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Materials for engineering, 3rd Edition - (Malestrom)

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6<br />

Composite materials<br />

6.1 Introduction<br />

A composite consists of a matrix material, within which is a dispersion of<br />

one or more phases of another material. In successful composites, the product<br />

has a combination of properties which is superior to those of the individual<br />

components. Although the dispersed phase can be in the <strong>for</strong>m of particles, in<br />

many important examples it is in the <strong>for</strong>m of fibres, and fibre composite<br />

materials <strong>for</strong>m an important subset of this class of <strong>engineering</strong> materials.<br />

Natural composite materials such as wood and bone have been used by<br />

mankind <strong>for</strong> many thousands of years: these are based on fibres of cellulose<br />

and collagen, respectively. The advantages of deliberately combining materials<br />

to obtain improved properties have been recognized since biblical times –<br />

the fifth chapter of the book of Exodus refers to the importance of the<br />

incorporation of straw in the manufacture of bricks – and many similar<br />

examples may be quoted throughout the history of civilization.<br />

There are many reasons <strong>for</strong> making composites: the incorporation of fibres<br />

into brittle ceramics referred to above produces a composite of enhanced<br />

toughness. Fillers, such as aggregate in concrete, reduce the overall cost of<br />

the product, and additionally improve the compressive strength. The second<br />

phase may furthermore be a gas, as in the manufacture of foamed products<br />

of low density.<br />

On the basis of strength and stiffness alone, fibre-rein<strong>for</strong>ced composite<br />

materials may not be superior to metals of similar strength, but when the<br />

specific modulus (i.e. modulus per unit weight) and specific strength are<br />

considered, then their use implies that the weight of components can be<br />

reduced. This is an important factor in all <strong>for</strong>ms of transport, where reductions<br />

of weight result in greater energy savings.<br />

We will first discuss how composite materials may be fabricated and then<br />

consider the extent to which their mechanical properties may be understood<br />

in terms of some simplified models.<br />

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