Axial-Torsion Behavior of Superelastic NiTi Tubes

Shape memory alloys have been studied extensively in pure tension, but careful studies of multi-axial behavior are far less common. To fill this gap, room temperature axial-torsion experiments on superelastic NiTi tubes have been performed. A set of 17 proportional strain-path experiments were used to characterize all four phase transformation surfaces: the onset surface and completion surface during loading, and the onset surface and completion surface during unloading. Careful analysis showed that the critical driving force for phase transformation was independent of strain path. This enabled us to correctly predict the transformation strain surface from the transformation stress surface. In the non-proportional strain path experiments, the specimens were either pulled first then twisted, or twisted first and then pulled. Significantly, the non-proportional responses during loading matched up well with the transformation surfaces previously characterized using proportional strain paths. This new observation suggests that the loading transformation surfaces are path independent, which substantially simplifies constitutive modeling.