Interlaminar structure and properties of carbon fiber/epoxy composites toughened with copolymerized nylon fiber veil after cryogenic treatment

In this paper, a research platform for cryogenic/room temperature cycles was built to explore the effects of cryogenic immersion time and cryogenic/room temperature cycle times on the mechanical properties of carbon fiber/epoxy(CF/EP)composites. Then, copolymerized nylon fiber veil (PAV) with different areal densities was inserted into the interlaminar of the composites, and the interlaminar toughness of the composites was studied after 16 h of cryogenic immersion in liquid nitrogen. The results demonstrate that the interlaminar fracture toughness of mode I(G_ⅠC), mode Ⅱ(G_ⅡC), tensile strength and flexural strength of CF/EP composites decline by 46.2%, 22.9%, 17.7% and 3.2%, after 16 h of cryogenic immersion without toughening. The G_ⅠC of CF/EP composites is increased by 49.1% and 114.0% at room temperature and after 16 h of cryogenic immersion, respectively, when PAV with 8 gsm areal densities were inserted. The G_ⅡC is increased by 140.2% and 178.0% at room temperature and after 16 h of cryogenic immersion, when PAV with 24 gsm areal densities were inserted. In addition, the insertion of PAV didn't significantly affect the flexural properties and interlaminar shear strength of CF/EP composites. Our findings suggest that the toughening mechanism of PAV is mainly the pull-out of nylon resin, plastic deformation of matrix, and the deflection of initiating cracks. This study improves the interlayer toughness of CF/EP composites after cryogenic treatment, which is of positive significance for promoting the compositing process of cryogenic fuel tanks.