Past II

The combinations of two or more different materials are called composite materials. They usually have unique mechanical and physical properties because they combine the best properties of different materials. For example, a fibre-glass reinforced plastic combines the high strength of thin glass fibres with the ductility and chemi­cal resistance of plastic. Nowadays composites are being used for structures such as bridges, boat-building etc.

Composite materials usually consist of synthetic fi­bres within a matrix, a material that surrounds and is tightly bound to the fibres. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibres made of a ceramic mate­rial such as carbon or glass embedded in a plastic matrix. Usually the fibres make up about 60 per cent by volume. Composites with metal matrices or ceramic matrices are called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.

Continuous-fibre composites are generally required for structural applications. The specific strength (strength-to-density ratio) and specific stiffness (elastic modulus-to-density ratio) of continuous carbon fibre PMCs, for example, can be better than metal alloys have. Composites can also have other attractive properties,

Unit5

such as high thermal or electrical conductivity and a low coefficient of thermal expansion.

Although composite materials have certain advan­tages over conventional materials, composites also have some disadvantages. For example, PMCs and other com­posite materials tend to be highly anisotropic —that is, their strength, stiffness, and other engineering proper­ties are different depending on the orientation of the com­posite material. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. The designer who uses composite materials in structures subjected to multidirectional forces, must take these anisotropic properties into account. Also, forming strong connections between separate composite material com­ponents is difficult.

The advanced composites have high manufacturing costs. Fabricating composite materials is a complex proc­ess. However, new manufacturing techniques are devel­oped. It will become possible to produce composite mate­rials at higher volumes and at a lower cost than is now possible, accelerating the wider exploitation of these materials.

Vocabulary:

fibreglass [faibaglcus] — стекловолокно fibre ['faibo'] — волокно, нить reinforced [ri: m'fo: st] — упрочненный expansion [iks'pa^n/sn] — расширение matrix ['meitnks] — матрица ceramic [si'raemik] — керамический specific strength [spa'sifik'strenO] — удельная проч­ность