Hodgkinson Mechanical Testing of Advanced Fibre Composites

1. Auflage 2000
ISBN: 978-1-85573-891-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 384 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

Testing of composite materials can present complex problems but is essential in order to ensure the reliable, safe and cost-effective performance of any engineering structure. This essentially practical book, complied from the contributions of leading professionals in the field, describes a wide range of test methods which can be applied to various types of advanced fibre composites. The book focuses on high modulus, high strength fibre/plastic composites and also covers highly anisotrpoic materials such as carbon, aramid and glass.Engineers and designers specifying the use of materials in structures will find this book an invaluable guide to best practice throughout the range of industrial sectors where FRCs are employed.

Autoren/Hrsg.

Hodgkinson, J M

Inhaltsverzeichnis

1;Front Cover;1
2;Mechanical Testing of Advanced Fibre Composites;4
3;Copyright Page;5
4;Table of Contents;6
5;Preface;12
6;List of contributors;14
7;Chapter 1. Introduction;18
7.1;References;20
8;Chapter 2. General principles and perspectives;21
8.1;2.1 Mechanical testing in perspective;21
8.2;2.2 Formal framework for mechanical test methods;27
8.3;2.3 Special features of the mechanical testing of composites;30
8.4;2.4 Nature and quality of test data;36
8.5;2.5 Mechanical tests for long-fibre composites;41
8.6;2.6 Concluding comments;50
8.7;References;51
8.8;Bibliography;52
9;Chapter 3. Specimen preparation;53
9.1;3.1 Introduction;53
9.2;3.2 Laminate production;53
9.3;3.3 Quality checking;56
9.4;3.4 Specimen manufacture;56
9.5;3.5 Strain gauging;58
9.6;3.6 Summary;59
9.7;References;59
10;Chapter 4.
Tension;60
10.1;4.1 Introduction;60
10.2;4.2 Testing equipment;67
10.3;4.3 Specimen details;73
10.4;4.4 Test procedure;79
10.5;4.5 Data reduction;81
10.6;4.6 Material and sample preparation;84
10.7;4.7 Practical example;87
10.8;4.8 Future developments;88
10.9;References;90
11;Chapter 5. Compression;92
11.1;5.1 Introduction;92
11.2;5.2 Types of test;93
11.3;5.3 Standards;99
11.4;5.4 Specimen preparation;100
11.5;5.5 Specimen configurations;102
11.6;5.6 Execution and problems;104
11.7;5.7 Typical results;106
11.8;5.8 Conclusions;114
11.9;References;114
12;Chapter 6. Shear;117
12.1;6.1 Introduction;117
12.2;6.2 Test methods;118
12.3;6.3 Summary of test methods;135
12.4;6.4 Comparison of data;135
12.5;6.5 Recommendations and concluding remarks;135
12.6;Acknowledgements;139
12.7;References;139
13;Chapter 7.
Flexure;141
13.1;7.1 Introduction;141
13.2;7.2 Three-point and four-point flexure tests;142
13.3;7.3 Comparison of recommended test methods;145
13.4;7.4 Failure modes;150
13.5;7.5 Typical data;150
13.6;7.6 Steel versus soft lined rollers;155
13.7;7.7 Through-thickness flexure;157
13.8;7.8 Conclusions;158
13.9;References;158
14;Chapter 8. Through-thickness testing;160
14.1;8.1 Introduction;160
14.2;8.2 General issues;161
14.3;8.3 Tensile test methods;163
14.4;8.4 Compression test methods;173
14.5;8.5 Shear test methods;177
14.6;8.6 Concluding remarks;184
14.7;Acknowledgements;184
14.8;References;185
15;Chapter 9.
Interlaminar fracture toughness;187
15.1;9.1 Introduction;187
15.2;9.2 Terminology and typical values;187
15.3;9.3 Overview of test methods and standards;190
15.4;9.4 Mode I testing;195
15.5;9.5 Mode II testing;211
15.6;9.6 Mixed mode I/II;217
15.7;9.7 Multidirectional laminates;221
15.8;9.8 Conclusions;223
15.9;References;224
16;Chapter 10.
Impact and damage tolerance;228
16.1;10.1 Introduction;228
16.2;10.2 Impact testing;228
16.3;10.3 Damage tolerance – compression after impact (CAI) tests;245
16.4;10.4 Boeing test methods and related variants;246
16.5;10.5 Data interpretation;252
16.6;10.6 Standardisation status;258
16.7;10.7 Future trends;260
16.8;References;261
17;Chapter 11.
Fatigue;265
17.1;11.1 Introduction;265
17.2;11.2 Basic test philosophy;266
17.3;11.3 Machines and control modes;271
17.4;11.4 Presentation of data;273
17.5;11.5 Monitoring fatigue damage growth;273
17.6;11.6 Potential problems;278
17.7;11.7 Fatigue life prediction;281
17.8;11.8 Post-fatigue residual strength;283
17.9;References;283
18;Chapter 12. Environmental testing of organic matrix composites;286
18.1;12.1 Introduction;286
18.2;12.2 Why environmental testing?;286
18.3;12.3 Environmental threats to composites;287
18.4;12.4 Standard tests;288
18.5;12.5 Sample conditioning;292
18.6;12.6 Experimental approaches;293
18.7;12.7 Determination of sorption behaviour;295
18.8;12.8 Lowering of Tg by absorbed liquids;296
18.9;12.9 How do composites perform in adverse environments?;297
18.10;12.10 Diffusion of liquids into composites;301
18.11;12.11 Classification of absorption categories;305
18.12;12.12 Edge corrections;306
18.13;References;308
19;Chapter 13.
Scaling effects in laminated composites;310
19.1;13.1 Introduction;310
19.2;13.2 Background;311
19.3;13.3 Investigation of failure;311
19.4;13.4 Practical application examples;321
19.5;13.5 Specialised scaling techniques in composites;325
19.6;13.6 Concluding remarks;328
19.7;References;329
20;Chapter 14. Statistical modelling and testing of data variability;331
20.1;14.1 Introduction;331
20.2;14.2 Importance of looking at data plots;331
20.3;14.3 Basic statistics;333
20.4;14.4 Distribution of sample statistics;334
20.5;14.5 Testing for differences between samples;334
20.6;14.6 Comparing several samples simultaneously;342
20.7;14.7 General linear model (GLM);348
20.8;References;356
21;Chapter 15. Development and use of standard test methods;357
21.1;15.1 Introduction;357
21.2;15.2 Development of test methods;358
21.3;15.3 Validation of test methods;360
21.4;15.4 Sources of standards and test methods;364
21.5;15.5 Harmonisation of composite test methods;369
21.6;15.6 Recommended mechanical test methods;372
21.7;References;372
22;Bibliography – selected ISO standards;373
23;Appendix A: Contact details for standards organisations;374
24;Index;376

