This paper investigates the effects of interfacial crack size and impact damage size on the shear properties and failure mechanisms of a typical marine sandwich composite made from glass reinforced polymer (GRP) skins and a poly(vinyl chloride) (PVC) foam core. An abrupt decrease in the static shear strength and fatigue resistance occurred when the interfacial crack length between the skin and core exceeded ～ 20--30 mm due to the failure mechanism changing from wrinkling of the GRP skin to shear cracking of the foam core. The static shear failure loads of the composite containing inter facial cracks between 20 and 90 mm in length were used to test the accuracy of two models for predicting the strengths of defected sandwich composites. The shear properties were also reduced when the impact damage area was increased, although the magnitude of the reduction was determined by the impact energy.
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