Impact damage resistance and tolerance of Z-pinned composite laminates

The study examines the effect of Z-pinning on the impact and post-impact behaviour of composite laminates. Unpinned and Z-pinned [02/902]s and [0/±45/90]s carbon/epoxy samples were subjected to low-velocity impacts inducing damage states ranging from barely visible damage to full penetration and subsequently tested in compression to examine the influence of Z-pinning on the resistance and tolerance to impact damage. The fracture modes induced by impact and the damage mechanisms controlling the residual compressive properties of the laminates were examined by radiographic and visual inspections. It was found that Z-pinning does not improve the resistance of the laminates to the initiation of delamination under impact loadings. However, Z-pinning enhances the resistance to delamination growth and improves the residual compressive strength of the laminates for impacts of sufficiently high energy. The beneficial effect of Z-pins may be attributed to their bridging action across delaminated interfaces, which reduces the delamination driving force and improves the resistance to local buckling under compressive loads.