An experimental set-up to perform uniform tensile tests of superelastic NiTi tubes or wires

Loading and unloading of tensile tests of superelastic NiTi tubes or wires exhibit forward and reverse stress plateaus. During these plateaus, strain fields along the tensile specimens are non-uniform. Some regions are highly deformed, being mostly in martensitic state, while the other regions are weakly deformed, being mostly in austenitic state. This non uniform deformation of superelastic tensile tests prevents to deduce the actual behavior in tension of superelastic NiTi alloys, even if local deformation fields are analyzed. The knowledge of the actual behavior in tension is yet critical to establish accurate and representative constitutive equations which are implemented in finite element simulation of devices using superelastic NiTi.

In this work, an experimental set-up aiming at deforming uniformly thin tubes or small wires during loading and unloading was designed. This set-up is used to study the superelastic behavior of a Ti-50,9%at Ni tube of inner and outer diameters 0.5 and 0.6mm respectively. The tube deformation was observed by Digital Image Correlation (DIC).

Superelastic cyclic tests were performed using the proposed experimental set-up. DIC measurements confirms the deformation uniformity along the tube both during loading and unloading. Experiments allow to deduce the actual forward and reverse tensile stress-strain curves.