Fatigue Testing of Superelastic NiTi Wires Thermally Activated for Shape Memory Effect

A few years ago, a unique way to use shape memory alloys for actuation purposes was proposed, involving repeated heating of wires in the superelastic state. Named high-performance shape memory effect (HPSME), this behavior was supplemented with preliminary test data on the fatigue life of the wires cycled under constant-stress loading. The present paper investigates the HPSME of superelastic wires under thermomechanical cycling for an extended range of loading conditions: "constant stress with limited maximum strain," "linear stress-strain variation with limited maximum strain," and "constant strain." The tests confirm that the HPSME generates higher stresses (around 1000 N/mm(2) against the usual 200-250 N/mm(2)) at temperature well above the activation temperature of the classical SMAs. However, the net stroke of the wires rapidly decreases with the number of cycles and the fatigue life hardly achieves a few thousands of cycles, especially under constant-strain loading. Based on the experimental findings, the recourse to the HPSME is strongly advised for low cycle or one-shot devices but not particularly encouraged for applications involving sustained cycling.