Biaxial failure behavior and a mechanism-informed criterion for 2D triaxially braided composites

Two-dimensional triaxially braided composites (2DTBCs) exhibit pronounced anisotropy and complex failure behavior under biaxial loading, where strain-path dependence and fiber coupling lead to nonlinear and asymmetric strength responses. To address the limitations of classical failure models, this study develops a refined mesoscale finite element framework that captures the progressive damage evolution and stress redistribution across interacting fiber systems. Simulations reveal a systematic transition in failure modes—from axial-dominated fracture to coupled axial-transverse damage and ultimately to shear-driven collapse in the bias tows—as strain ratios and axial loading modes vary. Based on these observations, a mechanism-informed, piecewise failure envelope is proposed, integrating a modified Tsai–Wu formulation in coupling regimes with a maximum strain criterion elsewhere. This hybrid approach improves predictive accuracy and enhances physical interpretability for multiaxial strength assessment in complex braided architectures.