Tension-Torsion Experiments on Superelastic Shape Memory Alloy Tubes

Shape memory alloys have been studied extensively in pure tension, but careful studies of multi-axial behavior are far less common in the published literature. Here, we present room temperature tension-torsion experiments on superelastic NiTi tubes, using stereo digital image correlation (DIC) for the first time to measure the strain on the tube surface. DIC can accurately measure large strains, which permitted us to capture the mechanical response at the beginning and end of the phase transformation. These responses were then used to generate the first measurements of the saturation stress and strain surfaces in the published literature. 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.