Effect Of Thermalmechanical Coupling On Fatigue Behaviors Of NiTi Shape Memory Alloys

Shape memory alloys(SMAs)have been widely used in biomedical,transportation and aerospace fields due to their unique shape memory effect and superelasticity under different thermo-mechanical loads,as well as corrosion resistance and excellent biocompatibility.In engineering applications,superelastic shape memory alloy components often withstand cyclic loading during service,so studying the fatigue properties of shape memory alloys is critical to structural reliability design and safety assessment.Since the phase transformation process of superelastic shape memory alloy during loading and unloading involves the accumulation of dissipation and the absorption(release)of latent heat,they will cause strong thermalmechanical coupling phenomenon during deformation.To this end,thermodynamic behaviors,fatigue crack initiation,fatigue crack growth and crack tip martensitic transformation of the superelastic NiTi shape memory alloy under cyclic loading were systematically studied and analyzed.Finally,the mechanisms of the effect of thermalmechanical coupling of shape memory alloy on the fatigue behaviors are described.Specifically,the following research contents were carried out in this paper:(1)The evolution of thermodynamic behaviors during cyclic loading are systematically studied by carrying out uniaxial cyclic tensile test under stress control on NiTi alloy wire.The experimental results show that the degradation of the recoverable strain during cyclic loading is caused by the collective effect of untransformed austenite after phase transformation,residual martensite after complete unloading and plastic deformation produced in the latest reverse transformation.In addition,it is found that the residual stress accumulation during cyclic loading makes the martensite more stable,and the phase transformation temperature of the shape memory alloy increases with the increasing of the loading cycles.(2)The effect of thermalmechanical coupling on pseudoelastic behavior and low cycle fatigue life of shape memory alloy is investigated through fatigue tests on NiTi alloy wire.Thermalmechanical coupling under different control modes has different effects on fatigue crack initiation.For strain-controlled loading,when the maximum strain of the load is the same,the degree of phase transformation is the same.At this time,the amount of austenite-martensite interfaces and martensite-martensite interfaces are the same as the potential source of fatigue crack initiation.However,as the loading frequency increases,the increase in temperature level makes the austenite more stable,and increases the stress associated with the austenite-martensite interfaces.Hence,the crack initiation is accelerated and the fatigue life decreases.For stress-controlled loading,the reduction of potential crack source and the increase of interfacial stress determine the fatigue crack initiation process of shape memory alloy,so the fatigue life is affected by the combination of maximum stress and loading frequency.Finally,based on total strain energy,a new low cycle fatigue life prediction model considering thermomechanical coupling is proposed,and the accuracy of the prediction model is verified by comparing the experimental results and prediction results under different load ratios.(3)The effect of thermalmechanical coupling on fatigue crack propagation is revealed by carrying out the fatigue crack propagation experiments at different loading frequencies on the edge notched specimens of NiTi alloy with pre-cracked.The results show that with the increasing of the loading frequency,the crack tip temperature at the maximum load increases,the crack growth rate slows down,and the fatigue crack growth life increases.In addition,the critical stress intensity factor amplitude slightly increases as the loading frequency increases,indicating that increase of the temperature caused by the increase of the loading frequency has a toughening effect on the shape memory alloy.(4)The model considering thermalmechanical coupling and thermal expansion effect is established,and the user material subroutine is developed.Temperature fields and phase transformation regions on mode I crack tip of the NiTi alloy at different loading rates are numerically simulated by finite element calculation.The accuracy of the model is verified by comparing the experimental and numerical temperature of the crack tip at the maximum load point.The numerical results of the martensitic transformation region around the crack tip show that the phase transition region reduces with the increasing of the loading frequency.