Effect of Loading and Constraining Conditions On the Functional Fatigue Life of Niti Shape Memory Wires

Shape memory alloys (SMAs) are increasingly used for the construction of simple, silent and compact solid-state actuators characterized by high power density. The rational design of shape memory alloy actuators requires reliable data on the fatigue strength of the material under cyclic thermal activation (thermomechanical or functional fatigue). Test results on SMAs under functional fatigue are scarce in the technical literature and the few data available are mainly limited to constant-stress loading applied to the material. Since the SMA elements used within actuators are normally biased by elastic springs or by another SMA element, their stress state is far from constant in operation. Also, the variation of the external load contributes to increased variability of stresses in the internal SMA elements. The mismatch between actual working conditions and laboratory arrangements leads to suboptimal designs and undermines the prediction of the actuator lifetime. This paper aims at bridging the gap between experiment and reality. Four test procedures are planned, covering most of the typical situations occurring in practice: constant-stress, constant-strain, constant-stress with limited maximum strain and linear stress-strain variation. The paper describes the experimental apparatus specifically designed to implement the four loading conditions and presents fatigue results obtained on commercial NiTi wires (0.15 mm diameter) tested under the four protocols. The constant-stress loading leads to the longest lifetimes of the wires while the constant-strain test is the most demanding condition on the material. Constant-stress tests with limited maximum strain and linear stress-strain variations produce life results in between the two limit situations of constant stress and constant strain.