Shear fracture of unidirectional composites without initial cracks

Abstract

A number of different types of shear failure in unidirectional composites are considered. It is shown that there is apparently insufficient energy to allow fracture unless large initial cracks are present. However, sufficiently large defects are not normally found in practice. This is explained in terms of the formation of a plastic zone which allows relative deformation, providing energy to drive the fracture without a crack actually being present. Finite element analysis has been undertaken of a unidirectional stepped plate loaded in the fibre direction. Assuming linear elastic behaviour, the strain energy release rate for mode II delamination from the corner of the step increases from zero up to a constant value with increasing crack length. By modelling the plastic zone with non-linear springs it is shown that significant energy is available from the start of crack propagation and therefore that large initial cracks are not required for fracture to occur. Similar effects can be expected in other cases of shear fracture, and may explain why fracture behaviour is apparently consistent with the presence of large initial cracks even though such defects may not in fact exist.