Leseproben

3 Specimen preparation
F.L. Matthews 3.1 Introduction
There are a number of subsidiary, but vital, issues that are complementary to the main activity of mechanical testing. These issues, taken together, constitute the preparatory work required to produce test specimens of adequate quality. If insufficient attention is given to any of these activities, the results from a particular test could be invalidated. The following remarks relate to the use of specimens of high performance composites fabricated from continuous preimpregnated fibres, the subject of this text. The four stages considered are: laminate production; quality checking; specimen manufacture; application of strain gauges. The final three stages would, of course, apply to any material. 3.2 Laminate production
Thin sheets, known as laminates, usually 1 or 2 mm thick for coupon specimens, are manufactured from layers of fibres preimpregnated with partially cured (if epoxy-based) resin prepreg. The matrix is usually an epoxy, but BMI (bismaleimide) and thermoplastic prepregs are also used. It should be noted that the following discourse relates mainly to epoxy prepregs (owing mainly to their popularity). It should, however, be pointed out that the preparation of laminates with thermoplastic matrices is in many ways a similar but more straightforward process, because the plastic resin is not required to cure, but simply ‘melts’ at a suitably high temperature and resolidifies when cooled. A single prepreg layer is usually 0.125 or 0.25 mm thick and the fibres are either continuous and parallel (unidirectional), or in the form of a woven fabric. The prepreg is supplied as ‘tape’, normally 0.3 m wide (but suppliers having width preferences, woven materials being generally wider than unidirectional products), sandwiched between protective layers of paper or plastic and wound on a reel. If epoxy, the prepreg should be kept in a freezer until it is required; if thermoplastic, low temperature is not a requirement but it is advisable to store the material in a clean, light-free environment. Shelf-life (for epoxies) is normally around 18 months and will be clearly stated by the supplier; thermoplastics, on the other hand, generally degrade very slowly at ambient temperatures. If the prepreg has exceeded its lifetime it can probably still be used for a further six months, at least. However, its suitability should be checked by moulding a test panel, or by checking the cure state of the matrix resin using differential scanning calorimetry (DSC). Appropriate lengths are cut from the reel and placed on top of each other with the fibres in each layer oriented relative to one another in a predetermined sequence. Hand tools, such as a ‘Stanley’ knife drawn against a hard edge, are usually satisfactory for cutting. Fabric prepreg can be cut using shears or scissors. Where available, a rotary knife or water jet could be used. The protective layers are removed before each layer is placed on that previously laid down, and the layer carefully smoothed out to prevent air entrapment. It is essential that the layers are aligned with reference to a datum, since even a few degrees’ misalignment can cause a dramatic effect on mechanical properties. With properly prepared prepreg the edge of the protective backing sheets can be used as a reference. Care must be taken to ensure that twisted or knotted fibre bundles, or prepreg areas containing gaps between bundles, are not included in the laminate. Following completion of the layup, the stack of prepreg layers is prepared for curing in the case of epoxies, or consolidation for thermoplastics. The epoxy resin, which forms the matrix of the composite, is formulated for autoclave curing; the whole curing process lasts several hours and involves a combination of vacuum, raised temperature (to 120 or 175 °C for epoxies, often higher for thermoplastics) and raised pressure. The prepreg layers are contained within a sealed ‘blanket’ as illustrated in Fig. 3.1. 3.1 Arrangement for producing laminates by autoclaving. To prevent the laminate sticking to the base and caul (pressure) plates, the latter can be coated with release agent, or layers of release fabric or a polymeric film are inserted between the plates and the prepreg. A disadvantage of the second approach is that an impression of the fabric is left on the surface of the laminate, thus making it difficult to detect the fibre orientation in the surface layers with the naked eye. As an alternative to autoclave curing it is possible to use a heated press, in which case it is necessary to monitor separately the state of resin gelation, or a press-clave. The latter device, illustrated in Fig. 3.2, is placed in a heated press, in combination with a separate high pressure supply and a vacuum source. A heated press, with facilities for rapid cooling of its platens, would be used for processing advanced thermoplastic prepregs. Clearly the size of the laminate that can be produced will be determined by the size of press available. 3.2 Layout of a press-clave. 3.3 Quality checking
