A rate dependent tension-torsion constitutive model for superelastic nitinol under non-proportional loading; a departure from von Mises equivalency

In this work, a modified 3D model is presented to capture the multi-axial behavior of superelastic shape memory alloys (SMAs) under quasi-static isothermal or dynamic loading conditions. General experimental based equivalent stress and strain terms are introduced and improved flow rule and transformation surfaces are presented. The 3D constitutive equations are found for both isothermal and dynamic loading states. An extended experimental study is conducted on NiTi thin walled tubes to investigate the performance of the model. The proposed approach is shown to be able to capture the SMA response better than the original model in tension-torsion loading conditions.