A MULTI-SCALE CONSTITUTIVE MODEL OF NiTi SHAPE MEMORY ALLOYS

To characterize the macroscopic deformation behavior of NiTi shape memory alloys and capture the microstructural evolution during the deformation, a multiscale constitutive model is proposed by employing a "bottom-up" modeling strategy (including grain scale, polycrystalline scale and macroscopic scale). At the grain scale, two different inelastic deformation mechanisms, i.e., martensite transformation, transformation-induced plasticity and their interactions are considered. The thermodynamic driving forces for these two inelastic deformations mechanisms are derived based on the constructed Helmholtz free energy and Clausius-Duhem inequality. Thermodynamics-consistent evolution equations for internal variables are proposed. To estimate the interactions among the grains in the polycrystalline aggregate and obtain the scale transition rule from grain scale to polycrystalline scale, a uniform deformation gradient criterion is employed. Then, The polycrystalline model is implemented into the finite element framework by combining the constitutive model with the finite element method. And a series of experiments are performed to characterize the deformation behaviors of on NiTi shape memory alloy thin plates (including the rectangular, trapezoidal, parabolic, hourglass-shaped plates and the plate with holes). The global force-displacement responses under various displacement amplitudes are measured. Comparing the simulated results obtain by the proposed model with the experimental data, it is shown that the proposed multiscale model not only accurately describes the macroscopic deformation behavior of the plates but also reveals the deformation and microstructural evolution information at different spatial scales.