Numerical Investigation on the Effect of Inclusions on the Local Fatigue Strain in Superelastic NiTi Alloy

The important role of inclusions for the fatigue behavior of Nitinol and the related service lifetime for medical devices is stated by numerous studies. Besides the well-known size effect on the fatigue limit, the corresponding crack initiation was observed preferably at particle-void-combinations. However, the detailed relationship of several geometrical inclusion properties and the resulting fatigue load remains not clear. To shed a light on this effects relationship, a numerical investigation was performed with a superelastic material behavior on a macroscopic framework. In the scope of this study, two-dimensional unit cells with fully embedded particles or particle-void-assemblies of different shapes and different relative orientations with respect to the load direction were evaluated. Additionally, those unit cells were subjected to different global strain amplitudes and mean strain levels. The careful evaluation of the results revealed a hierarchy of parameter effects on the fatigue strain. Besides the trivial relationship between global applied and local resulting fatigue load, the inclusion shape and the orientation were observed to show a strong effect on the local fatigue strain.