An experimental and numerical investigation into tensile fatigue failure of composite laminates containing wrinkles and cut plies

Fibre-reinforced polymer composites are generally seen as more fatigue resistant than metals. However, layup features such as discontinuous plies and/or manufacturing-induced defects such as wrinkles, can initiate fatigue damage and reduce the overall performance of composites. Through an extensive experimental programme and an advanced progressive damage model, this paper investigates the influence of defects and features, both in isolation and in combination, on the tensile static and fatigue performance of quasi-isotropic IM7/8552 laminates. The numerical model describes delamination and intralaminar matrix cracking using cohesive elements that follow a mixed-mode static and fatigue formulation. Experiments and modelling correlate well regarding both the ultimate static strength and S-N curves. The ultimate static strength shows a clear decreasing trend in the order of pristine, wrinkle, cut-ply and combined cut-ply & wrinkle. The cut-ply and wrinkle plus cut-ply show a similar influence on the fatigue life; both are more detrimental than wrinkle. The decrease of the fatigue life due to the defects/features can be up to two decades at a loading level corresponding to half of the pristine ultimate strength. This study implies that, for the IM7/8552 material, a wrinkle in a region of terminating plies does not add significant reduction to fatigue life.