The influence of stress ratio of cyclic loading on properties and mechanism of nitinol fracture

The shape memory alloy (nitinol) was studied under low-cycle fatigue at a frequency of 0.5 Hz, different levels of maximum stresses σmax and stress ratio R (0.1 and 0.5) in the loading cycle. It was shown that regardless of the level of σmax, the energy dissipated during deformation of nitinol at R = 0.1 always exceeded that inherent to it at R = 0.5, and decreased more intensively with an increase in the number of loading cycles. As a result, nitinol specimens loaded at lower R heated more, stabilizing nitinol in the austenitic state. It was believed that at lower R this is a factor that inhibits the stress-induced transformation of austenite into martensite. Differences in the mechanism of fracture of nitinol at different R were studied.