MATRIX CRACKING INITIATED BY FIBER BREAKS IN MODEL COMPOSITES

Fracture of resin in a composite material can be initiated by a tensile break in a fibre. This process has been investigated for a simple model composite, consisting of two inextensible rods placed along the axis of a cylindrical elastic block and touching in the centre. The rods represent a broken fibre. Energy release rates, G, were calculated by finite element methods for a circular crack growing outwards from the point where the rod ends separated as they were pulled apart. Results are compared with experimental observations on cracking of a silicone rubber cylinder containing two steel rods. It was found that a crack grew outwards under increasing load until its radius reached a certain size, approximately half-way to the surface of the resin cylinder. At this point, G reached a minimum value and then increased. Simultaneously, the crack accelerated and the sample broke. Forces required-to propagate the crack were successfully predicted by linear elastic fracture mechanics at all stages of crack growth and for a wide range of fibre and sample radii. In particular, good agreement was obtained with the maximum force that the model system could support, i.e. the breaking load. When the sample was surrounded by a rigid tube, representing neighbouring fibres surrounding the broken one, growth of a crack required an increasing load at all stages. The sample finally fractured when the broken fibre pulled out with resin still attached to it. Application of these results to unidirectional fibre-reinforced materials is discussed.