An experimental study of the superelastic behavior in NiTi shape memory alloys under biaxial proportional and non-proportional cyclic loadings

This paper presents the results of a series of experiments to investigate the superelastic cyclic stress–strain responses of NiTi shape memory alloys under tension–torsion biaxial loading conditions. The uniaxial tension and torsion experiments were also conducted to make comparisons. Experiments were controlled by axial displacement and torsional angle in sine wave form with fixed maximum values. Saturation is reached after 30 cycles. The evolutions of equivalent stress–strain curves as well as the separated tensile and shear stress–strain curves during cycling are analyzed. Dissipated energy density, characteristic stresses and strains as functions of deformation cycles are paid attention. Results show that the mechanical responses are significantly affected by the loading path. The stress–strain behaviors under proportional loading are similar to those under uniaxial loading. With the increase of the out-of-phase angle during the non-proportional loading, the special phase transformation exhibition totally disappears in the equivalent stress–strain curves.