Study of fatigue damage behavior in off-axis CFRP composites using digital image correlation technology.

This study investigates the fatigue behavior of off-axis carbon fiber reinforced polymer (CFRP) composites under varying stress levels, with a focus on both tensile-tensile and compressive-compressive loading modes. We conduct a comprehensive analysis of energy dissipation and stiffness across various loading conditions, highlighting the critical role of fiber deflection effects in the recovery of tensile-tensile fatigue properties. Utilizing digital image correlation (DIC) technology, we identify both commonalities and distinctions in crack propagation pathways and failure mechanisms between these two fatigue scenarios. In the case of tensile-tensile fatigue, the predominant damage mechanism involves the development of multiple interlaminar shear cracks. These cracks initiate debonding at the fiber/resin interface, propagating from macrocracks at the edges to microcracks at the center, ultimately culminating in fiber pull-out failure. Conversely, in compressive fatigue, damage occurs centrally in the form of intralaminar shear cracks. As damage accumulates, these cracks progressively propagate along the fibers towards the edges, accompanied by localized fiber buckling, ultimately resulting in compressive failure. Furthermore, we determine a critical compressive strain threshold, which serves as a pivotal indicator of failure in compressive-compressive fatigue testing for off-axis CFRP composites.