Cumulative Fatigue of Nitinol due to Multiple Applied Cyclic Strains

Nitinol continues to be used extensively for cardiovascular devices given its super-elastic behavior and favorable corrosion and fatigue performance. Although nitinol fatigue is often assessed by calculating a factor of safety based on strain amplitude and determined fatigue strength, such an approach is limited in several aspects. These limitations include not directly accounting for the full statistical distribution of the fatigue strength, having limited capability of combining multiple loading modes and magnitudes from various activities of daily living, and not considering the role of stress in nitinol fatigue. Therefore, the purpose of this study was to evaluate the fatigue performance of nitinol wires under a range of conditions that included (A) cycling the same specimen at a range of strain amplitudes and (B) incorporating different levels of applied stress.

Nitinol wire apex specimens were used for the testing. The specimens were first crimped, after which they were cycled at a range of strain amplitudes. Different levels of crimp strain were used to induce varying levels of applied stress for the same apparent cyclic strain conditions. For a given test, strain amplitude was increased in 5 million cycle increments until either specimen failure or final run-out occurred at 20 million cycles. The testing results demonstrated a cumulative effect of multiple cyclic strain states applied to the specimens and that the stress state of the material can affect the fatigue performance of the nitinol. In addition, statistical methods of combining strain amplitudes from different loading modes are presented.