Superelastic Behaviour of Niti Wires Under General Cycling Conditions: Thermomechanic 1-D Modelling

A 1-D thermomechanical model aimed to represent the behavior of Ni-rich ultrafine grained NiTi wires upon pseudoelastic cycling was developed. The three more significant features associated to the pseudoelastic behavior of NiTi wires were taken into consideration for its formulation. These are: the localized character of the stress induced transformation which is accompanied by the nucleation and propagation of a discrete number of transformation fronts; the thermal effects associated with the heat of transformation and the Clausius-Clapeyron type of dependence of critical stresses with temperature; the functional fatigue effects resulting in the decrease of critical stresses and the accumulation of permanent deformation during cycling. The model assesses the mechanical response by evaluating the local conditions at the different moving interfaces that might be present at a certain time. It was implemented in MATLAB and numerical simulations have been found to exhibit good agreement with experimental results obtained under similar conditions. It is therefore considered that the model here proposed constitutes a good conceptualization of the complex pseudoelastic behavior of NiTi wires that allows reproducing the fundamental aspects of actual observed behaviors