Usage of composite materials is ubiquitous in the modern world. While global tonnages are still well below those of steel, they now find a wider range of applications and their value is starting to become comparable to that of steel products. As low weight and energy efficiency become increasingly important, this trend is likely to accelerate. In this chapter, the objective is to identify some of the issues involved in commercial exploitation of composites. This is done by means of case studies drawn from various industrial sectors. The examples cover a range of composite type, engineering complexity, manufacturing route, market size and competitive position relative to more traditional materials.

An important aspect of composite materials concerns the technology by which they are produced. Depending on the nature of matrix and fibre, and the required architecture of fibre distribution, production at reasonable cost and with suitable microstructural quality can present a challenge. In most cases, manufacture of the final component and production of the composite material are carried out at the same time. This gives scope for optimal fibre placement and distribution of orientations, but also requires that the mechanical requirements of the application be well understood and that the processing route be tailored accordingly. Fabrication procedures for most commercially important (fibre-reinforced polymer) composites are technically mature, but there are some types of composite for which processing routes are still under development.

The usage of composite materials continues to expand rapidly. The current world-wide market value is not easy to estimate, but is certainly more than US$100 billion. Composites now constitute one of the broadest and most important classes of engineering materials – second only to steels in industrial significance and range of applications. There are several reasons for this. One is that they often offer highly attractive combinations of stiffness, strength, toughness, lightness and corrosion resistance. Another is that there is considerable scope for tailoring their structure to suit service conditions. This concept is well illustrated by biological materials such as wood, bone, teeth and hide, which are all composites with complex internal structures that have been designed (via evolutionary processes) to give mechanical properties well suited to the performance requirements. This versatility is, of course, attractive for many industrial purposes, although it also leads to complexity that needs to be well understood if they are to be used effectively. In fact, adaptation of manufactured composite structures for different engineering purposes requires input from several branches of science. In this introductory chapter, an overview is given of the types of composites that have been developed.

The previous three chapters cover the elastic behaviour of composites containing aligned fibres that are, in effect, infinitely long. Use of short fibres (or equiaxed particles) creates scope for using a wider range of reinforcements and more versatile processing and forming routes (see Chapter 15). There is thus interest in understanding the distribution of stresses and strains within such composites, and the consequences of this for the stiffness and other mechanical properties. In this chapter, brief outlines are given of two analytical models. In the shear lag treatment, a cylindrical (short fibre) reinforcement is assumed, with stress fields in fibre and matrix being simplified (leading to some straightforward analytical expressions). It introduces important concepts concerning load transfer mechanisms, although it is not very widely used for property prediction. The Eshelby method, on the other hand, is based on the reinforcement being ellipsoidal (anything from a sphere to a cylinder or a plate): the analysis is more rigorous, but with the penalty of greater mathematical complexity. The model is only briefly described here. Its use also introduces an important concept – that of a misfit strain, which is helpful in areas well beyond those of the mechanics of conventional composite materials.

In the previous chapter, it was shown that an aligned composite is usually stiff along the fibre axis, but much more compliant in the transverse directions. Sometimes, this is all that is required. For example, in a slender beam, such as a fishing rod, the loading is often predominantly axial and transverse or shear stiffness are not important. However, there are many applications in which loading is distributed within a plane: these range from panels of various types to cylindrical pressure vessels. Equal stiffness in all directions within a plane can be produced using a planar random assembly of fibres. This is the basis of chopped-strand mat. However, demanding applications require material with higher fibre volume fractions than can readily be achieved in a planar random (or woven) array. The approach adopted is to stack and bond together a sequence of thin ‘plies’ or ‘laminae’, each composed of long fibres aligned in a single direction, into a laminate. It is important to be able to predict how such a construction responds to an applied load. In this chapter, attention is concentrated on the stress distributions that are created and the elastic deformations that result. This involves consideration of how a single lamina deforms on loading at an arbitrary angle to the fibre direction. A summary is given first of some matrix algebra and analysis tools used in elasticity theory.

In the previous chapter, procedures were described that allow prediction of the elastic response of a single lamina when loaded at an arbitrary angle to the fibre axis. It was shown that such uniaxial sheets tend to be highly anisotropic, with much greater stiffness when loaded parallel to the fibres than at significant angles to them. Moreover, other aspects of the elastic response are very different in different directions. For these reasons, it is common to stack laminae into bonded sets (laminates), making the elastic properties (and also the strength) more isotropic, and also opening up the possibility of tailoring the properties of a component to the loads that it will experience in service – potentially a major advantage of composites when compared with more conventional materials. In this chapter, the treatment of individual laminae is extended to the case of a laminate with an arbitrary stacking sequence, supplying an analytical tool of considerable value in the design of composite materials.

The previous chapter covered factors affecting strength, in terms of the stresses at which damage and failure occur in composites. In many situations, however, it is the energy that is absorbed within the material while fracture takes place that is of prime importance. A tough material is one for which large amounts of energy are required to cause fracture. Some loading configurations, such as a component being struck by a projectile, provide only a finite amount of energy that could cause failure. In fact, there are many situations in which toughness, rather than strength, is the key property determining whether the material is suitable. In this chapter, a brief outline is given of the basics of fracture mechanics, with particular reference to the energetics of interfacial damage. This is followed by an appraisal of the sources of energy absorption in composites. Finally, progressive crack growth in composites is examined under conditions for which fast fracture is not energetically favoured (sub-critical crack growth).