Balance interlaminar improvement and in-plane adverse impact of hexagonal semi-embedded fine Z-pin reinforced polymer composite

This article aims to minimize the adverse impact of Z-pins on the in-plane properties of unidirectional carbon fiber reinforced polymer (CFRP) composites while enhancing their interlaminar fracture toughness by using CF Z-pins. A novel method was developed for implanting fine Z-pins of 0.1 mm diameter with semi-embedded length in the thickness center of laminate in an ultra-low distribution density of hexagonal array pattern. Mechanical properties and failure mechanisms of different Z-pinned composites were analyzed. Taking traditional CFRP as a reference, the semi-embedded Z-pinned CFRP has considerably increased (by 123%) propagation GIC and negligibly reduced in-plane mechanical properties. Correspondingly, the Z-pinned CFRP with full-thickness embedded length shows a rise of 244% in the propagation GIC and a reduction of less than 9% in the in-plane properties. The failure modes indicate that pull-outs of Z-pins and their fractures both contribute to the enhancement of the fracture toughness of Z-pinned laminates, which are influenced by the distribution density and the embedded length of pins. It manifests that shortening the embedded length of pins can narrow the scope of waviness in planar fibers and relieve the induced stress concentration, which is conducive to improving the retention ratio of in-plane mechanical properties of Z-pinned composites. Finally, with 0.10% areal density, a better balance between interlaminar and in-plane properties of Z-pinned composites is successfully achieved for those with semi-embedded pins. Moreover, the Z-pin enhancement effect on interlaminar toughness is limited by its lower areal distribution density