Superelastic shape memory alloy undergoes phase transformation when subjected to uni-axial tensile loading, transforming from its initial phase austenite to martensite. This stress-induced first-order phase transformation involves heterogeneous latent heat release and absorption, hysteresis heat release and heat transfer between the specimen and the ambient air, which in turn influences the temperature of the specimen and the mechanical responses and deformation processes due to the material’s intrinsic mechanical nonlinearity and thermomechanical coupling. The temperature variations, stress-strain oscillations and damping capacities during cyclic phase transformation in the superelastic NiTi polycrystalline and CuAlNi single crystalline shape memory alloys are examined in this thesis....[ Read more ]

Superelastic shape memory alloy undergoes phase transformation when subjected to uni-axial tensile loading, transforming from its initial phase austenite to martensite. This stress-induced first-order phase transformation involves heterogeneous latent heat release and absorption, hysteresis heat release and heat transfer between the specimen and the ambient air, which in turn influences the temperature of the specimen and the mechanical responses and deformation processes due to the material’s intrinsic mechanical nonlinearity and thermomechanical coupling. The temperature variations, stress-strain oscillations and damping capacities during cyclic phase transformation in the superelastic NiTi polycrystalline and CuAlNi single crystalline shape memory alloys are examined in this thesis. Monotonic loading-unloading and cyclic loading tests with different deformation frequencies in different ambient conditions were performed to investigate the effect of deformation frequency and the ambient condition. It was found that both the temperature and the stress-strain curves vary significantly with the frequency and the number of cycles. For each frequency, steady-state cyclic thermal and mechanical responses of the specimen can be reached after a transient stage, exhibiting stabilization. A theoretical analytical model was built based on the highly simplified lumped heat transfer analysis and the Clausius-Clapeyron relationship, with an incorporation of the thermomechanical coupling effect in the responses of the material. It was found that, for given material properties and specimen geometry, all such frequency-dependent variations in temperature, stress and hysteresis loop area are essentially determined by the competition between the time scale of the heat release (i.e., the phase transition) and the time scale of the heat transfer with the ambient. The results emphasize that the time scale of external loading and the time scale of heat transfer must be clearly specified when one characterizes and models the cyclic stress-strain relationship of the SMA materials.

Keywords: Phase transition; Shape memory alloy; Temperature and stress oscillations; Hysteresis loop area; Thermomechanical coupling; Ambient condition; Time scales of phase transition and heat transfer.