Composite Engineering carbon fibre component design and engineering

A brief explanation of prepreg technology

Composite, made up of distinct parts or elements. [Etymon, Latin. Compositus]

When we mention composites we are referring to a homogenous material, which combines:

Advanced composite fibres, such as aramid / kevlar / polyethylene, carbon, ceramic, S and E glass fibre's.

A thermoset resin matrix, which can either be epoxy, polyester, thermoplastic, vinyl ester and phenolic types.

This then forms the composite laminate. The fibres are the primary load carriers, the resin matrix supports the fibres and transfers the load between fibres.

In some instances, an alternative to a solid laminate can be a sandwich structure. This is created by placing a rigid core material between two laminates. It results in a very stiff and lightweight panel, which in theory is structurally similar to a steel I beam. The core materials utilised are aluminium and Nomex honeycomb, rigid plastic foams and balsa core.

The majority of the components designed by Composites Engineering utilise what are commonly referred to as prepreg's. These are manufactured from continuous fibres which have been infused with a thermosetting resin system, creating a pliable and tacky sheet of material. The precise specification of the fibres, their orientation and the resin matrix can be specified to achieve the optimum lamina performance. Prepreg's can be manufactured with unidirectional or woven fibres and the volume of fibres per square metre can also be specified according to requirements.

To form a component, a mould is created from a pattern and the composite materials that will form the component are then laminated on to the surface of the mould. The composite materials are positioned in the mould in locations specified on the design drawings. The orientation of the fibres is critical as this is instrumental in carrying the loads applied to the component.

At various stages during the process the lay-up is consolidated by covering the component with a nylon film which is sealed to the periphery of the mould with a strip of mastic. A vacuum is then applied to the mould surface and atmospheric pressure consolidates the materials. When the construction has been completed this step is repeated and the mould is then placed in an autoclave (a heated pressure vessel) for curing.

Following a computer controlled curing cycle the component is then removed from the mould. The final construction stages depend upon the configuration of the component It may need to be trimmed by hand, and it may possibly be CNC machined. If it forms part of a larger composite structure, an additional step may possibly be to bond the assembly together with structural adhesive.

Properties of composite fibres and engineering metals

As an illustration of the various aproximate values of the fibres that we incorporate into composite structures, compared with common metals, we have prepared the following charts which will open in a new window.

 Tensile strength Typical density Specific modulus Tensile modulus

The above charts illustrate the various properties of the bare fibres. In a composite structure these are impregnated with a resin system to form the composite laminate. The ultimate performance will be dictated by the resin system and the level of adhesion, the fibre volume fraction and the orientation of the fibres relative to the load paths.