Introduction

Fiber-reinforced composites are a valuable class of engineering materials because they can exhibit both high stiffness and strength simultaneously, in contrast to more homogeneous materials which are generally brittle and defect sensitive. In fiber composites, the inherent lack of toughness of the reinforcing fiber, or its sensitivity to microstructural defects, is overcome by the local redundancy of the composite structure, so that its strength may be utilized effectively. Individual fibers are relatively weakly coupled by the matrix so that failure of one fiber does not generally precipitate immediate failure of the composite as a whole, allowing high strength and stiffness to be achieved in the fiber direction.

The tensile failure of a fiber-reinforced material is a complex process which involves an accumulation of microstructural damage. Unlike homogeneous brittle materials, fiber composites do not contain a population of observable pre-existing defects, one of which ultimately precipitates failure. Instead, an accumulation of fiber or matrix fractures develops as the material is loaded and this constitutes a ‘critical defect’ in a macroscopic view of the fracture. Fracture mechanics may successfully account for the strength of single fibers, but it is inadequate to extend its application to unidirectional fiber composites when the overall behavior is dominated by the probability of defects in fibers propagating under the stress concentrations surrounding previous fiber fractures as well as the probability of defects in the matrix which are responsible for the multiplication of transverse cracks. Consequently, the statistical process of damage development in composites needs to be emphasized (Manders, Bader and Chou 1982).