Despite its unusual flow properties, recent cyclic testing of Nitinol shows aspects of the fracture behavior which adhere to traditional fatigue tenets for ductile metals. Specifically, fatigue fractures nucleate at microstructural heterogeneities resulting in failure at locations other than as-predicted using FEA. The FEA analyses are not incorrect – they just don’t account for microstructural heterogeneities, such as inclusion stringers that can act as crack nucleation sites in high-cycle fatigue. The macroscopic fracture plane may contain the entire length of the inclusion stringer or a section thereof, indicative of the (probabilistic) potency of the defect acting as a nucleation site for fatigue cracking. Inclusions in Nitinol become ‘strung-out’ as a result of axi-symmetric drawing techniques used to produce the tubing used for laser-cutting stents. The directionality of the ‘stringers’ are common in wrought metals and result in anisotropy of fatigue performance in the final product.

Analyses of superelastic Nitinol have always assumed crystallographic and microstructural isotropy in modeling operational conditions and prediction of failure locations. Data clearly shows SE Nitinol can exhibit a high degree of both crystallographic and microstructural anisotropy. With Nitinol being the material of choice for use in evermore demanding applications such as the frame for transcatheter valves (tCV) to treat valvular pathologies, ensuring long-term device durability requires a sophisticated approach to analyzing cyclic data and predicting fatigue life.

New predictive capabilities of metals enable incorporation of anisotropies on fatigue life – termed “Lifing” in the Aerospace industry. Reliable engineering practices also must consider multiple failure modes (FMEA). This presentation delineates the methodology we are using to achieve a more sophisticated description and better predictability of most probable locations of fatigue cracking in medical devices. Such improvements in analytical techniques also provide a basis for device design changes to optimize device properties – such as fatigue life.