Micromechanisms of crack extension in composites

Abstract

Typical engineering composites consist of brittle fibres, such as glass or carbon, in a weak, brittle, plastic matrix such as epoxy or polyester resin. An important characteristic of these composites, however, is that they are surprisingly tough, largely as a result of their heterogeneous nature and the manner of their construction. During deformation, microstructural damage is widespread throughout the composite, but much damage can be sustained before load-bearing ability is impaired. Beyond some critical level of damage, failure may occur by the propagation of a crack which usually has a much more complex character than cracks in homogeneous materials. Crack growth is inhibited by the presence of interfaces both at the microstructural level between fibres and matrix and at the macroscopic level as planes of weakness between separate laminations in a multiple laminate. The fracturing of a composite therefore involves not only the breaking of the load-bearing fibres and the weak matrix, but a complex combination of excursions along these weak interfaces. A variety of mechanisms has been suggested to explain the overall level of toughness in a composite material. These are discussed in the light of experimental results on a range of composite types and in relation to the common materials and environmental variables which affect their mechanical properties.