Constitutive Equation of NiTi Superelastic Wire

Abstract

During the superelastic deformation process, because of the involvement of the austenite-martensite phase transformation, a superelastic wire will experience a self-heating or self-cooling process due primarily to the latent heat of the material. As the strain rate increases, and conditions become more adiabatic, the self-heating and self-cooling will cause a temperature rise upon loading and will drop upon unloading. As a consequence, an apparent effect of the strain rate on the superelastic behavior in the shape-memory alloys with a large diameter or more adiabatic conditions will be noticed. In the present paper, a constitutive stress-strain-strain rate equation is proposed to describe the self-heating behavior. In order to verify the model, a series of experiments have been conducted to study the effect of the strain rate, wire diameter, and adiabatic condition on the superelastic behavior of the shape-memory alloy wire. As will be shown later, the proposed equation can predict the behavior of the superelastic wire accurately, and the prediction is in good agreement with the experimental data.