Mode II interlaminar fatigue properties of z-pinned carbon fibre reinforced epoxy composites

Improvements to the mode II delamination fatigue resistance of carbon fibre–epoxy composite by through-the-thickness reinforcement with z-pins are experimentally investigated. The effects of the volume content and diameter of z-pins on the mode II delamination crack growth rate and fatigue strengthening mechanisms of composites under interlaminar cyclic shear loading are investigated. Paris-like curves show the threshold cyclic stress intensity value to initiate mode II fatigue-induced growth of delaminations is increased by z-pinning (up to one order of magnitude). The delamination crack growth is also slowed by z-pinning (up to two orders of magnitude). The mode II interlaminar fatigue resistance is improved by z-pins forming a large-scale extrinsic bridging traction zone along the delamination. Fractographic analysis reveals the z-pins fail under mode II fatigue loading by debonding, longitudinal splitting and transverse shear cracking.