| Tension-Tension Fatigue Testing of Nitinol Wires | ||
| Mr. Steven Walak, Boston Scientific Corporation, 480 Pleasant Street, Watertown, MA 02472
Nitinol stent wires were subjected to zero and non-zero mean strain tension-tension fatigue tests. The non-zero mean strain testing protocol established the one million cycle endurance limits for selected mean strains between 4.00% and 7.75%. Tests were performed at 15 and 30 Hz with photonic sensor feedback used to control test sample strain. Fine wire capstan grips minimized local grip stress and facilitated fracture in the sample gauge section. Test results confirmed recent literature results that indicate traditional Goodman and Soderberg linear life prediction models are not appropriate for nitinol wire devices. Data from zero mean strain rotary bend and tension-tension fatigue tests showed a large drop in cyclic strain tolerance between 0.00% and 1.00% mean strain. The large drop in cyclic strain tolerance observed at 1.00% correlates well with the onset of the strain induced austenite to martensite phase transformation. Cyclic strain endurance limit data showed a trend toward increased cyclic strain tolerance with increasing mean strain from 1.00% to 6.00%. In this region, the cyclic strain endurance limit increased from a low of 0.20% to a high of 0.35% as mean strain was raised. At 7.75% mean strain the cyclic strain endurance limit decreased to 0.15%. | ||