A thermo-mechanically coupled constitutive model for temperature- and rate-dependent cyclic functional degradation of NiTi shape memory alloys

A three-dimensional thermo-mechanical constitutive model has been developed to characterize the superelasticity (SE) degradation in NiTi shape memory alloys (SMAs) under cyclic loading, accounting for a broad spectrum of ambient temperatures and loading rates. The proposed model integrates multiple inelastic deformation mechanisms, including martensite transformation, transformation-induced plasticity, austenite plasticity, and martensite plasticity. It is developed in the context of irreversible thermodynamics and finite deformation. The driving forces of each inelastic deformation mechanism are determined using the established Helmholtz free energy and the inequality of energy dissipation. The internal heat production and temperature balance equation are derived from the principle of energy conservation. Based on the derived driving forces, equations for the thermodynamic-consistent evolution of internal variables are presented. The rationality of the model is validated by comparing the predicted outcomes with experimental data. It is demonstrated that the SE degradation of NiTi SMAs at various ambient temperatures (from 313 K to 393 K) and loading rates (from 5 × 10−4 s−1 to 6 × 10−2 s−1) is effectively captured, and the evolutions of temperatures along with the activation of inelastic deformation mechanisms are also predicted. The proposed model provides a theoretical framework for designing SMA-based devices under complex loading conditions.