Cumulative Nitinol Fatigue with Statistical Evaluation

Nitinol is used extensively for cardiovascular devices given its super-elastic behavior and favorable fatigue performance. Although nitinol fatigue is often evaluated by calculating a factor of safety based on strain amplitude and a selected fatigue strength percentile, such an approach is limited. These limitations include not directly incorporating the complete statistical distribution of the fatigue strength and having limited capability of combining multiple loading modes and magnitudes, which almost always occur in implanted medical devices. Examples of multi-mode and variable loads include (A) cyclic loads driven by multiple activities of daily living, (B) simultaneous loading modes occurring at different frequencies, and (C) altered loads from combination products due to revision surgeries. Therefore, the purpose of this study was to evaluate the cumulative fatigue performance of nitinol by (A) cycling individual specimens under a range of strain amplitudes and (B) evaluating several statistical methods to evaluate nitinol fatigue under multi-load scenarios.

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. For a given test specimen, up to three levels of strain amplitude were incorporated. The percentage of cycles for each strain amplitude was also varied between the tests. The test results demonstrated a cumulative effect of multiple cyclic strains on nitinol fatigue. Predictive capability of statistical fits that incorporate either a linear and nonlinear relationship between log of strain amplitude and log of cycles to failure, as well as different cumulative damage models are presented.