IMPROVING THE MODE II DELAMINATION BRIDGING PERFORMANCE OF FIBROUS COMPOSITE Z-PINS

Composite Z-pins have been traditionally manufactured employing carbon fibres combined with a bismaleimide (BMI) resin. These materials allow a substantial enhancement of the mode I interlaminar fracture toughness of Z-pinned laminates to be achieved. Nonetheless, the improvements in mode II toughness are much more modest, due to the inherent brittleness of the combination of carbon/BMI. In order to increase the mode II dominated toughness of Z-pins, fibrous Z-pin rod-stocks were manufactured employing matrix and fibre materials with mechanical properties dissimilar to those of carbon/BMI. The novel through-thickness reinforcement candidates were inserted in quasi-isotropic glass fibre/epoxy laminates and their single-pin delamination-bridging performance was characterised under a mode II dominated loading configuration. Polybenzoxazole (PBO) fibre pins showed an increase in average energy dissipation per pin by as much as 113 mJ (2000% increase) compared to the 5mJ dissipated by traditional carbon fibre/BMI pins. The effect of twisting the fibre yarns was characterized by including Z-pins manufactured out of twisted PBO fibres. These showed 10-fold increase in performance compared to carbon fibre, albeit a reduction of 50% compared to unidirectional PBO.