Choosing the manufacturing process

Choosing materials for the reinforcement

Although glass fibres are by far the most common reinforcement, many advanced composites now use fine fibres of pure carbon. Carbon fibres are much stronger than glass fibres, but are also more expensive to produce. Carbon fibre composites are light as well as strong. They are used in aircraft structures and in sporting goods (such as golf clubs), and increasingly are used instead of metals to repair or replace damaged bones. Even stronger (and more costly) than carbon fibres are threads of boron.

Polymers are not only used for the matrix, they also make a good reinforcement material in composites. For example, Kevlar is a polymer fibre that is immensely strong and adds toughness to a composite. It is used as the reinforcement in composite products that require lightweight and reliableconstruction (e.g., structural body parts of an aircraft). Composite materials were not the original use for Kevlar – it was developed to replace steel in radial tyres and is now used in bulletproof vests and helmets.

Making an object from a composite material usually involves some form of mould. The reinforcing material is first placed in the mould and then semi-liquid matrix material is sprayed or pumped in to form the object. Pressure may be applied to force out any air bubbles and the mould is then heated to make the matrix set solid.

The moulding process is often done by hand, but automatic processing by machines is becoming more common. One of the new methods is called pultrusion (a term derived from the words 'pull' and 'extrusion'). This process is ideal for manufacturing products that are straight and have a constant cross section, such as bridge beams.

In many thin structures with complex shapes, such as curved panels, the composite structure is built up by applying sheets of woven fibre reinforcement, saturated with the plastic matrix material, overan appropriately shaped base mould. When the panel has been built to an appropriate thickness, the matrix material is then cured.

In many advanced composites (such as those used in the wing and body panels of aircraft), the structure may consist of a honeycomb of plastic sandwiched between two skins of carbon fibre reinforced composite material. Such sandwich composites combine high strength, and particularly bending stiffness, with low weight. Like everything to deal with aircraft, they can be very costly! Probably composites will never totally replace traditional materials like steel, but in many cases they are just what we need. And no doubt new uses will be found as the technology evolves. We haven’t yet seen all that composites can do.

17. Having learnt all the necessary words and word-combinations which are the specific features for composites you can characterize each of the following ones:

a) reinforced concrete; b) fiberglass; c) metalloceramics; d)sandwich composite.

18. Which of the features are common for these composites?

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