Understanding the Fatigue Behaviour of NiTiCu Shape Memory Alloy Wire Thermal Actuators

Abstract

This paper deals with the fatigue behaviour of NiTiCu shape memory alloy (SMA) wire actuators on thermo-mechanical cycling (TMC). Cyclic loading in SMA actuators is invariably associated with both functional and structural fatigue. The characteristic of the actuators such as austenite (hot shape) remnant deformation and recovery strain undergo changes upon TMC. These in turn result in continuous change in strain response (functional fatigue) during application. It has been shown that the functional fatigue can be minimized by adopting TMC at higher stress than that of the working stress prior to the application. On the other hand, failure of the actuators by fracture (structural fatigue) due to cyclic stress/strain is inevitable. Study shows that the fatigue life of the actuators is strongly dependent on the type of loading and the temperature range of operation. This has been explained in terms of damage accumulation, crack initiation and fracture behaviour.