How does the design process differ between composites and metals?

When designing a composite component or structure, there are many possibilities for engineering the material itself to create more efficient structures. The matrix material, the fibres, and the arrangement of the fibres in the material, can be optimised to carry the required structural loads, in a way which is not possible for isotopic materials such as metals.

Composite material suppliers have developed a vast range of polymer composite resins and fibres. Each component of a polymer composite system that a supplier offers, has been optimised for some particular characteristic, for example, a physical property, an improved resistance to a hostile environment or an ability to work with a specific manufacturing process. The size of this range of materials means that the initial selection process can be more difficult than when working with other engineering materials such as metals. However, the range of materials offered by suppliers can be reduced rapidly to identify candidate materials by applying established elimination and ranking techniques. Indeed, this range of candidate materials presented to the design engineer is one of the key strengths of working with composites. If an existing material system can not be identified that will meet requirements, then composite materials design from a fundamental level can be approached with new resin and fibre combinations.

Selection of candidate resins and fibres and the nature of the reinforcement is, however, only one stage of the material design process. The arrangement of the reinforcement in the matrix opens up further routes to optimised material performance. In some applications stiffness and strength requirements can be met by using random orientation of fibres within the matrix. For higher performance components, directional fibres may be used and structural laminates may be built up from differently aligned layers of reinforcement.

Component shape, reinforcement form and cost are the dominant factors when selecting a manufacturing process. When designing with composites the manufacturing route will impact on the mechanical performance of the structure and it is therefore essential that the selection of materials and the selection of a manufacturing process must be considered in parallel in any design process.

There are cases where standard structured profiles exist and this facilitates rapid solutions to fundamental engineering problems. However, it is usual that optimisation of a complete structure is required in the design process. Again, this should be seen as an area of strength for composites as, through optimisation at the design stage, higher performance and lower maintenance structures can be obtained.

Key to the design process is the need to obtain accurate materials data. When designing with isotropic materials such as metals, simple modulus and strength data is available from most "good bookshops", but for most composite material systems, these data are generally not available. Manufacturers carry some design data but data sets from these sources are usually incomplete. The composites design engineer requires these data at an early stage of the design process. There are many standard tests for obtaining composite material properties. When validated data is not available for the material in question standard mechanical tests can be carried out.