In a material which contains sites of more ductile material, the crack opens by deforming the softer material instead of breaking the bonds of the more brittle parts. The inclusion of less brittle components in a material, may therefore act as crack stoppers. There are special advantages to be gained if the resilient component is a fibre. When fibres are dispersed in a matrix or binder the properties of the composite material may be a useful blend of both fibres and binder. Even if a few fibres break in the path of a crack, they do not completely lose their contribution to resisting tensile forces, which they would do if not bound in a matrix.
Composite materials are both strong and light in comparison with single phase materials.
One of the requirements for a successful composite is good adhesion between the fibres and the matrix. While glass fibres bond well to resins, they do not bond to builders cement. The adhesion of some fibres can be improved by coating the fibre with an agent which will adhere to the matrix. The ceramic particles in dental restorative composites are coated with a silane coupling agent to improve their adhesion to the resin matrix. Another requirement is that the fibres should be long.