The Relationship between Tensile and Flexural Strength of Unidirectional Composites

Weibull theory predicts a higher strength in bending than in tension. However it assumes that failure initiates from a critical defect, whereas many unidirectional composites fail gradually in bending. A model has been developed of the composite as a bundle, each element of which consists of a small bundle of fibres. The resin is assumed to stabilise the bundle against buckling under compression, whilst allowing the individual elements to split and behave independently in tension. This model also predicts higher strength in bending than in tension, with a ratio of similar order of magnitude to that pre dicted by Weibull theory. The new model provides an explanation for the different strengths in bending and tension which is consistent with the progressive, noncatastrophic failure process which is often observed in bending tests of unidirectional composites.