Rotary Bend Fatigue of Nitinol to One Billion Cycles

Nitinol implants, especially those used in cardiovascular applications, are typically expected to remain durable beyond 10(8) cycles, yet literature on ultra-high cycle fatigue of nitinol remains relatively scarce and its mechanisms not well understood. To investigate nitinol fatigue behavior in this domain, we conducted a multifaceted evaluation of nitinol wire subjected to rotary bend fatigue that included detailed material characterization and finite element analysis as well as post hoc analyses of the resulting fatigue life data. Below approximately 10(5) cycles, cyclic phase transformation, as predicted by computational simulations, was associated with fatigue failure. Between 10(5) and 10(8) cycles, fractures were relatively infrequent. Beyond 10(8) cycles, fatigue fractures were relatively common depending on the load level and other factors including the size of non-metallic inclusions present and the number of loading cycles. Given observations of both low cycle and ultra-high cycle fatigue fractures, a two-failure model may be more appropriate than the standard Coffin-Manson equation for characterizing nitinol fatigue life beyond 10(8) cycles. This work provides the first documented fatigue study of medical grade nitinol to 10(9) cycles, and the observations and insights described will be of value as design engineers seek to improve durability for future nitinol implants.