Influence of electric current on the thermo-mechanical static and fatigue properties of shape memory NiTi wires

NiTi shape memory wires are fostered to be utilized as actuators due to their strain recovery capability involved with phase transformation and the high mechanical properties. Since large strains are involved with the phase transformation, fatigue is a crucial issue that needs to be carefully investigated. Fatigue of superelastic alloys is usually analyzed through S-N curves obtained by imposing alternated stresses. Differently, actuators based on shape memory alloys are usually subjected to a bias load and thermal cycles drive the actuation. This work presents an experimental characterization of the thermo-mechanical static and fatigue properties of shape memory NiTi wires intended for actuation purposes. A series of original results are presented. The wires' stroke capability is found to depend on the heat rate and temperature associated with a varying magnitude of the current utilized to perform the thermal cycle exploiting the Joule's effect. Alongside a reduction of the actuation time associated with increasing current, results show that the stroke increases with the current, and with it the alternated strain associated with a single thermal cycle. The ultimate stress under quasi-static loads is also found to be influenced by the current, as it linearly diminishes with it, hence with the operating temperature. Above all, the most interesting results relate to fatigue, as it is found that the influence of the applied current on thermo-mechanical fatigue is even larger than the effect of the applied bias load. Further, functional fatigue in terms of reduction of the recoverable strain upon thermal actuation has not been found for any loading condition.