Can polymer composites directly replace metals?

Many metal articles or components can instead be made from composites, but there are important differences which mean that direct substitution should be made with care.

Most engineering materials are essentially isotropic. That is, they have the same properties such as strength and modulus, in any direction. There may be 'grain' in some metals due to the manufacturing process, but it is only in critical applications that this matters. Most machining or casting processes do not have to take directional differences into account.

Most composites will have very different properties in different directions. This is because, although the matrix material is isotropic, the reinforcement is not. Carbon fibres may be up to 100 times stronger under tension than they are in shear, and the stiffness may differ in the two directions by similar ratios. The properties of the composite will reflect the properties of the reinforcement, so that it can have greatly different properties in different directions. This is exploited in design as manufactured articles rarely require to be equally strong in all directions, and composites can achieve this by particular arrangements of the reinforcement. However, a different design procedure is required for composites compared to that required for metals.