AbstractsEngineering

NiTi polycrystalline shape memory alloy (SMA) has many applications due to its shape memory and superelastic properties that originate from the thermoelastic martensitic phase transition. Stress-induced phase transition of the polycrystal creates heterogeneous temperature and deformation distribution in the material. This thesis investigates the loading-rate dependence on temperature and deformation oscillations in superelastic NiTi SMA. Through synchronized measurement of the specimen’s surface morphology (as a qualitative measure of deformation heterogeneity), temperature field, and stress-strain histories over the frequency range of 0.004-4 Hz, it was found that the domain pattern, the temperature profile and the stress-strain curves strongly depend on the loading frequency and the cycle number. For each frequency, these responses experienced asymptotic changes cycle by cycle before reaching the stabilized repeatable responses. The local temperature at all points in the specimen oscillated with the same period and eventually reached their respective steady states of oscillations. The experimental data of rate-dependent average temperature and hysteresis loop area were also discussed with a lumped heat transfer model. It’s shown that such rate dependence was actually due to the effect of temperature and the intrinsic thermomechanical coupling of the material and was governed by the competition between the time scale of heat release and that of heat transfer to the ambient. Afterwards, the Digital Image Correlation (DIC) technique was applied to quantitatively characterize the rate-dependent deformation of the material by monotonic loading-unloading tests with strain rate from 0.0003/s to 0.98/s. The DIC results clearly reproduced the domain evolution process and showed that, as the strain rate increased, the domain number would also increase but the strain gradient at the domain interface would decrease. As a result, the overall deformation became more homogenous. The corresponding deformation mode changed from the localized nucleation-growth mode to the spinodal decomposition mode. The change of deformation behavior with strain rate was the result of competition between bulk free energy and interface free energy in the material system. Keywords: NiTi shape memory alloy, loading-rate effect, temperature and deformation fields, time scales of heat release and heat transfer, thermomechanical coupling, deformation mode, free energy.