Effect of training conditions and extended thermal cycling on nitinol two-way shape memory behavior

Shape memory alloys (SMA) are a class of materials proposed to be used as actuator elements in smart structures. They undergo a reversible martensitic phase transformation during thermal cycling. In this work, 55-nitinol (55% nickel-45% titanium shape memory alloy) was investigated to determine the effect of training conditions, bias stress and extended thermal cycling on the transformational behavior. Thin wires (0.19 mm diameter) were trained using a previously developed methodology [1] to exhibit two-way shape memory (TWSM). The wires were placed in a cryogenically cooled apparatus and heated by electrical resistance while the wire deformation was measured. Several different training schedules were used by varying the amount of prestrain and number of training cycles. The recovery strain and transformation temperatures were measured throughout extended thermal cycling (up to 10 000 cycles). A bias stress was also used to enhance the amount of retention of prestrain in the wires. In all cases, stabilization of TWSM behavior occurred within 2000 thermal cycles.