Mode I delamination fatigue resistance of unidirectional and quasi-isotropic composite laminates reinforced with rectangular z-pins

This paper presents an experimental study on the mode I delamination properties of circular and rectangular z-pins in hot-cured carbon fiber/epoxy laminates. Specimens with two different ply stacking sequences (unidirectional and quasi-isotropic) are used to investigate the effect of different pin geometries on the interlaminar fracture toughness and fatigue resistance. Due to a change in the debonding behavior during the cooling stage after the curing process in an autoclave, the use of rectangular z-pins results in an increased mode I fracture toughness compared to circular pins in both unidirectional and quasi-isotropic hot-cured laminates. Furthermore, Paris curves obtained from delamination fatigue resistance tests reveal that rectangular z-pins are more effective at increasing delamination resistance than circular z-pins in quasi-isotropic laminates. In unidirectional laminates, a change in the cross-section geometry of the pins only slightly affects the mode I fatigue resistance of z-pinned composite laminates.