Half-thickness z-pin effect on the interlaminar and the intralaminar properties of polyimide z-pin reinforced polymer composites

This paper is intended to diminish the detrimental impact of z-pins on intralaminar properties of carbon fiber reinforced polymer (CFRP) composites when strengthening their interlaminar fracture toughness by using polyimide z-pins. The microstructural observation indicates that the z-pin pre-hole implanted (ZPI) process can effectively decrease the initial damages compared with the traditional process. The experimental results indicate that z-pins with the different length can significantly improve the mode I fracture toughness (G I ) of specimens. And the G I of specimens with full-thickness z-pins is twice larger than that of specimens with half-thickness z-pins, which attribute to the larger bonded area between z-pin and laminates. However, the fracture of z-pinned specimens with the larger G I has displayed as multiple delamination cracks, which is not conducive to maintaining the intralaminar performance of specimens. Compared with unpinned specimens, the flexural strength of specimens with half-thickness z-pins has a retention of more than 95%. Meanwhile, the plastics strain energy of specimens with half-thickness z-pins is twice larger than that of specimens with full-thickness z-pins. In short, the ZPI process and half-thickness z-pins could achieve the desirous equilibrium of mechanical properties between the interlaminar and the intralaminar.