S. Walak, Boston Scientific Corporation, Watertown, MA
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%.
Identifying a “safe” strain range for nitinol medical implants exposed to high cycle fatigue conditions is extremely important. Goodman and Soderberg plots used to predict the fatigue endurance limit of traditional metallic materials are known to dramatically over estimate safe strain levels in superelastic nickel titanium alloys. A unique strain controlled tension-tension fatigue test was developed to determine the cyclic strain tolerance of small diameter nitinol wires at 4.00, 6.00 and 7.75% mean strain. Rotary bend tests established the cyclic strain endurance limit at zero mean strain.
Test results showed a large drop in cyclic strain tolerance between 0.00% and 1.00% mean strain. Cyclic strain tolerance was relatively insensitive to increasing mean strain between 1.00% and 6.00%. A trend toward decreasing cyclic strain tolerance was observed at 7.75% mean strain. The resulting “map” of safe and unsafe strain regions was consistent with literature data generated on simulated nitinol tube “stent specimen” fatigue tests and Finite Element Analysis (FEA).
The strain controlled tension-tension fatigue test was shown to be a useful method for directly evaluating the cyclic strain sensitivity of nitinol wires at positive mean strains and frequencies up to 30 Hz.
