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In the previous chapter, some background was provided about types of reinforcement and their distribution within different matrices. Attention is now turned to predicting the behaviour of the resulting composites. The prime concern is with mechanical properties. The reinforcement is usually designed to enhance the stiffness and strength of the matrix. The details of this enhancement can be rather complex. The simplest starting point is the elastic behaviour of a composite with aligned long (continuous) fibres. This arrangement creates high stiffness (and strength) in the fibre direction. However, it is also important to understand the behaviour when loaded in other directions, so the treatment also covers transverse loading. In this chapter, and in the following one, perfect bonding is assumed at the fibre/matrix interface. Details concerning this region, and consequences of imperfect bonding, are considered in Chapter 6.
Composites are essentially materials comprising two or more distinct constituents, integrated into a single entity. An important aspect of composite theory concerns the properties that the material exhibits, expressed in terms of those of the constituents and the architecture of the integration. A case of interest is that of a two-constituent system in which one of them is just a void – possibly a vacuum, although more commonly a gas phase. Of course, voids have properties that are substantially different from those of constituents in conventional composites. For example, the stiffness will be effectively zero and the conductivity will tend to be very low. In practice, many materials contain at least some porosity, with the potential to affect certain properties, but in most cases it would not be considered appropriate to classify them as composites. However, very high porosity levels (say, >~30–40%) can justify treatment as a separate type of (composite) material. Sometimes the term ‘foam’ is used in such cases, although the word does carry connotations that would not necessarily apply to all highly porous materials. In this chapter, some composite theory approaches are applied to such materials and information is provided about their ‘microstructure’ (pore architecture), production and potential benefits.
In this chapter, an overview is provided of the types of fibre and matrix in common use and of how they are assembled into composites. Many types of reinforcement, mostly fibres, are available commercially. Their properties are related to atomic structure and the presence of defects, which must be controlled during manufacture. Matrices may be based on polymers, metals or ceramics. Choice of matrix is usually related to required properties, component geometry and method of manufacture. Certain composite properties may be sensitive to the nature of the reinforcement/matrix interface; this topic is covered in Chapter 7. Properties are also dependent on the arrangement and distribution of fibres, i.e. the fibre architecture, an expression that encompasses intrinsic features of the fibres, such as their diameter and length, as well as their volume fraction, alignment and spatial distribution. Fibre arrangements include laminae (sheets containing aligned long fibres) and laminates that are built up from these. Other continuous fibre systems, such as woven configurations, are also covered. Short fibre systems can be more complex and methods of characterising them are also briefly described.
There has been a prodigious level of interest in graphene over recent decades, and also in (the closely related) carbon nanotubes (CNTs). This has included quite a strong focus on their mechanical properties, with various claims in particular being made about high strength levels, which relate to their extremely fine scale. Since the scope for their usage in isolation is relatively limited, at least in terms of exploitation of high strength levels, a lot of attention has been directed towards the production and use of composite materials containing them as reinforcement. Unfortunately, most of the hopes originally expressed have not been fulfilled. In fact, the mechanical properties of such composites have in all cases been inferior to those of conventional carbon fibre composites. This is partly due to severe difficulties in manufacturing composites with relatively high levels of well-dispersed, well-aligned reinforcement. However, this is not the only problem, since many of the original expectations were based on incomplete understanding of the issues involved in defining the strength of a material, with particular reference to the role of toughness. These CNT-reinforced composites tend to have a low toughness, as a direct consequence of their very fine scale. In this chapter, the issue of scale-related effects is first addressed in general terms, followed by information about some specifics of using fine-scale reinforcement (particularly CNTs).
The behaviour of composite materials is often sensitive to changes in temperature. This arises for two main reasons. First, the response of the matrix to an applied load is often temperature-dependent; and second, 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 influence the response of the material to an applied load. Thermal cycling can thus have strong effects on, for example, creep characteristics. 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.
The elastic behaviour of long and short fibre composites is described in Chapters 4–6. This involves considering the stresses in individual plies of a laminate (under an external load) and stress distributions within and around short fibres. This information is now used to explore how a composite material suffers microstructural damage, potentially leading to ultimate failure of some sort. There are two distinct aspects to these (highly important) characteristics. First, there is the onset and development of microstructural damage (mainly cracking of various types) as a function of applied load. Second, there are the processes that cause absorption of energy within a composite material as it undergoes such failure and fracture. The latter determine the toughness of the material and are treated on a fracture mechanics basis in Chapter 9. In the present chapter, attention is concentrated on predicting how applied stresses create stress distributions within the composite and how these lead to damage and failure. The treatment is largely oriented towards long fibre composites (particularly laminates), and also towards polymer-based composites, although most of the principles apply equally to discontinuous reinforcement and other types of matrix.
Certain types of multi-component configuration, while not constituting conventional composite materials, can nevertheless be treated using the approaches and methodologies of composite theory. These include layered systems, particularly the simplest one of a (planar) substrate with a coating (‘deposit’) on one face. This is effectively a real version of the ‘slab model’ that is commonly used to predict composite properties such as stiffness, conductivity, etc. Such property prediction is, of course, valid for a substrate/coating system, although often of limited interest. However, the slab model can be extended in directions that are potentially more useful for coatings. In particular, the creation of curvature, which has only been touched on so far, in the context of (asymmetrical) laminates, can be predicted and utilised (for example to measure residual stresses in coatings). A central concept here concerns the misfit strain, which is used in earlier parts of the book (particularly as it relates to the Eshelby method).
Composite materials, or at least materials that could be regarded as composites, are widespread in nature. This is, of course, a reflection of the many gains in ‘efficiency’ that can be made by integration of two or more constituents. Moreover, the development of artificial composite materials, for mechanical and/or other purposes, has benefited considerably from insights gained by examining bio-composites, and by their direct utilisation. The kingdoms of both plants (wood, grasses, straw, etc.) and animals (bone, skin, teeth, marine shells, corals, etc.) offer many examples of highly successful materials that are essentially composites. Their importance relates not only to lessons about structure–property relationships, but also to the issue of degradation and recycling. While the ‘rotting’ of wood is often regarded as its Achilles’ heel, viable recycling strategies are increasingly required for all materials (and manufactured composites are often perceived as being unsatisfactory in this respect). It is clearly not appropriate in a book of this type to provide great detail about natural materials, or indeed about recycling, but a few of the main principles and issues involved are briefly summarised here.
Previous chapters have mainly concerned 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 local plasticity, debonding, cracking or sliding – in fact, no elastic or inelastic processes of any description. In practice, such phenomena may take place at or close to the interface, depending on its structure and the stresses generated there. These processes can influence the onset and nature of subsequent failure. Before treating the strength and fracture of composites (Chapters 8 and 9), it is helpful to consider the interface region in detail and examine how its response can be characterised and influenced. The meaning and measurement of bond strength are therefore outlined here. This is followed by information about the formation of interfacial bonding in various systems and the scope for its control.
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.