Tensile strength and damage mechanisms of three-dimensional carbon fiber/epoxy resin braided composites with defects

Three-dimensional(3D) braided composites are high-performance composite materials, which can overcome the shortcomings of traditional laminates that are prone to delaminate, have higher structural integrity, excellent mechanical properties and strong damage resistance and are widely used in aerospace and other fields. In order to support the reliable design of the structures, the tensile properties and damage evolution of 3D four-directional braided composites(3D4DBC) with initial defects were systematically investigated through a comparison between tensile tests and multi-scale high-fidelity simulations. By establishing the finite element models with multi-scale defects, and adopting the Tsai-Wu tensor theory in conjunction with the Mises equivalent stress criterion, a refined analysis of the axial tensile progressive damage process was accomplished. Experimental and simulation results indicate that the stress-strain curves of 3D4DBC are basically consistent in the linear segment. Braiding angle significantly affects the mechanical properties and damage mechanisms of the materials. Composites with smaller braiding angles exhibit higher tensile strength and modulus, with the primary damage pattern being axial tensile fracture of the yarns. In contrast, those with larger braiding angles demonstrate improved ductility, and their dominant failure mechanism shifts to shear failure of the yarns. The presence of defects influences the damage evolution process, which leads to significant nonlinearity in the stress-strain curve and ultimately degrades the tensile stiffness and strength of the composites. This study not only elucidates the failure mechanisms of 3D4DBC, but also yields insights with practical implications, establishing a theoretical framework for the structural optimization and engineering application of 3D braided composites.