Cyclic Response And Fatigue Failure Of Nitinol Under Tension-Tension Loading

Cyclic mechanical response and fatigue damage evolution were investigated in superelastic Nitinol in the mixed austenite-martensite state. Tapered dogbone-shaped specimens were loaded in tension under displacement control. In order to mimic the load history of a transcatheter device, test specimens were first loaded to a fully martensitic state and then partially unloaded to create a mixture of martensite and austenite on the lower stress plateau. Subsequently, sinusoidal displacement cycles were applied at three different loading frequencies. The taper results in a single continuous martensitic band in the gage section during cyclic loading for ease of observation. Digital image correlation (DIC) and high-speed video recording were used to document strain distribution, loading rate dependence, and fatigue crack initiation. Fatigue life data up to 10 million cycles were obtained for loading at 40 Hz. DIC-based strain maps show that mean and alternating strains vary across the gage section, and that at high displacement amplitudes, alternating strains are localized near the austenite-martensite transition region. Fatigue crack initiation was documented in real time through high-speed video recording and occurred preferentially near the austenite-martensite transition region. Fatigue life data show a fatigue limit and a sharp transition between low cycle failures and high-cycle runouts. The fatigue limit in tension corresponds to the threshold for activating cyclic phase transformation at a macroscopic scale and can be estimated approximately using the upper and lower stress plateaus and the effective elastic modulus, all of which can be obtained from the tensile stress-strain curve.