Comprehensive Study of Fatigue Performance of Superelastic Nitinol

Two main melting techniques are used for production of Nitinol materials: Vacuum Arc Remelting (VAR) or combination of Vacuum Induction Melting (VIM) with VAR process (VIM/VAR). These melting methods lead to formation of different inclusion types with a range of inclusion size and area fraction depending on the process and purity of the materials. Moreover, new alloy grades have been recently developed with limited comprehensive fatigue study compared with existing materials on the market. In the current study, superelastic wire samples and diamond-shaped surrogates were manufactured from Nitinol wire and tubes, respectively. These samples were originally produced from Ingots melted using VAR only or VIM/VAR techniques and different grades of Nitinol (standard vs high purity grades). Results showed that there is a correlation between inclusion size/area fraction and fracture probability and type of fatigue test. In addition, a new model for predicting the fatigue resistance of superelastic Nitinol was developed based on the data. The model takes the non-metallic inclusion size and distribution as the independent variables and furnishes the strain amplitude that leads to a specified fatigue life for Nitinol. This model is a novel tool that will be useful in the design of NiTi components such as implantable medical devices. The model will provide a reliable method to assess the suitability of a particular NiTi alloy for a specific application when the strain-based boundary conditions are known.