Textural effect on the fatigue response of superelastic nitinol

It is well known from the Nitinol literature that crystallographic texture plays an important role in the austenite-to-martensite transformations. In particular, Robertson, et al. determined that wrought Nitinol tube, sheet and rod have predominant a {221}<1-22> texture. The theoretical transformation strain, calculated with a modified Taylor factor, was in good agreement with experimental results. Specifically, mechanical test specimens that were oriented 45˚ to the drawing/rolling axis resulted in a lower transformation strain. Our current investigation extends these monotonic results to fatigue conditions. Diamond-shaped specimens were photoetched from superelastic SE-508 sheet and mechanically cycled in displacement control to 10 Million total cycles or to fracture. The implications of these results are discussed in terms of optimizing the durability of medical device designs.