Inversion of the thermomechanical response in nitinol under cyclic loading: an analytical interpretation based on the thermoelastic effect theory

The superelastic behaviour of nitinol is crucial for the design of collapsible and self-expanding cardiovascular implants. Once these are expanded into the host anatomy, the material is predominantly in the austenitic configuration in the majority of the structure, and the cyclic loads acting on the devices are primarily due to small blood pressure variations occurring during the cardiac cycle. Nevertheless, only very few studies have explored the temperature evolution during small cyclic loading of nitinol in a stable austenitic state, reporting an unusual response, where the thermoelastic signal is in phase with the sinusoidal loading wave, rendering the common fundamental law of the thermoelastic effect inapplicable. In this study, infrared thermography (IRT) was employed to investigate the thermomechanical behaviour of an austenitic nitinol specimen under cyclic sinusoidal loading, with increasing amplitude and average strain values. An inversion of the thermomechanical response of nitinol was observed experimentally and explained analytically adopting the higher-order thermoelastic theory. The understanding of the austenitic temperature modulation with the local level of stress allowed to define an IRT approach suitable to quantify the stress levels, knowing the material thermal response and the ratio between mean and amplitude of the applied load.