Impact damage initiation in composite materials

A series of low velocity impact tests have been carried out on a (0°, 90°) glass fibre reinforced epoxy resin in order to investigate the influence of varying key impact parameters on the damage initiation threshold. Initial tests have confirmed observations made by previous researchers, that is that the impact force required to initiate damage, Pcrit, varies linearly with t3/2, where t is the target thickness. This relationship has been shown to apply for test temperatures between 23 and 90 °C. The experimental evidence suggests that the influence of test temperature on damage initiation is complex, although the evidence does suggest that the initiation force increasing with temperature in the thinner laminates. It has also been shown that this threshold does not exhibit any significant target size dependency, for the range of plate geometries investigated here. A final series of tests to investigate the influence of impactor geometry have shown that Pcrit increases with indentor diameter, with the most significant changes being observed in the thinner laminates.The damage initiation threshold was predicted using two previously-published models, one based on the interlaminar fracture toughness of the composite and the second on the interlaminar shear strength, ILSS. Here, it was shown that the latter can successfully predict the variation of Pcrit with both target size and impactor geometry.