A discussion about multi-axial fatigue criteria for NiTinol cardiovascular devices

Nickel-Titanium (NiTinol) alloys exploit a typical super-elastic behavior which makes them suitable for many biomedical applications, among which peripheral stenting, requiring the device being subjected to the high mobility of the lower limbs. Unfortunately, this complex environment can lead to the device fatigue fracture with likely other more severe complications, e.g. restenosis. Standards require to experimentally verify stent fatigue life behavior, without giving indications on how to select the loads to be applied for resembling most critical in-vivo conditions. Moreover, different multi-axial fatigue criteria have been originally developed for standard metals to predict the behavior under cyclic loads, but none of them is specifically formulated for NiTinol. This paper presents a numerical study having two aims: i) understanding how non-proportional loading conditions due to combination of axial compression, bending and torsion induced at each patient gait on the femoro-popliteal artery affects the implanted stent stress/strain distribution; ii) understanding how stent fatigue life prediction may be affected by the choice of the fatigue criteria. Accordingly, two different peripheral stent geometries, resembling commercial ones, were analysed under different sets of loading conditions. The cyclic deformations induced over the device structure by macroscopic loads are interpreted through four different fatigue approaches recently used in Nitinol fatigue analyses: Von Mises, Fatemi-Socie, Brown-Miller and Smith-Watson-Topper. The comparison between the outputs highlights that they are strongly influenced by the loading path, recognizing the major role in fatigue due to the combined torsional and bending actions. On the other hand, the choice of the fatigue criterion impacts on the fatigue life prediction.