Mechanical Testing of Advanced Fibre Composites

J M Hodgkinson

• Publisher: Woodhead Publishing
• Year: 2000
• Print ISBN: 9781855733121
• eBook ISBN: 9781855738911
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & 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.

1

Introduction

J.M. Hodgkinson

In the mind of the general public the term ‘composite materials’ is largely either misunderstood or not understood at all. There is a reasonable idea of what might be expected of some other materials and what they might be used for. Steel is used for fabricating the skeleton of many buildings (or inside the concrete) and for most automobile body shells; copper is used for electrical wiring; aluminium is used a lot in aeroplanes; plastics (of what-ever colour) are used for almost everything. However, even with these isotropic homogeneous materials there is little real understanding of why they are used for particular applications. This is not an unreasonable situation. Most people have more to concern themselves about in their lives than why a specific screw could be made from steel, brass or a plastic. It does not matter whether people understand, or not. Quite rightly the expectation is that the goods that they purchase, or make use of in some way, are fit for purpose. This is where the mechanical and other types of materials testing comes in.

In order to design a structure or component so that it is efficient and fit for purpose, the shape of each subcomponent needs to be decided upon, taking into account the material it is to be made from. This means that careful consideration must be given to the intimate relationship between how the component is supposed to perform in service and the properties of the material from which it is made. This can be a tricky balancing act even with isotropic homogeneous materials but substantially more difficult when attempting to make use of materials which are not isotropic and not homogeneous.

How does one go about deciding what a material is capable of, mechanically speaking? Well, first one needs to know what the beast one is dealing with is made of, and in this book we are concerned with what are generally termed advanced fibres in a plastic matrix. The fibres involved in the discussion are carbon, aramid and glass, normally continuous rather than short fibres. The resins considered are epoxies and a variety of thermoplastics. For the most part, but not exclusively, we are concerned with laminates of these fibres and resins. Given any knowledge of the way that homogeneous materials react to the application of loads of varying types, it does not take much thought to come to the conclusion that these fibre-reinforced plastics are an entirely different breed, and that considerable thought and experimentation might be needed to describe, adequately well, their mechanical properties.

This is what this book is all about. We need to be able to establish how these materials react to all types of loading, be they tensile, compressive or shear, of short-term or long-term duration, or cyclic, in the presence of high or low temperatures, or other environments which might significantly modify their behaviour, in the same way that we can for homogeneous materials. Designers can then make use of the information to create structures which perform within the design requirements. These structures include large parts of military and civil aircraft, racing cars, automobiles, buses, coaches, lorries, railway and military vehicles, boats, ships and other marine vehicles, a wide variety of sports, home, office, recreational and other leisure goods and, increasingly, civil engineering structures.

The tests which can be carried out to ascertain the behaviour of these materials depend on testing machines which have been designed and built, not necessarily with this particular range of materials in mind, but are generally adequate for the purpose. Quite frequently it is the subtesting equipment (i.e. testing jig), specimen design and other experimental arrangements which address the special reqirements of these materials.

Subsequent chapters in this book describe the specimen design and how the tests might be carried out, as far as possible to best practice, under different loading regimes, with due regard given to the statistical analysis of the data produced and progress in the development of test methods from initial conception to full international acceptance.

During the period of this book’s development there have been numerous initiatives by standards organisations worldwide to update existing methods and produce new standard test methods to satisfy (or at least to attempt to satisfy) the particular requirements of advanced fibre-reinforced plastic matrix composite materials. The ‘push’ for these better, or new, test methods to be developed, refined, written into standardised form and finally adopted, preferably at international level, has come from the ‘grass roots’, largely (but not exclusively) driven by the aerospace industry.

