Evaluation Of Different Fatigue Criteria For NiTi Cardiovascular Devices

NiTinol (NiTi) is widely used in biomedical fields. In particular, several cardiovascular minimally-invasive devices exploit NiTi pseudo-elastic behavior. Notwithstanding the high performance of NiTi peripheral stents or cardiovascular valves, the fatigue behavior is still an open issue. Stents and valve foresee cyclic loadings due to leg or heart movements that may induce multi-axial solicitations. During cyclic loadings, the material is subjected to elastic cycles in austenitic phase in some regions, while it is partially transformed in martensite in other regions where it may elastically cycle from the lower plateau to the upper one or vice versa. Numerical analyses are a useful tool to study the device fatigue behavior, but numerical results need to be correctly interpreted. Damage parameters commonly used in the fatigue analysis are first principal strain or equivalent Von Mises strain, valid for proportional loadings. However, the cyclic loads applied in-vivo induce multi-axial non-proportional solicitations. This study aims at comparing different fatigue criteria to numerically evaluate the fatigue performance of various geometries (stent, valve) in different loading conditions. The comparison highlights that all the fatigue criteria gives the same critical zone but they provide different results in terms of failure risks. Hence, a validation of the correct prediction have been obtained through comparison with experimental tests helping in the selection of the most suitable criterion for NiTi cardiovascular devices.