Analysis Of Effects Of Strain Rate On Mechanical Properties Of NiTi Shape Memory Alloys

Superelastic polycrystalline NiTi shape memory alloys under tensile loading accompany the strain localization and propagation phenomena. Experiments showed that the number of moving phase fronts and the thermo-mechanical behaviors, such as propagation stress and stress hysteresis as well as damping capacity, were very sensitive to the loading rate due to the release/absorption of latent heat and the material’s inherent temperature sensitivity of the transformation stress. In this paper, a theoretical approach based on the heat diffusion equation is used to study the temperature evolution of one-dimensional superelastic NiTi specimen under different loading rates and boundary conditions with moving heat sources or a uniform heat source. Comparisons of temperature variations with different boundary conditions show that the heat exchange with the ambient conditions, especially at the boundaries plays a major role in the inhomogeneous temperature profile that directly relates to localized deformation and other thermo-mechanical responses. Analytical relationship between the front temperature of a single phase front, the inherent Clausius-Clapeyron relation (sensitivity of the material’s transformation stress with temperature), heat transfer boundary conditions and the loading rate is established to analyze the nucleation of new phase fronts. Moreover, a simple model is proposed to analyze the heat exchange between the specimen ends and the grips of the test machine, which can reduce to the classical Dirichlet boundary conditions with long effective length while comparison between the case of short effective length and the Neumann boundary conditions is also given. Finally, this paper derives a a linear model and a nonlinear model to study the rate-dependent stress-strain curve, especially the damping capacity, of one-dimensional NiTi SMAs by using the rate-independent driving force in the micromechanics inspired constitutive model. Compared with experimental data and previous work, both models use fewer material parameters and can easily describe the trend of damping capacity with respect to the strain rate. For their clear physical reason and simple mathematical expression, both models are useful tools for the dynamic design and simulation of superelastic SMAs in practice.