thickness testing 143
w r broughton
8.1 Introduction 143
8.2 General issues 144
8.3 Tensile test methods 146
8.4 Compression test methods 156
8.5 Shear test methods 160
8.6 Concluding remarks 167
Acknowledgements 167
References 168
9 Interlaminar fracture toughness 170
p robinson and j m hodgkinson
9.1 Introduction 170
9.2 Terminology and typical values 170
9.3 Overview of test methods and standards 173
9.4 Mode I testing 178
9.5 Mode II testing 194
9.6 Mixed mode I/II 200
9.7 Multidirectional laminates 204
9.8 Conclusions 206
References 207
10 Impact and damage tolerance 211
p j hogg and g a bibo
10.1 Introduction 211
10.2 Impact testing 211
10.3 Damage tolerance – compression after impact (CAI) tests 228
10.4 Boeing test methods and related variants 229
10.5 Data interpretation 235
10.6 Standardisation status 241
10.7 Future trends 243
References 244
11 Fatigue 248
p t curtis
11.1 Introduction 248
11.2 Basic test philosophy 249
11.3 Machines and control modes 254
11.4 Presentation of data 256
11.5 Monitoring fatigue damage growth 256
11.6 Potential problems 261
11.7 Fatigue life prediction 264
11.8 Post-fatigue residual strength 266
References 266
12 Environmental testing of organic matrix composites 269
g pritchard
12.1 Introduction 269
12.2 Why environmental testing? 269
12.3 Environmental threats to composites 270
12.4 Standard tests 271
12.5 Sample conditioning 275
12.6 Experimental approaches 276
12.7 Determination of sorption behaviour 278
12.8 Lowering of Tg by absorbed liquids 279
12.9 How do composites perform in adverse environments? 280
12.10 Diffusion of liquids into composites 284
12.11 Classification of absorption categories 288
12.12 Edge corrections 289
References 291
13 Scaling effects in laminated composites 293
c soutis
13.1 Introduction 293
13.2 Background 294
13.3 Investigation of failure 294
13.4 Practical application examples 304
13.5 Specialised scaling techniques in composites 308
13.6 Concluding remarks 311
References 312
14 Statistical modelling and testing of data variability 314
l c wolstenholme
14.1 Introduction 314
14.2 Importance of looking at data plots 314
14.3 Basic statistics 316
14.4 Distribution of sample statistics 317
14.5 Testing for differences between samples 317
14.6 Comparing several samples simultaneously 325
14.7 General linear model (GLM) 331
References 339
15 Development and use of standard test methods 340
g d sims
15.1 Introduction 340
15.2 Development of test methods 341
15.3 Validation of test methods 343
15.4 Sources of standards and test methods 347
15.5 Harmonisation of composite test methods 352
15.6 Recommended mechanical test methods 355
References 355
Bibliography – selected ISO standards 356
Appendix – contact details for standards organisations 357
Index 359

Preface
Mechanical property data are essential in the design process if structures are to perform as intended – reliably and cost-effectively for their full life. However, there are no data without testing at some stage. But what tests should be carried out to give the required data? How, precisely, should the tests be conducted, and who says so? What does the data actually mean? How reliable are the data produced? Are data obtained from small test specimens meaningful when large structures are being designed? What effect will the operating environment have? Fortunately most, if not all of these questions have been answered in the case of isotropic solids, giving a starting point for the development of mechanical test methods for more complex materials such as advanced fibre composites. This book attempts to set out the current position with regard to these potentially highly anisotropic materials, which are finding repidly increasing applications despite their complexity.

The expression ‘advanced fibre composites’ probably means different things to different people. To many it might encompass only carbon and a small group of thermoplastic fibres including aramid and polyethylene, to the exclusion of glass fibres. However, in some industrial applications, glass fibres, whilst not necessarily being deemed as advanced in any particular sense, are the only fibres which can fulfil the specific design and environmental requirements. So perhaps the term ‘advanced’ in this context is really application driven. As far as this book is concerned much of the discourse surrounds high modulus, high strength fibre/plastic matrix composites, but not exclusively so, it is high performance which is the key. It has been left to the author(s) of each chapter to judge for themselves, from their own interests and experience, precisely what to include. It is in any case quite clear that, for most of the mechanical test methods described, relatively minor modifications allow perfectly good results to be obtained across the whole range of fibre/matrix combinations, from the most exotic to the most humble.

This book has developed out of a short course of the same title which has normally run on a yearly basis at Imperial College (London University) since 1989.The course has now seen over 250 delegates ‘graduate’; they have come from a wide variety of industry sectors and from all over the world.The course has been supported by experts in their field from Queen Mary and Westfield College (London University), the National Physical Laboratory, the Defence and Evaluation Research Agency and City University. I am indebted to these colleagues, and those from Imperial College, who have not only taught on the course but have also given up a great deal of valuable leisure time providing their copy for the book. A special thankyou goes to Professor Geof Pritchard, the only contributor to the book who hasn’t taught on the course, which has a section on Environmental Effects but, in comparison to the book chapter, is probably woefully inadequate. In recognition that not everybody has the same interests in life, this book is organised in chapters dealing with particular types of test (tension, compression, shear, etc.), allowing the reader to ‘dip in and out’ as he/she wishes. It is my hope that the reader finds the book both informative and interesting and that it encourages best practice as it is currently known, across the various industrial sectors making use of fibre-reinforced plastic matrix composites.

It is as well to remember that a bad test is not worth doing and that even the best test can be done badly. It is all in the detail.


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