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Carbon Fibre

PLASTIC COMPOSITES
Carbon fibre has higher heat resistance, tensile strength and durability than glass fibre. When combined with a precise amount of thermosetting resin, such as epoxy, it has exceptional strength to weight ratio. Carbon fibre twill is the most common weave.

High performance composites are used to make lightweight structures that would otherwise not be feasible. The most talked about composite in the bicycle industry is carbon fibre and epoxy resin (CRP). Other high performance composites include glass fibre reinforced plastic (GRP), aramid fibre reinforced plastic (ARP) and metal fibre reinforced plastic (MRP).

Additives, fillers and reinforcement are used to enhance the properties of plastics. They are used to improve specific mechanical properties, such as stiffness and durability. Carbon and aramid fibres are very expensive, so every effort should be taken to minimise material consumption while maximising strength.

Properties
A range of fillers is used to improve the mechanical properties of plastics including talc, minerals, fibers and textiles. Fibre reinforced composite materials have superior strength for their weight, several times greater than metal, and different grades are suitable for everything from racing cars to bike frames.

Various weaves are available to provide different strength characteristics. The direction of weave will affect the mechanical properties of the part. For high performance products this is calculated using finite element analysis (FEA) prior to manufacture. Certain weaves have better drape and so can be formed into deeper profiles. However, fibre alignment is critical; just five degrees of movement can reduce strength by up to 20%. Unlike metals that deform under load, composites shatter, making them vulnerable under certain loads.

Core materials are used to increase the depth of the part and thus increase torsional strength and bending stiffness. The role of the core material is to maintain the integrity of the composite skin. Examples of core materials include DuPont Nomex honeycomb (their trademark for aramid sheet), polyurethane foam and aluminium honeycomb.

Processing
Thermoplastics can be injection moulded, extruded and compression moulded with all of these fillers. Fibre reinforcement may be less than 1 mm long in thermoplastic moulding. Long and continuous strand fibre reinforcement is incorporated into thermosets by compression moulding, composite laminating, filament winding and 3D thermal laminating. The types of fibre used include glass, aramid, carbon and more recently hemp and jute.

Relevant Applications
Bike Frames
Fibre reinforced plastics are relatively new materials in bike frame construction. The high strength to weight, stiffness and resilience of composites makes them very desirable materials. The weave and binder resin can be adjusted to suit the unique stresses and strains on a bike frame, thus producing stiffness and flex where necessary. However, the loads on a bike frame are very complex, even for the most advanced FEA programs, and the full potential of composites has yet to be realised.