PHASE FIELD MODELLING OF FATIGUE FRACTURE IN SHAPE MEMORY ALLOYS CONSIDERING THERMO-MECHANICAL COUPLING EFFECT

NiTi shape memory alloys (SMAs) have found extensive application across numerous emerging industrial fields due to their unique macroscopic mechanical behaviour and exceptional thermoelastic effects. However, the poor fatigue life and complex fracture behaviour exhibited by these alloy materials during service represent major obstacles to their large-scale development. This work presents a unified derivation within a thermodynamic framework for both the macroscopic thermo-mechanically coupled constitutive model and the fatigue fracture phase field model for SMAs, ensuring the thermodynamic consistency. Employing the fatigue fracture phase field model, this study conducts an in-depth investigation into the fatigue fracture behaviour of SMA under thermo-mechanical coupling, particularly focusing on the oscillatory temperature field at the crack tip during crack propagation at the macroscopic scale. The fracture phase-field model is employed to theoretically investigate the relationship between temperature oscillations at the crack tip and martensitic phase transformation behavior in SMAs, along with the thermal hysteresis effect observed during cyclic loading, the thermal accumulation effect emerging with increased loading frequency, and the fatigue crack propagation behavior at different temperatures. Regarding numerical implementation, this study completed the finite element adaptation of the fatigue fracture phase field model of SMAs. A residual-controlled staggered algorithm is employed to circumvent the non-convexity issue of the phase field model's energy functional, with convergence conditions set to enhance computational stability. Simulation results indicate that under cyclic loading, martensitic phase transformation occurs at the SMA crack tip, releasing latent heat. This latent heat undergoes thermal exchange with the temperature field, inducing a thermal hysteresis effect. At different loading frequencies, temporal discrepancies arise between the latent heat release at the SMA crack tip and the thermal exchange process, leading to thermal accumulation phenomena. The model proposed herein provides a theoretical tool for subsequent research into the macro-fatigue fracture behaviour of SMAs.