Inclusions Size-based Fatigue Life Prediction Model of NiTi Alloy for Biomedical Applications

Current standards consider the size and distribution of inclusions in semi-finished material, but do not place requirements on final biomedical devices made of NiTi shape memory alloys. In this paper, we analyze this by comparing the fatigue performances of NiTi superelastic wires obtained by different processes through a simple bilinear model of fatigue response in terms of strain life. The fracture surfaces of failed wires are analyzed through SEM microscopy and data regarding the presence of particles, and their morphology is recorded and analyzed using Type-I extreme value distribution. The results show a strong correlation between the fatigue limit of wires (in terms of strain) and the predicted extreme values of inclusions at fracture origin. Then, following the concept of treating the inclusions as ‘small cracks,’ a simple relationship between fatigue limit strain range and inclusion size is proposed based on ΔK_th data from the literature. The model is compared with the fatigue data obtained from the tested wires.