Variation of electrical resistance in superelastic NiTi for sensor applications

Abstract

Nickel-Titanium (NiTi) is a most commonly known as a heat-activated shape memory alloy. However, the material sometimes displays a constant-temperature property called "superelasticity." A superelastic material is one which can undergo very high reversible strains due to stress-induced change in crystal structure. In the case of Superelastic NiTi, Martensitic transformation occurs. The two crystal structures differ to the extent that the gradual phase transformation is coupled to a gradual change in resistivity. In fact, resistive sensing is a common characterization technique for shape memory alloys. The unique material properties of superelastic NiTi could also be the basis for creating a resistive sensor that is sensitive enough to measure small displacements, and robust enough to measures large displacements. This study focuses on NiTi which displays superelastic behavior above room temperature. To assess the material's potential as a strain sensing medium, the NiTi wire is shape-set into coil springs which amplify the sensor's net deformation. The relationship between strain and resistance is measured. The study shows that various aspects of the strain-resistance response, including non-linear hysteretic behavior and temperature dependence of electrical resistivity, pose challenges to sensor design. Though the accuracy of the spring sensors is still under development, several recommendations are made with regard to effective device design. In addition, the design of a one-axis strain rate sensor, which differentiates between only two modes of behavior, is explored.