Nitinol Fatigue: The Effect Of The Initial Loading Strain On The Fatigue Life Of A Wire Formed Stent Specimen

This study investigates the effects of initial loading strain on the fatigue life of a Nitinol cardiovascular stent. Cyclic tension-tension fatigue data was obtained to 4 x 10⁸ cycles for stent test specimens with initial loading strain values of 4%, 7%, & 9%. Pre-strained samples were cycled across a range of mean strains and strain amplitudes. In cardiovascular stent applications, a typical Nitinol stent undergoes a loading process of inserting the stent into a vascular delivery catheter. This process reduces the stents larger native diameter to a catheter inner diameter so that the device can be introduced through small peripheral vasculature. As the stent is reduced in diameter, the strains present in the stent increase. A common thought for the range for this initial loading strain is to avoid the plastic deformation region of the stress-strain curve and keep the initial loading strain below 6-7%, with lower loading strains being preferred; however, the data shows there is an increase in fatigue life as the initial loading strain shifts from 4% to 9%. This information differs from the common belief that lower loading strains are better for fatigue life. The data can also be used to generate predictive fatigue life curves.