Effect of plastic deformation and temperature on the functional fatigue behavior of large diameter superelastic Ni-Ti shape memory alloys

The superelasticity of shape memory alloys (SMA) can be used to provide self-centering and/or energy dissipation characteristics to structures including buildings, bridges, automobiles, and aircrafts. The functional fatigue behavior of SMA is important because it affects the stiffness, strength, strain recovery and energy dissipation of the material. This study investigated the functional fatigue behavior of large diameter Ni-Ti SMA bars under different levels of plastic deformation and different ambient temperatures. Differential scanning calorimetry was used to measure the martensitic transformation temperatures. Cyclic loading with a 1% strain increment was applied to investigate the maximum recovery strain, i.e. the superelastic limit. Low-cycle fatigue loading with different applied peak strains (2%, 3%, 4% and 5%) was performed at different temperatures (-40 degrees C, -10 degrees C, 10 degrees C, 25 degrees C and 50 degrees C). The effects of plastic deformation, testing temperature, and number of cycles on the stress-induced martensitic phase transformation, degradation of superelastic properties, and fatigue life were studied. The superelastic properties, such as the changes in the stress-strain curves, elastic modulus, yield stress, damping ratio and recovery strain, were analyzed. It was shown that the functional fatigue resistance (in terms of degradation in the superelastic properties and fatigue life) of Ni-Ti SMA reduced as the applied peak strain increased, particularly when the applied peak strain was higher than the superelastic limit. Additionally, when Ni-Ti SMA was subjected to combined plastic deformation and higher than room temperature, the functional fatigue resistance reduced as the temperature increased.