Novel hybrid material z-pins for balanced mode I to mode II delamination bridging

Effective through-Thickness Reinforcement (TTR) for prepreg-based laminated composite structures can be achieved by use of Z-pins. Z-pins made from carbon-fibre reinforced rods provide a substantial enhancement of interlaminar fracture toughness in Mode I regimes, whereby the TTR undergoes complete frictional pull-out. However, the performance of carbon-fibre Z-pins in mode II is severely limited by their tendency to experience brittle failure. On the other hand, metal (e.g. steel) z-pins strongly enhance the interlaminar fracture toughness in Mode II-dominated regimes, as they can resist large sliding displacements because of their ductile response. Nevertheless, untreated metal TTR usually experiences low friction during pull-out, hence providing a limited toughening effect for mode I delamination. This paper presents a novel TTR architecture achieved by hybrid z-pins, which consist of a ductile core embedded into a carbon-fibre composite sheath. The key objective is to achieve a “balanced” toughening performance across the full mode-mixity range, combining the benefits provided by monolithic carbon-fibre and ductile metal TTR. The delamination-bridging performance of the novel hybrid z-pins is characterised using an Arcan-type rig, for regimes spanning from pure Mode I to pure Mode II. It is demonstrated that the novel hybrid z-pins provide a substantial enhancement of the interlaminar fracture toughness across the full range of mode-mixity conditions, in contrast to monolithic pins made of single-material constituents.