The fracture toughness of conventional materials and composite systems containing non-fracturing reinforcing elements

Abstract

A preliminary study has been made of the fracture surface energies and modes of failure of unidirectionally reinforced fibrous composites containing bicomponent (duplex) reinforcing elements with non-fracturing cores. Local high stress concentrations have been produced by the use of severely notched specimens of the Tattersall and Tappin type which have been tested in three-point bending. Comparisons have been made with conventional materials tested under similar conditions. It has been shown that the duplex fibre-reinforced specimens are able to support a substantially constant load up to deformations at which conventional materials have either failed completely or are able to support only relatively negligible bending loads. The effective fracture surface energies of experimental duplex fibre systems subjected to this degree of deformation are of the same order of magnitude, on a weight for weight basis, as those of the toughest available metal alloys. It is observed that when the permanently deformed duplex fibre systems are subjected to further deformations of a cyclic nature within the relevant elastic range of the material, relatively large amounts of energy are absorbed by frictional losses during decoupling and recoupling of the core-sheath interface. Possible reasons for this are discussed. The influence of transverse reinforcement on the generation and development of secondary matrix cracks has also been investigated.