On the Increase in Fracture Energy with Thickness in Delamination of Unidirectional Glass Fibre-Epoxy with Cut Central Plies

The effect of thickness on delamination is investigated by means of static tension tests on unidirectional glass fibre-epoxy specimens with cut central plies. It is found that the delamination stress decreases with increasing thickness, but by significantly less than predicted on the basis of classical fracture mechanics. This indicates that the fracture energy is not constant and therefore cannot be considered to be a material prop erty. Tests on specimens with 8 cut and 32 continuous plies showed an increase of 69% in the fracture energy compared with specimens with 1 cut and 4 continuous plies. Tests on specimens with different ratios of cut to continuous plies suggest that it is the number of cut plies and the magnitude of the applied stress which influence the fracture energy. It is postulated that the fracture energy is a function of the size of a plastic zone which is dependent on the specimen geometry and applied stress. Caution therefore needs to be exercised in using values of fracture energy in situations different from the ones under which they were measured.