What is carbon fibre?
Carbon fibre is a composite material, consisting of strong material fibres and a resin. It is sold either as a dry fabric, in sheets or as a narrow tape, where resin is added as part of the fabrication process - or - as a fabric that is already impregnated with resin (pre-preg), where the fabrication process simply activates the resin to cure the final part.
Carbon fibre fabric.
Types / weight of fabric?
All types of carbon are sold as either a fabric, tape or straight tow (long strands of fibres). The fabrics are usually woven with strands laid in two perpendicular directions (warp and weft) with various weave patterns (commonly plain weave or 2 x 2 twill), or strands aligned in one direction, known as Uni-Directional (or ‘UD’). Woven fabric or UD material can incorporate fibres of different weights, which is usually expressed as grams per square metre. For example the most commonly available fabric is a 200gsm 2x2 twill weave.

Using uni-directional fibres can offer an advantage in some applications because the designer is better able to control the location and orientation of fibres in the final part. For example, if a part required twice the number of fibres in one direction than in its perpendicular direction, using a woven fabric would add more fibres in the perpendicular direction than is necessary. This would add (unnecessary) strength, but also additional weight. UD fibres also benefit from being laid ‘flat’ on a microscopic level, compared to woven fibres which need to kink slightly to accommodate the weave pattern.
Strips of UD fabric.
High modulus or high strength?
The carbon fibres themselves are available in different grades of tensile strength and tensile modulus (stiffness), although as with most things there is always a trade off. Many frame manufacturers’ marketing departments insist their frames are made from only the very best ‘high modulus’ carbon, which provides a very stiff frame. Generally the higher the modulus of the carbon the lower its strength, which would make a very stiff frame which was brittle and weak. The reality is that most carbon products use a range of different fibres to achieve the desired material properties.
Most carbon products use a range of fibres to achieve the required stiffness and strength.
Other considerations.
When designing a carbon fibre part it is important to consider the characteristics of the final part and how that can be achieved with different types, weights and weaves of fibres. Carbon can only achieve its maximum strength in the direction the fibres are laid and therefore the orientation of the fabric or tape used is crucial. Part of producing a design is the lay up schedule, which determines where and how specific fibres or fabrics are laid into or onto the mould. The choice of fibres is also informed by the fabrication technique used, for example it may be easier to use a woven fabric with a wet lay up technique even though that may not provide the optimum material strength
A precise lay up schedule is part of the design of any carbon part.