Experimental observations on rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy

Abstract

Based on the strain-controlled cyclic tension-unloading tests at various strain rates (3.3×10−4–3.3×10−2/s), the effect of strain rate on the uniaxial cyclic deformation of a super-elastic NiTi shape memory alloy (SMA) was investigated. It is concluded that apparent degeneration of super-elasticity occurs during the cyclic loading, i.e., the residual strain and transformation hardening increase, but the start stress of forward transformation and the maximum responding stress decrease with the increasing number of cycles and strain rate, and finally reach to their saturated states after certain cycles. Although the dissipation energy per cycle decreases progressively during the cyclic loading and is saturated after certain cycles, it does not change monotonically with the increasing strain rate. Moreover, the temperature oscillation is observed due to the internal heat production from the inelastic dissipation and transformation latent heat, and the extent of temperature variation increases monotonically with the increasing strain rate, which is the physical nature of rate-dependent cyclic deformation of the NiTi SMA.