Materials for engineering, 3rd Edition - (Malestrom)

Composite materials 189
‘Smart’ fibre composites
The idea of embedding sensors into composites during the manufacturing
process dates from the 1980s. Research activity in this area has increased
significantly in recent years, with several aims.
(i) By embedding sensors which can be integrated during the cure process,
it becomes possible to improve significantly the manufacture of advanced
composites by measuring parameters such as strain, pressure and
temperature.
(ii) When the component is in service, incorporated sensors would be able
to monitor fatigue cracking, corrosion, overload, etc.
The development of smart composites is likely to accelerate the application
of advanced fibre composite materials, for it is extremely difficult to incorporate
the same capabilities in competitive materials.
6.2.5 Metal matrix composites
The matrix in a metal matrix composite (MMC) is usually an alloy, rather
than a pure metal, and there are three types of such composites, namely,
(i)
(ii)
(iii)
dispersion-strengthened, in which the matrix contains a uniform
dispersion of very fine particles with diameters in the range 10–
100 nm,
particle-reinforced, in which particles of sizes greater than 1 µm are
present, and
fibre-reinforced, where the fibres may be continuous throughout the
length of the component, or less than a micrometre in length, and
present at almost any volume fraction, from, say, 5 to 75%.
Production of MMCs
The MMCs can be classified into two broad categories, those in which the
metallic matrix is introduced in a solid, particulate form, and those in which
the metal is melted.
Powder metallurgy. Conventional powder metallurgical techniques, in
which the individual phases are mixed together in particulate form, are
important. After homogenization of the mix, the blended powders are pressed
in an appropriate mould to form a ‘green compact’ of high porosity and low
strength and finally sintered at high temperature (in a protective atmosphere),
often under external pressure (‘hot pressing’), to form the final, dense,
composite. Examples of this type of product are high-speed cutting tools and
mining drills composed of particles of tungsten carbide (WC) in a matrix of