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THE COMPOSITE HAS CHARACTERISTICS QUITE DIFFERENT FROM THOSE OF THE SEPARATE MATERIALS

TECHNICAL & PRACTICE

Bryan Harris examines the structural use of fibre composites, which is illustrated with a case study on the carbon-fibre SNOFA overleaf.

Conventional materials are never as strong as we expect because they contain defects of various kinds which cannot be eliminated in practical manufacturing. The strength of bulk glass and other ceramics is determined not by their strong covalent bonds, but by the tiny pores or sharp cracks that exist on the surface or in the interior. The strength of any sample of a glass or ceramic is determined by the size of the largest defect or crack which it happens to contain, and the strength of the best bulk ceramic will rarely exceed about one thousandth of its theoretical strength.

If flaw sizes can be reduced by control of the manufacturing process, the strength of the material will be raised and its variability reduced. One of the most effective ways of doing this is to produce the material as a fine fibre, and this has been achieved in the case of glass, carbon, and polymers. If these strong fibres are embedded in a matrix of some other material, such as a polymeric resin or cement paste, the resulting composite is a structural material with characteristics quite different from those of the separate components, and can be tailored to suit specific requirements.

The main reinforcing filaments currently used in structural composites are glass, carbon, and polymeric fibres, including aromatic polyamides like Kevlar and polyethylene. Most of these commercial fibres are obtainable in a variety of forms, including continuous tows, woven or braided tows, and chopped bundles.

These materials have profiles of properties quite different from those of other engineering materials. They can be made in forms that are highly anisotropic or they may be made isotropic and they must be used accordingly. The best (and most expensive) materials have high strengths and rigidities, but only in one direction. One of their most attractive features is that many composites exhibit very high levels of toughness. Some of the advantages of fibre-reinforced plastics are as follows:

high strength/weight and stiffness/weight ratios compared with steel and concrete;

rapid installation without heavy lifting equipment;

resistance to harsh environments (hot, cold, wet, chemical);

flame retardants can be added: fibre-reinforced plastics exhibit ablative properties in fire (like space-shuttle tiles) which increases the time available for evacuation of a burning structure;

transparency to electromagnetic radiation;

good impact and blast resistance;

wide variety of surface appearances/finishes/colours (including optical transparency, if required);

good-quality products normally require little maintenance (although they are not 'maintenance-free'); and - excellent means of repairing/strengthening other materials by adhesive bonding.

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