Fatigue Behavior of Superelastic Nitinol: Effects of Melting Methods, Microstructure, and Loading Conditions

This study investigates the fatigue behavior of superelastic Nitinol from different ingot manufacturing methods (VAR and VIM+VAR), forms (rods and tubing), and purity levels (standard and high purity), using wire and diamond-shaped surrogate samples. Wire samples were evaluated using rotary bending fatigue (RBF) testing, while diamond-shaped surrogates underwent tension–tension fatigue testing with a 6% pre-strain and a 3% mean strain to simulate the crimping and deployment conditions of the device. Finite Element Analysis (FEA) was employed to determine the machine displacement necessary to achieve target strain values during testing of diamond samples. Fatigue tests were performed at 37°C and up to 10 million cycles across a range of alternating strain levels. Microstructural and fracture surface analysis was conducted to assess the influence of non-metallic inclusions (NMIs) on fatigue properties and crack initiation sites. Strain-based fatigue limits were determined, revealing that high purity materials consistently outperformed standard materials, regardless of melting method, form, or loading condition. However, fatigue performance in high purity Nitinol was found to be sensitive to the stressed volume; depending on the loading condition, fatigue performance was significantly affected by maximum inclusion size and/or by inclusion area fraction. These findings provide insight into the complex relationship between microstructure, loading, and fatigue performance, and support the design and reliability assessment of Nitinol-based medical devices.