Thermo-Mechanically Coupled Cyclic Deformation Of Super-Elastic Niti Alloy And Its Constitutive Model Under Uniaxial Loadings

As a new type of smart materials, the super-elastic NiTi alloy has attracted more and more attention, and in some practical engineering applications they are always subjected to cyclic loading at different frequencies in some practical engineering applications. It was shown that the martensite transformation and its reverse occurred in the super-elastic NiTi alloy exhibit obvious rate dependence. However, the rate dependence caused by the thermo-mechanical coupling effect is studied deficiently at present. In this work, the experimental observations and constitutive modeling on the thermo-mechanical coupling deformation of the super-elastic NiTi alloy performed under the uniaxial cyclic loading. The main researches are listed as follows:1) Experimental research:(1) Cyclic transformation behaviors of super-elastic NiTi alloy micro-tubes are investigated in the strain rate range of 3.3 × 10-4/s～3.3× 10-2/s in the uniaxial case. The results show that the different temperature rises are induced by the internal heat generation and a cycling hardening can be observed during the cyclic transformation at different loading rates; and the dissipation energy shows a non-monotonic relationship with strain rate before and after a critical strain rate.(2) The uniaxial transformation ratcheting of super-elastic NiTi alloy micro-tubes is investigated in the stress rate range of 1MPa/s～80MPa/s. The results show that the rate-dependence induced by the thermo-mechanical coupling effect is similar to that obtained at different strain rates. In the same stress level, the transformation ratcheting strain increases, bt the dissipation energy decreases with the increasing stress rate.(3) In the temperature range of 20℃～120℃, strain and stress-controlled cyclic experiments show that during the cyclic transformation, forward and reverse transformation stresses increase linearly if the temperature is lower than 80 ℃; but, as the temperature is higher than 80℃, a non-linear relationship occurs between the stress and temperature; also the residual strain and transformation ratcheting strain evolve more quickly at higher temperature.2) Constitutive model:(1) In the framework of generalized visco-plasticity, the Graesser model is proposed to predict the cyclic transformation of super-elastic NiTi alloy by taking the evolutions of the forward and reverse transformation stresses, residual strain and cyclic transformation induced plasticity into account. This model is then extended to describe the rate-dependent behavior of the alloy by introducing rate-dependent evolution parameters. These models are validated by comparing the experiments and predictions.(2) In the framework of thermodynamics, a thermo-mechanically cyclic constitutive model is proposed to describe the thermo-mechanical coupling effect of super-elastic NiTi alloy by modifying the martensitic volume fraction evolution equation and considering two kinds of internal thermal mechanisms:i.e., the intrinsic mechanical dissipation and the transformation latent heat, as well as introducing the evolution equation of internal stress to reflect the decreased transformation stress during the cyclic transformation. The model is verified by comparing the simulated stress-strain curves and temperature evolutions with the experimental ones.