Effects of carbon-fibre Z-pins on the through-thickness tensile strength of curved composite laminates under four-point bending

This study investigates the influence of discrete through-thickness reinforcement, i.e. Z-pins, on the through-thickness tensile strength (TTS) of curved laminates through four-point bending experiments. Three types of samples are considered: unpinned, and Z-pinned with 0.27 % and 0.54 % areal densities. HexPly® IM7/8552 carbon/epoxy unidirectional prepreg with 0/±45 layup and 0.28 mm diameter T300/BMI pins were employed to manufacture the specimens. The Z-pinned laminates have comparable TTS with the unpinned samples in terms of the first observable load-drop. However, the TTS corresponding to the ultimate load-drop for low-density and high-density Z-pinned samples are 29 % and 38 % lower than for the unpinned ones, respectively. Z-pinned samples show less scatter than unpinned ones in terms of the ultimate TTS values. All specimens failed suddenly, i.e. with no evident damage progression. This implies that Z-pins were not able to form a progressive bridging zone to dissipate mechanical energy. Through a detailed meso-scale finite element analysis (FEA), it was found that the high through-thickness tensile residual stress in the Z-pin neighbourhood generated from the cool-down stage of cure is an important factor in causing the reduction in TTS of Z-pinned laminates. CT scan images of tested Z-pinned specimens show that the carbon-fibre pins experience fracture inside the laminates.