A Fracture-Mechanics Based Approach to Fatigue of Nitinol Tube

The traditional approach to biomedical device design for resistance to fatigue failure is based on a total-life philosophy for predicting safe in vivo operating conditions. Although this approach is extremely useful for determining safe applied loads and displacements, it cannot predict the critical flaw size that may eventually lead to a cumulative damage and possible failure in an implanted device subjected to millions of pulsatile cyclic loads. Indeed, there is a dearth of relevant data in the literature on such fracture-mechanics based approaches to fatigue, and that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, namely bulk Nitinol bar and sheet. The results presented herein document the fatigue and fracture response in Nitinol tubing, similar to that used for medical device manufacture, which has undergone a series of shape-setting procedures to flatten the material. The resultant At temperature is ~25-30°C, identical to self-expanding Nitinol stents. The fatigue behavior at various load ratios (R = 0.1, 0.5, and 0.7) is presented and shows higher fatigue thresholds than previous literature sources have reported for bulk Nitinol material. Our objective is to combine the well-characterized total-life predictions with fatigue/fracture data from compact-tension specimens at controlled flaw sizes in order to aid the development of stents with enhanced structural longevity.