PhD Thesis Presentation
The cyclic compressive responses of polycrystalline pseudoelastic NiTi shape memory alloy (SMA) are investigated under frequency ranging from 0.0007Hz to 50Hz with a controlled maximum strain of 4.2%. The cyclic compressive fatigue behaviors of this material are investigated under stress levels from 800MPa to 1200MPa 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 frequency dependent coupling between phase transition (PT) and heat transfer and above which the macroscopic plastic deformation of austenite phase gets involved due to heat accumulation in the transient stage and interacts with PT and heat transfer. The accumulation of the plastic strain 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 absence of plastic strain. Theoretical modelling is performed to quantify those steady-state responses resulting from the interactions among PT, heat transfer and plastic strain.
For the compressive fatigue behaviors, with stress intensity factors in the order of 1~0.001MPa*m0.5 , the life span of all the samples with no pre-existing hole exceed 1 million cycles, among which a fatigue life of 20 million cycles under 800MPa is recorded with stable thermomechanical responses and an average coefficient of performance (COP) of 17.6. The fatigue life is significantly reduced with the increasing hole size due to the great increase in the stress intensity factor. 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.
(Supervisor: Prof. Qingping Sun)