Behaviour in torsion of superelastic NiTi

Abstract

Torque–strain curves have been established at 20 and 37°C and for up to 100 strain cycles for NiTi tube with an A _f temperature of—8.5°C. The performance mirrors the behaviour in tension and compression. In the first few strain cycles, the curves exhibit the familiar flag shape, but this gradually changes into a sigmoidal curve on repeated cycling. Changes are rapid at first, but after ∼ 40 cycles a steady state condition is approached closely. Attempts to analyse the performance in shear in relation to that in tension and compression draw attention to the problems involved in trying to establish flow or yield criteria for the material.