*Invited* On the Life Fatigue of Superelastic Nitinol

Lifetime prediction of medical components that are subjected to cyclic mechanical motion is critical for the design and optimization of all devices manufactured from Nitinol. Medical devices, many of which are permanently implanted and experience millions to billions of in vivo cycles, dominate the current Nitinol market. The safety and durability of these devices may be measured by their resistance to fatigue. Descriptions of mechanical fatigue on a microscopic, and even macroscopic, level for these Nitinol based medical devices are complicated and remain incomplete. In particular, an accurate description of the fatigue properties under different loading conditions is still uncertain due to the path dependence of the phase transformation. The source of the complications is due to the uncertain role of the transformation under cyclically varying deformations and the complexity of the various phases in Nitinol. First, we will review the fatigue behavior of Nitinol under several loading modes, e.g., tension, bending, torsion, for “standard” specimen geometries. We will then focus on the effects of pre-straining, transformation and testing temperatures, as well as phase composition on the total life fatigue of superelastic Nitinol under a simple tension-tension loading condition. Finally, the effect of inclusions (size, distribution, composition, area fraction) on Nitinol materials processed with standard VAR, standard VIM-VAR, and high purity VAR methods will be addressed in both wire form and newly designed diamond-shaped subcomponents cut from tubing. This presentation will demonstrate the path dependency and energy-related fatigue behavior of Nitinol, and offer tools for optimization and tailoring of Nitinol components for fatigue applications.