Bi-Axial 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. Here, we present room temperature bi-axial experiments on superelastic NiTi tubes, using stereo digital image correlation (DIC) for the first time to measure the strain on the tube surface.  The mechanical responses from both proportional and non-proportional strain paths were measured. The proportional responses 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.  In the non-proportional strain path experiments, the specimens were either pulled first then twisted, or twisted first then pulled.  Significantly, the non-proportional responses during loading matched up well with the transformation surfaces we previously characterized.  This new observation suggests that the loading transformation surfaces are path independent, which could substantially simplify constitutive modeling.

The full field nature of DIC was also important in this work. The strain fields revealed propagating transformation fronts in pure tension, no propagating fronts in pure torsion, and a progression of behaviors in between, similar to the optical microscopy observations of Sun and Li [1]. By quantitatively measuring the strain fields with DIC, however, we found new features. In tension-dominated tests, the transformation front appeared as a near discontinuity in not only the axial strain field, but also the shear strain field. Also, the shear strain fields in pure torsion were not uniform. Although there were no propagating fronts, torsion caused vertical columns of shear strain to gradually appear and disappear during phase transformation.

[1] Sun, Q. P. and Li, Z. Q. Phase Transformation in Superelastic NiTi Polycrystalline Micro-Tubes Under Tension and Torsion--From Localization to Homogeneous Deformation. International Journal of Solids and Structures, 39(13-14):37973809, 2002.