The manufacturing process, if not properly controlled, can introduce defects into the laminate. Typical defects are voids (small cavities in the resin), delaminations (unbonded areas between layers) or, unusually, longitudinal cracks (lack of bonding between fibre and matrix). Voids can be caused if the prepreg is not allowed to warm to room temperature before laying-up, thus introducing moisture into the prepreg stack. Delaminations can be caused by entrapped air or the inclusion of pieces of backing sheet. Longitudinal splitting and delamination can occur in multidirectional laminates as a result of thermal stresses induced during cooldown from the curing temperature. All the above defects will degrade mechanical properties, particularly in compression, shear and flexure. It is, therefore, important that their presence is detected so that faulty laminates can be discarded. The standard method of detection is to use ultrasonic C-scan, which is good at detecting inclusions, porosity and delaminations, or, possibly, X-ray techniques, which can detect through-thickness cracks. 3.4 Specimen manufacture
Specimens, as defined by the relevant standard, or test to be carried out, are cut from the laminates using a diamond-tipped saw. The normal blade has 600 grit, but a cleaner cut, with less damage to the laminate, is obtained with 800 grit. In the latter case the blade can become clogged with debris and frequent cleaning may be required. Laminates produced by autoclaving will have a feathered edge which must be removed. It is clearly vital that edges produced after trimming, which effectively act as a datum for subsequent specimen cutting, are correctly aligned with the fibres in the layers. A commonly used method for establishing the 0° direction of a cured laminate prior to cutting is to split off a narrow strip of material along this direction (in multidirectional laminates this can be done if the 0° layer is made slightly wider), but it has been shown that this approach may not be sufficiently accurate, and a preferred method1 is to mark the outermost ply by scoring across in the 90° direction in a non-stressed region. High temperatures are generated during dry cutting, which can cause local degradation and damage at the machined edge. This can be largely prevented by the use of a coolant (water), but subsequent drying-out steps must be taken to remove any absorbed liquid from the specimen. Generally, specimen blanks are machined oversize, final dimensions being achieved by grinding. Drilling is readily achieved with tungsten carbide or diamond tipped bits, the laminate being supported by a (sacrificial) backing plate. It is advisable to use a drill bit tip angle of 55–60° for thin laminates and 90–100° for thick sections, rather than the usual 120°. Other operations are best achieved by grinding. Clearly, appropriate support is needed for thin laminates if through-thickness shaping is to be carried out. Kevlar fibre-reinforced materials need special attention. Owing to the nature of the fibre it is difficult to avoid a ‘furry’ edge. Specially adapted impregnated wheels can be obtained for cutting. Another alternative is to use a high pressure water jet. For many tests, for example tension and compression, it is often necessary to bond end-tabs to the specimen; this is done to diffuse the gripping loads and prevent failure at the specimen ends. According to the particular requirements, the tabs may be of aluminium alloy, GFRP (glass-fibre reinforced plastic) or CFRP (carbon-fibre reinforced plastic). When the tabs are of composite, the preferred method is to stick strips to the trimmed laminate before cutting into specimens. This approach is not only quicker, but it also ensures alignment of tabs and specimen. When the tabs are metal this approach cannot be adopted and the tabs must be bonded to individual specimens, using a jig to give accurate positioning. Surfaces where end-tabs are to be bonded should be abraded in order to remove surface contamination, whilst taking care not to damage the outermost fibres. This is done most easily, particularly if the laminate surface is rough, by grit blasting, the only objection to this method being that the surface may itself become contaminated, either by grease carried by the grit or embedment of the grit. Surfaces not needing to be abraded can be...