The behaviour of composite materials is often sensitive to changes in temperature. This arises for two main reasons. Firstly, the response of the matrix to an applied load is temperature-dependent and, secondly, changes in temperature can cause internal stresses to be set up as a result of differential thermal contraction and expansion of the two constituents. These stresses affect the thermal expansivity (expansion coefficient) of the composite. Furthermore, significant stresses are normally present in the material at ambient temperatures, since it has in most cases been cooled at the end of the fabrication process. Changes in internal stress state on altering the temperature can be substantial and may strongly influence the response of the material to an applied load. Creep behaviour is affected by this, particularly under thermal cycling conditions. Finally, the thermal conductivity of composite materials is of interest, since many applications and processing procedures involve heat flow of some type. This property can be predicted from the conductivities of the constituents, although the situation may be complicated by poor thermal contact across the interfaces.

Composites make up a very broad and important class of engineering materials. World annual production is over 10 million tonnes and the market has in recent years been growing at 5–10% per annum. Composites are used in a wide variety of applications. Furthermore, there is considerable scope for tailoring their structure to suit the service conditions. This concept is well illustrated by biological materials such as wood, bone, teeth and hide; these are all composites with complex internal structures designed to give mechanical properties well suited to the performance requirements. 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 composite that have been developed.

The previous chapter covered factors affecting strength, which is related to the stresses at which damage and failure occur in composites. In many situations, the energy absorbed by the material under load is equally important. A tough material is one for which large amounts of energy are required to cause failure. In many loading configurations, such as when a component is struck by a projectile, only a finite amount of energy is available to cause failure. In other cases, such as with loads arising from temperature changes, only a finite degree of strain needs to be accommodated in order for the stresses to become small. In such situations, toughness, rather than strength, is the hey 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, slow crack growth in composites is examined for conditions where fast fracture is not energetically favoured.

In the previous two chapters, some background was given about the various types of reinforcement and the ways in which they may be distributed within different matrices. In this chapter, attention is turned to the problem of predicting the behaviour of the resulting composites. Prime concern is with the mechanical properties. The incorporation of the reinforcement is usually aimed at enhancing the stiffness and strength of the matrix. The details of this enhancement can be rather complex and difficult to describe with complete rigour. The simplest starting point is to consider the elastic behaviour of a composite with continuous fibres, all aligned in the same direction. Aligned composites are normally used to exploit the stiffness (and strength) parallel to the fibres. However, it is also important to understand the way they behave when loaded in other directions. The treatment therefore includes the behaviour under transverse loading. In this chapter and in the following one, the assumption is made that there is perfect bonding between fibre and matrix across the interface between them. The detailed nature of the interfacial region, and the consequences of imperfect bonding, are considered in Chapter 7.

The previous two chapters are concerned with the elastic behaviour of composites containing fibres which are, in effect, infinitely long. The preparation of composites containing short fibres (or equiaxed particles) allows scope for using a wider range of reinforcements and more versatile processing and forming routes (see Chapter 11). Thus, there is interest in understanding the distribution of stresses within such a composite, and the consequences of this for the stiffness and other mechanical properties. In this chapter, brief outlines are given of two analytical approaches to this problem. In the shear lag model, a cylindrical shape of reinforcements is assumed, and the stress fields in fibre and matrix are simplified so as to allow derivation of straightforward analytical expressions for the composite stiffness. The Eshelby method, on the other hand, is based on the assumption that the reinforcement has an ellipsoidal shape (which could range from a sphere to a cylinder or a plate). This allows derivation of an analytical solution which is more rigorous than that of the shear lag model, but with the penalty of greater mathematical complexity. In the treatment given here, attention is concentrated on the principle of the Eshelby approach; sources are suggested for readers needing more mathematical details.

The elastic behaviour of long- and short-fibre composites was described in Chapters 4 to 6. The stresses in the individual plies of a laminate under an external load and the stress distributions along short fibres were examined. This information is used to explore the ways in which a material suffers microstructural damage, leading to the ultimate failure of a component. There are two important aspects to this behaviour. Firstly, there is the deflection, degree of damage and ultimate failure of a component as a function of applied load. Secondly, there are the processes which cause absorption of energy within a composite material as it is strained. The latter determine the toughness of the material and are treated in Chapter 9. In the present chapter, attention is concentrated on predicting the applied stress at which damage and failure occur. The treatment is oriented towards long-fibre materials and laminates, and, in particular, towards polymer-based composites. Most of the principles apply equally to discontinuous reinforcement and other types of matrix. Some specific points concerning failure of such systems are dealt with in Chapter 9.

The preceding three chapters have dealt with the elastic behaviour of composites. Among the assumptions made in most of these treatments is that the interfacial bond is ‘perfect’. This means that there is no debonding, cracking or sliding – in fact, no elastic or inelastic processes of any description. In practice, many important phenomena may take place at the interface, depending on its structure and the stresses generated there. These processes tend to promote plastic deformation of the matrix and can also influence the onset and nature of failure. Before treating the strength and fracture behaviour of composites (Chapters 8 and 9), it is necessary to consider the interface and examine how its response can be characterised and influenced. In the present chapter, the meaning and measurement of bond strength are described. This is followed by an outline of the formation of interfacial bonds in various systems and a summary of the techniques used to influence the bonding characteristics.

Composite materials are used in a very wide range of industrial applications. In this chapter, the objective is to identify some of the considerations involved in commercial exploitation of composites. This is done by means of a few case studies and there is no attempt to present a systematic survey. The examples given cover a range of composite type, engineering complexity, manufacturing route, market size and competitive position relative to conventional materials. At the beginning of each case study, a list is given identifying the reasons for preferring a composite to more conventional engineering materials. Although the examples are spread over the full range of matrix types, the bulk of the annual composite production of around 10 million tonnes is currently in the form of PMCs. At the start of each example, a list is given of the requirements of the application.

In this chapter, the underlying science of fibres and matrices is described. Some specific examples are given to illustrate the key factors involved. A wide range of reinforcements, mostly in the form of fibres, is now available commercially. Their properties can be related directly to the atomic arrangement and the defect content of the reinforcement, which must be controlled in the manufacturing processes. Matrices may be based on polymers, metals or ceramics. The choice of matrix is related to the required properties, the intended applications of the composite and the method of manufacture. The properties of the matrix depend on microstructure which, in turn, depends on manufacturing route and subsequent thermal and mechanical treatments. Certain properties of the composite may be sensitive to the nature of the reinforcementi/matrix interface; this topic is covered in detail in Chapter 7.