Glass Fibre Reinforced Plastic (GFRP)

The first man-made fibre glass can be spun into cloth and used for fire-proof curtains or made into fibres which are able to transmit light over long distances.

The ultimate tensile strength of undamaged, very small diameter glass fibres is extremely high, although the strength is reduced significantly if the fibres are slightly damaged.

In its structural use, it is often merely referred to as glass fibre or fibreglass, when glass fibres (in various forms) are bonded together by appropriate resins.

When moulded with resin, the resulting composite is of considerably lower strength. Nevertheless, good GFRP structures are stronger than mild steel and on a simple strength-for-weight basis, it can be comparable to high tensile steel if the fibre form and lay-up is near optimum. However, it is considerably less stiff than steel or even aluminium.

A graphic example of GFRP flexibility is the enormous deflection, which takes place in the pole during a pole vault. As the glass fibres are about a hundred times stronger than the resin, it is obviously necessary to get as much fibre packed into the moulding as possible.

Non-structural items may be made from a percentage of chopped strand mat (i.e. glass fibres in a random, non- woven state). But where considerable strength is required, uni-directional glass cloth is used.

To provide all round strength, sheets of uni-directional cloth can be layered up at 90º to each other in a similar manner to the grain in plywood. Sometimes, such sheets are used as facings for an internal honeycomb of plastic-impregnated paper, to give a very efficient structure in terms of strength, stiffness and weight.

The glass fibre sheet material can be supplied with cloth already impregnated with resin and partially cured (‘Pre-preg’), in which case it is necessary to keep the material in refrigerated storage. Resin curing is usually done at elevated temperatures (120ºC-170ºC) with the GRP component in its mould. An autoclave is often used under pressure.

The main reasons for using GFRP are:

• In instances where metal cannot be used (e.g. for radar domes or other nonelectrical conducting applications).
• The ease and low cost of producing very complex shapes.
• To provide good strength/weight ratio.
• Its ability to produce selected directional strength.

The main disadvantage of glass fibre is that it lacks stiffness and as such, is not suitable for applications subject to high structural loadings.

Ceramic Fibres

Made by firing clay or other non-metallic materials, ceramic fibres are a form of glass fibre that are used in high-temperature applications. They can be used at temperatures up to 1650ºC and are suited for use around engine and exhaust systems. Ceramic fibres are heavy (and expensive) and are only used where no other materials are suitable.