Behaviour of strain–resistivity coupled measurements of Ti–Ni–Cu wires during thermal cycling under constant stress

When shape memory wires are submitted to thermal changes under constant load (thermomechanical cycles), the austenite to martensite or martensite to austenite changes lead to strain changes and electrical resistivity changes. In some favourable cases, a one to one linear correspondence exists between the strain (ε) and the relative resistivity (Δρ/ρ0) changes. This linear curve can be used for example in robotic devices for control position. The control position quality depends on the strain–resistivity relationship and especially on its linearity and hysteresis for repeated thermal cycles. The Ti–45.0Ni–5.0Cu (at.%) alloy is a good candidate for this kind of activator because of its one step transformation and simple resistivity behaviour. The present paper is focussed on the behaviour of Δρ/ρ0 ersus ε curves during thermal cycling of Ti–45.0Ni–5.0Cu wires submitted to constant stresses. The wires have been previously submitted to different thermomechanical treatments (cold drawing + heat treatments). The influence of the stress amplitude has been studied for each thermomechanical treatment. The results show that the Δρ/ρ versus ε curves depend on the stress amplitude as well as on previous thermomechanical treatments and that some optimal conditions can be deduced.