Experimental study of Z-pin fatigue: understanding of mode I and II coupon behaviour

Insertion of carbon fibre reinforced polymer rods (Z-pins), is an effective method for the through-thickness reinforcement (TTR) of laminated composites. Whilst Z-pins are primarily used to mitigate delamination under quasi-static loading and/or impact events, the effectiveness of TTR following long-term exposure to cyclic loads still needs to be properly understood. This paper aims to fill the aforementioned knowledge gap by addressing the effect of fatigue on the Z-pins themselves, both in mode I and mode II regimes. The fatigue tests considered here are carried out on single Z-pin coupons, as well as Double-Cantilever-Beam (DCB) and End-Loaded-Split (ELS) specimens. The results indicate that, for relatively small cyclic displacements, Z-pins tolerate fatigue well, exhibiting good residual performance post-fatigue. However, increasing the amplitude of the fatigue loads damages the Z-pins and, hence, it tends to reduce the TTR residual ability to mitigate delamination under further quasi-static or impact loading.