Effect of hydrostatic pressure on multilevel structure and bending progressive damage of 3D braided composites under simulated deep-sea environment

Abstract

Compared to metal materials and laminated composites, three-dimensional (3D) braided composites are showing innovatively great potential in deep-sea pressure shell applications owing to their light-weight, excellent out-of-plane mechanical properties and near-net forming capacity. In this paper, three-dimensional six-directional (3D6D) braided composites with different epoxy resins are designed and fabricated. The samples are put in a deep-sea pressure simulator at 10 MPa pressure for 80 h and experimentally test via the three-point bending method. A multi-level evaluating approach, namely nano-scale, micro-scale, meso-scale and macro-scale, is proposed. The effect of the hydrostatic pressure on multilevel structure and progressive failure behaviours are analyzed. Results indicate that, although the moisture absorption does not lead to the generation of new chemical bonds on the nanoscale in the carbon/epoxy composites, the moisture absorption has significant effects on the bending mechanical properties and damage initiation of 3D6D braided composites. Moreover, after deep-sea hydrostatic pressure, the strength of 3D6D braided composites decrease by 15.64–30.93 %, and the damage volumes increase by 0.039–0.405 %. Furthermore, the final failure mechanism of 3D6D braided composites after deep-sea hydrostatic pressure are largely governed by the fiber-matrix debonding and matrix cracking.