In this study, we developed a novel biaxial tensile fatigue testing technology for a cross-ply carbon fiber-reinforced plastic (CFRP) cruciform model specimen to simulate the laminated structure and stress state of a high-pressure hydrogen vessel fabricated using CFRP at the test specimen level and to predict the fatigue life of a hydrogen vessel with high accuracy. The biaxial tensile fatigue S-N diagram of the cross-ply CFRP specimen can be represented by a single straight line when arranged in terms of the apparent interlaminar shear stress. Observation of the initiation and extension behavior of damage under biaxial tensile fatigue using synchrotron radiation laminography shows that under cyclic biaxial loading, spherical microvoids are generated in the interlaminar resin at the corners of the CFRP intersections and at a site where splitting occurred near the center of the CFRP layer on the lower-load side. The microvoids coalesced, expanded, and progressed to peeling along the fiber/resin interface and delamination.
