The Role of Stress State on Nitinol Fatigue

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 the cyclic strains in the device, the stress state of material at the location of peak strain amplitude is known to effect fatigue performance. However, the role of stress in nitinol fatigue is still not well understood. Therefore, the purpose of this study was to evaluate the fatigue performance of nitinol wires under a range of loading conditions that generated differences in stress state through either residual stresses, the presence of a crimp loading prior to cycling, or test temperature.

Nitinol wire apex specimens with a wire diameter of 0.51 mm and an apex bend radius of 0.75 mm were used for the testing. Prior to the fatigue testing, the specimens were crimped to crimp strains of 0%, 6%, or 16%. These levels of crimp strain were utilized to induce different stress states in the high stress/strain region during cycling. In addition, specimens that were crimped to 6% strain were fatigued at a range of bath temperatures to further assess the effect of stress on the fatigue performance. After crimping, specimens were cycled to a run out of 10-million cycles at several strain amplitudes. Specimens were also examined metallographically to quantify inclusion content. The testing results demonstrated that the stress state of the material can affect the fatigue performance of nitinol under apparently similar cyclic strain conditions.