Functional fatigue of shape memory wires under constant-stress and constant-strain loading conditions

Shape memory alloys (SMAs) are increasingly used for the construction of simple solid-state actuators characterized by outstanding power density. The rational design of these actuators requires reliable data on the fatigue strength of the alloy under cyclic thermal activation (functional fatigue). The technical literature shows scanty test results for SMAs under functional fatigue. Furthermore, the few data available are mainly limited to the condition of constant stress applied to the material. Since the SMA elements used within actuators are normally biased by conventional springs or by another SMA element, their stress condition is far from constant in operation. The disagreement between actual working conditions and laboratory conditions leads to suboptimal designs and undermines the prediction of the life of the actuator. This paper aims at bridging the gap between experiment and reality. Four characteristic test conditions are envisioned, covering most of the actual situations occurring in practice: constant-stress, constant-strain, constant-stress with controlled maximum strain and cyclic-stress with controlled maximum strain. The paper presents the experimental apparatus specifically designed to implement the four test conditions. Fatigue results on a commercial NiTi wire (0.15 mm diameter) tested under constant-stress and constant-strain loading are also presented and discussed.