Effect of Loading Frequency on Temperature and Stress Oscillations in Cyclic Phase Transition of NiTi Shape Memory Alloy

Superelastic shape memory alloys (SMAs), when subjected to displacement-controlled cyclic deformation, exhibit distinctive temperature and stress oscillations. The amplitudes of the oscillations vary significantly with the deformation frequency and can even damage the material before the oscillations reach steady-state. This talk reports the effects of the deformation frequency on the cyclic response of a superelastic NiTi polycrystalline wire by synchronized measurement of stress and temperature over frequency range of 0.0004 ~ 1 Hz in stagnant air. It is observed that, for all the test frequencies, steady-state cyclic thermal and mechanical responses of the specimen can be reached after a number of loading cycles, exhibiting a kind of “thermal shake down”. The oscillations in temperature and stress follow the same frequency dependence and the amplitudes of the steady-state oscillations increased monotonically with the loading frequency and gradually reached saturation at high frequency. However, the steady-state average temperature and average stress increased more rapidly with the frequency and have no saturation. Analysis of the results shows that the temperature oscillation is directly related to the release/absorption of heat during cyclic phase transition, while the mean temperature rise of the specimen is caused by the accumulation of the dissipated mechanical heat (intrinsic hysteretic heating) of the phase transition. It is shown that such frequency-dependent oscillations of temperature and stress originate from the competition between the time scale of heat generation and the time scale of heat transfer with the ambient.