The cyclic compressive responses of polycrystalline pseudoelastic NiTi shape memory alloy (SMA) are investigated under frequency ? ranging from 0.0007?? to 50?? with a controlled maximum strain of ?_??? = 4.2%. The cyclic compressive fatigue behaviors of this material are investigated under stress levels of ?_???= 800??? , 950??? and 1200??? with and without fabricated holes of different sizes. For the cyclic compressive responses, we have found a critical frequency ?_???^?? , below which the temperature and stress oscillations are dominated by the ? dependent coupling between phase transition (PT) and heat transfer and above which the macroscopic plastic strain ?^? of austenite phase gets involved due to heat accumulation in the transient stage and interacts with PT and heat transf...[ Read more ]

The cyclic compressive responses of polycrystalline pseudoelastic NiTi shape memory alloy (SMA) are investigated under frequency ? ranging from 0.0007?? to 50?? with a controlled maximum strain of ?_??? = 4.2%. The cyclic compressive fatigue behaviors of this material are investigated under stress levels of ?_???= 800??? , 950??? and 1200??? with and without fabricated holes of different sizes. For the cyclic compressive responses, we have found a critical frequency ?_???^?? , below which the temperature and stress oscillations are dominated by the ? dependent coupling between phase transition (PT) and heat transfer and above which the macroscopic plastic strain ?^? of austenite phase gets involved due to heat accumulation in the transient stage and interacts with PT and heat transfer. The accumulation of ?^? causes the reduction of PT volume fraction, and thus the latent heat and hysteresis heat in the subsequent cycles. Eventually the thermomechanical responses are brought to a steady-state stage with the saturation of ?^?. Theoretical modelling is performed to quantify those steady-state responses resulting from the interactions among PT, heat transfer and ?^?. For the compressive fatigue behaviors, with stress intensity factors ?¹ in the order of 10⁰~10^-3???√?, the life span ?_? of all the samples with no pre-existing hole exceed 1 million cycles, among which a ?_? of 20 million cycles under ?_??? = 800??? is recorded with stable thermomechanical responses and an average coefficient of performance (COP) of 17.6. ?_? is significantly reduced with the increasing hole size due to the great increase in ?¹. Quasi-brittle splitting and chipping caused by compression-parallel cracks are the main failure modes of the samples under cyclic compression. The studies in this thesis establish a broader scenario of frequency dependent interactions among PT, heat transfer and plasticity in SMA and provide a solid mechanical base for the application of NiTi in solid-state cooling technology.

Keywords: NiTi shape memory alloy (SMA); Phase transition (PT); Cyclic compression; Frequency effect; Thermomechanical coupling; Plasticity; Fatigue crack; Fracture; Size effect