Impact of Processing-Induced Anisotropy and Strained Volume on the Fatigue Behavior of Nitinol Sheet for Medical Devices

The increasing use of Nitinol sheet for fabricating complex medical devices, such as transcatheter heart valve frames, necessitates a thorough understanding of its anisotropic mechanical properties. The sheet rolling process induces potentially significant martensitic texture differences that result in orientation dependence of material properties. This anisotropy is hypothesized to directly impact fatigue performance, as the severity of microstructural features are highly dependent on their orientation relative to the principal stress axis.

To investigate this, Resonetics diamond fatigue samples were cut from superelastic Nitinol sheet in multiple orientations, including parallel (longitudinal) and perpendicular (transverse) to the rolling direction. Strain-controlled fatigue testing was conducted with a 6% pre-strain and a fixed mean strain of 3% with varying strain amplitudes, simulating the loading conditions of a deployed medical device, to generate S-N curves for both orientations. To correlate fatigue behavior with microstructure, material characterization was performed. This included optical metallography to assess grain morphology and scanning electron microscopy (SEM) for fractographic analysis of failure initiation sites. Comparison is made to four-point bend fatigue testing data from the same raw sheet material and tube diamonds from similar material to address the importance of strain volume on fatigue performance across testing modalities and source material.

By integrating fatigue data with microstructural observations, this work seeks to clarify how sheet processing–induced anisotropy affects the durability of superelastic nitinol. The results provide practical guidance for device designers and manufacturers, supporting improved performance predictions and optimized material specifications for next‑generation implantable devices.