Materials for engineering, 3rd Edition - (Malestrom)

6
Composite materials
6.1 Introduction
A composite consists of a matrix material, within which is a dispersion of
one or more phases of another material. In successful composites, the product
has a combination of properties which is superior to those of the individual
components. Although the dispersed phase can be in the form of particles, in
many important examples it is in the form of fibres, and fibre composite
materials form an important subset of this class of engineering materials.
Natural composite materials such as wood and bone have been used by
mankind for many thousands of years: these are based on fibres of cellulose
and collagen, respectively. The advantages of deliberately combining materials
to obtain improved properties have been recognized since biblical times –
the fifth chapter of the book of Exodus refers to the importance of the
incorporation of straw in the manufacture of bricks – and many similar
examples may be quoted throughout the history of civilization.
There are many reasons for making composites: the incorporation of fibres
into brittle ceramics referred to above produces a composite of enhanced
toughness. Fillers, such as aggregate in concrete, reduce the overall cost of
the product, and additionally improve the compressive strength. The second
phase may furthermore be a gas, as in the manufacture of foamed products
of low density.
On the basis of strength and stiffness alone, fibre-reinforced composite
materials may not be superior to metals of similar strength, but when the
specific modulus (i.e. modulus per unit weight) and specific strength are
considered, then their use implies that the weight of components can be
reduced. This is an important factor in all forms of transport, where reductions
of weight result in greater energy savings.
We will first discuss how composite materials may be fabricated and then
consider the extent to which their mechanical properties may be understood
in terms of some simplified models.
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