Comparison of experimental and numerical data for NiTinol peripheral stents fatigue prediction

Nickel-Titanium alloys (NiTinol) are widely used for biomedical applications. Peripheral stents are almost exclusively composed of NiTinol, as its superelasticity is suited for mini-invasive insertion and durable effect. After crimping and deployment stents undergo multi-axial loads imposed by vascular and lower-limb movement (e.g. axial compression, bending, torsion). This complex mechanical environment could lead to metal fatigue and device fracture, with possible severe consequences (e.g. in-stent restenosis). Standard regulations require experimental verification of stent fatigue behavior for preclinical assessment, but no exact indications are provided to direct the load combination or the in-vivo worst case scenario. Moreover, different fatigue criteria were developed for common metals to predict fatigue endurance, but no criteria were specifically defined for the unique thermo-mechanical properties of NiTinol.

This study investigated the role of multi-axial non-proportional loading conditions on different stent geometries, looking at how they affect the stent stress/strain distribution along the device and how the fatigue prediction may be affected by the choice of different fatigue criteria (the standard Von Mises alternate approach and other critical plane approaches). Then, a preliminary experimental fatigue campaign was performed in agreement with the numerical simulations in order to compare the numerical predictions with the experimental results. The obtained outputs suggest that the critical plane approaches are more reliable than the standard Von Mises criterion.