Although it is clear that many other organisations were involved in these developments in the 1980s and 1990s (and might have been equally concerned about the dearth of appropriate standardisation for this class of materials), a key catalyst appears to have been the Composites Research Advisory Group (CRAG), which set about in the early 1980s to attempt to define what the best practice should be over a range of test methods. The Group reported the results of its preliminary deliberations in 1985 and in a final report¹ in 1988, but by this time numerous sections of industry, research organisations and university researchers (primarily but not exclusively in the UK) were making use of the recommendations. The CRAG recommendations were proposed to the British Standards Institution and subsequently had a considerable effect in the development of new international standards. From start to finish the process has taken the best part of 20 years to establish a fairly coherent and comprehensive body of standards at international level. One is tempted to suggest that this is an extraordinarily long time. It is also a time during which the influence of the aerospace industry on the future of composite materials has diminished somewhat. At least we are left with the legacy of the standards.

References

1. Curtis PT, ed. CRAG Test Methods for the Measurement of the Engineering Properties of Fibre Reinforced Plastics, Royal Aircraft Establishment, Technical Report 88012; February 1988.

2

General principles and perspectives

S. Turner

2.1 Mechanical testing in perspective

2.1.1 Overall objectives of mechanical testing

Humanity’s utilization of materials has always been supported by testing activities, which have developed over the centuries from crude tests of the fitness-for-purpose of service items to the modern science-based procedures that support all aspects of the science and technology of materials and their utilization. There is now a mutual dependency between advances in scientific knowledge and test method development, with first one and then the other providing an enabling facility for further progress in the development of versatile evaluation programmes capable of supporting various essential industrial operations. In the particular case of mechanical tests those operations include:

• quality control

• quality assurance

• comparisons between materials and selection

• design calculations

• predictions of performance under conditions other than those of the test

• indicators in materials development programmes

• starting points in the formulation of theories.

This list is a simplification, in that some of the functions overlap and several are linked by lateral connections which become effective at various stages in the conversion of materials into end-products. But, in isolation, these functions make different demands on the data, and therefore, the resources that are deployed need to be matched carefully to the demands of particular circumstances. For instance, quality control can usually be achieved by the use of simple test procedures provided that they reflect relevant mechanical characteristics of the product; the simplicity of the test procedure and precision of the data are usually deemed far more important than scientific rigour and accuracy whereas, in contrast, the priorities would be reversed for a procedure used to generate data for a design calculation.

Some test methods are multipurpose via a variety of operating procedures. Thus, a conventional tensile test operated under fixed conditions may serve a quality control function whereas, operated with controlled variation of influential factors such as temperature and straining rate, it may provide a first-order estimate of load-bearing capability. On the other hand, some test methods are uniquely dedicated to a single purpose and the data they yield could be misleading if used in a wider context.

There is another complication in materials testing. The property value derived from a mechanical test varies with the state of internal order of the tested item, which for many classes of material is sensitive to the production route and other factors. Each sample or test specimen is then unique, and derived data must be regarded as relating just to it, rather than to the material in general. The corresponding properties of the latter, or of other samples, have to be inferred. There are, therefore, far-reaching ramifications for the scope of test programmes, evaluation strategies, the mode of utilization of the data, design procedures and so on.

The variations in material state are commonly in the molecular or atomic orders which, after the processing stage, slowly change towards a state of greater order. In a fibre composite the molecular reordering process generally occurs in the matrix and at the fibre–matrix interfaces. However, the dominating source of variation is the spatial distribution of the fibres, which may change inadvertently during the manufacturing stage, or may be changed deliberately by the fabricator to induce a particular mechanical ef...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• List of contributors
• 1: Introduction
• 2: General principles and perspectives
• 3: Specimen preparation
• 4: Tension
• 5: Compression
• 6: Shear
• 7: Flexure
• 8: Through-thickness testing
• 9: Interlaminar fracture toughness
• 10: Impact and damage tolerance
• 11: Fatigue
• 12: Environmental testing of organic matrix composites
• 13: Scaling effects in laminated composites
• 14: Statistical modelling and testing of data variability
• 15: Development and use of standard test methods
• Index