The members of a class have features in
common: similar properties, similar processing routes, and, often,
similar applications.
Metals have relatively high
moduli. They can be made strong by alloying and by mechanical and
heat treatment, but they remain ductile, allowing them to be formed
by deformation processes.
Certain high-strength alloys (spring
steel, for instance) have ductilities as low as 2%, but even this is
enough to ensure that the material yields before it fractures and
that fracture, when it occurs, is of a tough, ductile type. Partly
because of their ductility, metals are prey to fatigue and of all the
classes of material, they are the least resistant to corrosion.
Ceramics and glasses, too, have
high moduli, but, unlike metals, they are brittle. Their ‘strength’
in tension means the brittle fracture strength; in compression it is
the brittle crushing strength, which is about 15 times larger.
And because ceramics have no ductility,
they have a low tolerance for stress concentrations (like holes or
cracks) or for high contact stresses (at clamping points, for
instance). Ductile materials accommodate stress concentrations by
deforming in a way which redistributes the load more evenly; and
because of this, they can be used under static loads within a small
margin of their yield strength. Ceramics and glasses cannot.
Brittle materials always have a wide
scatter in strength and the strength itself depends on the volume of
material under load and the time for which it is applied. So ceramics
are not as easy to design with as metals. Despite this, they have
attractive features.
They are stiff, hard and
abrasion-resistant (hence their use for bearings and cutting tools);
they retain their strength to high temperatures; and they resist
corrosion well. They must be considered as an important class of
engineering material.
Polymers and elastomers are at
the other end of the spectrum. They have moduli which are low,
roughly SO times less than those of metals, but they can be strong -
nearly as strong as metals. A consequence of this is that elastic
deflections can be large.
They creep, even at room temperature,
meaning that a polymer component under load may, with time, acquire a
permanent set. And their properties depend on temperature so that a
polymer which is tough and flexible at 20°C may be brittle at the
4°C of a household refrigerator, yet creep rapidly at the 100°C of
boiling water.
None have useful strength above 200°C.
If these aspects are allowed for in the design, the advantages of
polymers can be exploited. And there are many. When combinations of
properties, such as strength per-unit-weight, are important, polymers
are as good as metals.
They are easy to shape: complicated
parts performing several functions can be moulded from a polymer in a
single operation. The large elastic deflections allow the design of
polymer components which snap together, making assembly fast and
cheap. And by accurately sizing the mould and pre colouring the
polymer, no finishing operations are needed. Polymers are corrosion
resistant, and they have low coefficients of friction. Good design
exploits these properties.
Composites combine the
attractive properties of the other classes of materials while
avoiding some of their drawbacks. They are light, stiff and strong,
and they can be tough. Most of the composites at present available to
the engineer have a polymer matrix - epoxy or polyester, usually –
reinforced by fibres of glass, carbon or Kevlar.
They cannot be used above 250°C
because the polymer matrix softens, but at room temperature their
performance can be outstanding. Composite components are expensive
and they are relatively difficult to form and join. So despite their
attractive properties the designer will use them only when the added
performance justifies the added cost.
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