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There are a variety of other fibres which can be used in advanced composite structures but their use is not widespread. These include:
A low density, high tenacity fibre with good impact resistance but low modulus. Its lack of stiffness usually precludes it from inclusion in a composite component, but it is useful where low weight, high impact or abrasion resistance, and low cost are required. It is mainly used as a surfacing material, as it can be very smooth, keeps weight down and works well with most resin types.
In random orientation, ultra-high molecular weight polyethylene molecules give very low mechanical properties. However, if dissolved and drawn from solution into a filament by a process called gel-spinning, the molecules become disentangled and aligned in the direction of the filament. The molecular alignment promotes very high tensile strength to the filament and the resulting fibre. Coupled with their low S.G. (<1.0), these fibres have the highest specific strength of the fibres described here. However, the fibre’s tensile modulus and ultimate strength are only slightly better than E-glass and less than that of aramid or carbon. The fibre also demonstrates very low compressive strength in laminate form. These factors, coupled with high price, and more importantly, the difficulty in creating a good fibre/matrix bond means that polyethylene fibres are not often used in isolation for composite components.
A very high silica version of glass with much higher mechanical properties and excellent resistance to high temperatures (1,000°C+). However, the manufacturing process and low volume production lead to a very high price (14mm – £74/kg, 9mm – £120/kg).
Carbon or metal fibres are coated with a layer of boron to improve the overall fibre properties. The extremely high cost of this fibre restricts it use to high temperature aerospace applications and in specialised sporting equipment. A boron/carbon hybrid, composed of carbon fibres interspersed among 80-100mm boron fibres, in an epoxy matrix, can achieve properties greater than either fibre alone, with flexural strength and stiffness twice that of HS carbon and 1.4 times that of boron, and shear strength exceeding that of either fibre.
Ceramic fibres, usually in the form of very short ‘whiskers’ are mainly used in areas requiring high temperature resistance. They are more frequently associated with non-polymer matrices such as metal alloys.
At the other end of the scale it is possible to use fibrous plant materials such as jute and sisal as reinforcements in ‘low-tech’ applications. In these applications, the fibres’ low S.G. (typically 0.5-0.6) mean that fairly high specific strengths can be achieved.
Published courtesy of David Cripps, Gurit
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