A Thermodynamically Consistent Phase-Field Framework for Anisotropic Fatigue in Composite Structures

Fatigue crack evolution in fiber-reinforced composites exhibits strong anisotropy and nonlinearity, making predictive modeling challenging. This study proposes a thermodynamically consistent anisotropic phase-field fatigue fracture model that, for the first time, incorporates an entropy-production-based pathway into phase-field formulations for composites. In addition, the framework couples a history-dependent nonlinear fracture toughness degradation law with an anisotropic structural tensor, enabling a unified representation of fatigue crack evolution under cyclic loading. The model is implemented within COMSOL Multiphysics and validated through 2D and 3D simulations as well as experimental data. Results demonstrate that cracks preferentially propagate along the fiber orientation, while increasing fiber orientation angle significantly delays crack initiation and reduces growth rate. Fiber fracture toughness governs the transition between matrix-dominated, mixed-mode and fiber-dominated failure mechanisms, influencing crack trajectories and energy release patterns. Furthermore, the model accurately reproduces Paris law, while three-point bending simulations and mesh-convergence analyses confirm its numerical robustness and engineering adaptability. This framework, which can be extended to multiphysics effects such as viscoelasticity, viscoplasticity and thermo-mechanical coupling, offers a robust foundation for reliability assessment of advanced composite structures under complex service conditions.