Materials for engineering, 3rd Edition - (Malestrom)

Composite materials 191
the form of wire is fed into an atomising arc and the resulting metal vapour
is vacuum sprayed on to a drum with a fibre mat wound round it.
The disadvantage of MMCs produced by liquid metal techniques is that
they offer increased performance but at increased cost and this led to the
withdrawal of some major UK companies from the production of MMCs.
These materials have good prospects for the future, however, and it is expected
that they will experience steady incremental growth in their application in
performance-limited markets.
Continuous fibre MMCs have found some application in aerospace
structures, and automobile connecting rods have been manufactured from
aluminium reinforced with fibres of stainless steel. Discontinuous MMCs
with dispersions of silicon carbide fibres or particles can be shaped by standard
metallurgical techniques and these have found application in aluminiumbased
automobile engine components, for example in the selective
reinforcement of the ring area of diesel engine pistons. This reduces weight,
and increases wear resistance and thermal conductivity.
The addition of aluminium oxide fibres (Saffil) to aluminum increases the
wear resistance and stiffness, while reducing the coefficient of thermal
expansion, and this has also led to new uses of such MMCs in piston crowns
of internal combustion engines.
6.3 Cellular solids
In the cellular solids category we include cellular solids that occur in nature,
such as wood and bone, as well as man-made cellular polymers such as
foams. Foams are made by the entrapment of gas (either physically or
chemically introduced) while the polymer is liquid, and the resultant lightweight
products are used in thermal insulation, for providing buoyancy, and
also in load-bearing applications such as cushioning and padding.
Wood is the most widely used of all structural materials, since a ten times
greater volume of wood is used annually than of iron and steel. There are tens
of thousands of species of trees, each of which will possess its own mechanical
and structural characteristics, and furthermore there is a large variability in
the properties of a particular type of tree. Typical mechanical properties of a
few woods are given in Table 6.2, but from a design point of view it should
be borne in mind that such properties may vary by up to ± 20%.
Wood consists of cellulose, hemi-cellulose and lignin. Lignin is an
amorphous polymer and acts mainly as a matrix for the other constituents.
Cellulose consists of long-chain molecules, present in slender strands called
micro-fibrils. Hemi-cellulose is similar to cellulose and is partly amorphous
and partly oriented.
Commercially important trees come from botanical classes known as
‘softwoods’, which are from coniferous trees, and ‘hardwoods’ which are