Experimental investigations into delamination behavior of curved composite laminates reinforced by a pre-hole z-pinning (PHZ) method

In this paper, an experimental study is conducted on the resistance of delamination in curved carbon fiber/epoxy composite laminates reinforced by z-pins. The curvature of the specimens is designed according to that of the aero-engine fan blade roots. A cost-effective pre-hole z-pinning (PHZ) technique is employed to mitigate initial in-plane damage, and the impact of z-pin volume fraction and diameter on interlaminar fracture toughness and bridging behavior is determined through double cantilever beam (DCB) testing. The mode-mixity of z-pin is determined, and the fracture toughness of curved specimens is calculated based on Timoshenko curved beam theory. The results indicate that the mode-mixity varies depending on the location of z-pins. Z-pins are subjected to a mix of crack opening and crack sliding loads during the tests. The primary failure mode of z-pins is pull-out, with a minor portion of the pins experiencing fracture. Specimens with 0.8 vol% z-pins exhibit a fracture toughness that is approximately 567% higher than that of unpinned specimens. The delamination resistance capacity of z-pins is dependent on the mixed-mode ratio, due to the transition in delamination resistance mechanism and failure behavior of the z-pins.