Pre-strain and Mean Strain Effects on the Fatigue Behavior of Superelastic Nitinol Medical Devices

There is a growing body of evidence that “in situ” modification of the microstructure of Nitinol due to pre-strain and mean strain strongly influence the fatigue properties of implanted medical devices and often, counterintuitively, for the better. The purpose of this review paper is to focus on the experimental evidence and to support these finding with micro-mechanical and metallurgical references. Pre-strain effects are well recognized in Nitinol thermal actuator fatigue behavior and applied in medical device applications but the effects of mean strain remain controversial. Detailed reviews of the pertinent literature indicate that pre-strains intensify texture, increase martensite volume fraction, and induce plasticity that may enhance or degrade fatigue performance. The majority of experimental studies demonstrate that fatigue cycling with mean strains within the two-phase region leads to longer lives compared to cycling in the linear elastic region or at mean strains greater than the end of the stress plateau. These effects are described in terms of martensite stabilization, cyclic phase change, and the decrease in composite modulus with increasing mean strain. These factors combine to result in alternating strain between approximately constant upper and lower plateau stresses to induce phase change.