General Information

A sheet of fibre reinforced material is anisotropic, which means its properties depend on the direction of the fibres. Random direction fibres would result in a much lower strength than uni-directional fibres, laying parallel to the applied load. However, the strength and stiffness of a uni-directional lay-up would be very low with the applied load at 90º to the fibres, as this is primarily a test of the resin. Hence the usual practice of placing alternate layers is at 90º to each other.

Due to small variations in the size of the individual fibres and the final quality of the finished component (which can be affected by careless handling, variations in cleanliness or lay-up, voids, pressures, temperatures, etc.), there will be a greater scatter on final strength than on a conventional, metallic component. Therefore, due allowance on stress reserve factors is essential.

It has already been stated that composites usually have good internal damping characteristics and are less prone to vibration resonances. Where high strength combined with stiffness is required, a CFRP is used. But, when lesser levels of stiffness are necessary, GFRP or AFRP are used.

Composites have very low elongation properties and toughness. Aluminium alloy has a typical elongation-to-fracture value of 11%, whereas composites range from 3% for GFRP to 0.5% for CFRP.

To some extent, the maximum operating temperatures for GFRP, CFRP and Kevlar composites depend on the actual adhesives used. But are generally in the range 220ºC-250ºC.

Some composites, such as carbon fibre in a carbon matrix, have very high permissible operating temperatures (around 3000ºC). They are used for high-energy braking applications and as thermal barriers for space vehicles. Boron, Tungsten, Silicon Carbide and Quartz may also be used to provide fibres for high-temperature composites.