Fatigue life assessment and damage evolution in Z-pinned laminates

Abstract

An experimental investigation has been carried out to elucidate the damage evolution and failure mechanisms in unpinned and Z-pinned quasi-isotropic carbon fibre/epoxy laminates manufactured via a pre-hole pin insertion technique and subjected to in-plane tension-tension fatigue loading. X-ray computed tomography was used to analyze the internal configuration of samples subjected to several stress levels of the ultimate tensile stress (UTS) in order to identify and understand the internal damage evolution. The obtained results indicate that, at low cycles, the Z-pinned laminates exhibited poor fatigue behavior owing to the low ultimate tensile strength as compared to the unpinned laminates. The stiffness degradation plots and Micro CT images suggest that the Z-pin bridging inhibited extensive edge debonding which resulted in higher normalized stiffness for the Z-pinned laminates. Matrix microcracking initiated around the Z-pins and propagated through resin rich zones leading to poor load bearing ability and subsequent fibre splitting and rupture. It is worth noting that the failure modes of the quasi-isotropic laminates evolved from delamination dominated failure to the on-axial fibre failure.