A greater level of stiffness and strength is achieved with increases in the volume fraction of fibres in the composite. However, an upper limit of approximately 80% fibres exists when the amount of matrix is insufficient to fully surround the fibres.
Good bonding is required between the fibres and the matrix to ensure that load is properly transferred through the composite. Bonding is also required to ensure that the fibres do not separate from the matrix during loading.
Fibres are quite often coated with special agents to improve bonding and moisture resistance.
Fibres are quite often characterised by their aspect ratio l/d, where l is the fibre length and d is the fibre diameter.
The properties of the composite are generally superior for higher aspect ratios, i.e. a longer fibre or a smaller diameter.
The ends of fibres carry less of the load than the centre of the fibre. In a composite made of short fibres there are proportionally more ends implying that the reinforcing is not as effective.
Smaller diameter fibres also have less surface area and are therefore less likely to contain surface flaws that can lead to brittle failure.