Wire Rotary Bending Fatigue Characteristics of AltusTM Nitinol Compared Against Multiple Suppliers

Nitinol implantable devices are expected to withstand high cyclic loads under different strain conditions. Fatigue fracture samples often exhibit near-surface, non-metallic inclusions (NMIs) at crack initiation points that limit structural fatigue life. Improvements in the microcleanliness of Nitinol, i.e., reductions in frequency and maximum dimension of NMIs, have therefore been a topic of increased interest in the medical materials and medical device industries as a means of addressing the need for high cycle fatigue in implantable applications. This work aims to better understand the fatigue life of different superelastic Nitinol wire offerings from various melt and wrought material suppliers. In this study, wires were processed under similar finishing conditions including cold work and heat treatment with similar measured thermal and mechanical properties. The 125 µm diameter wires were characterized via rotary bending fatigue at strain amplitude levels of 0.8-1.3% up to 10⁷ cycles from various melt and wrought material suppliers including Fort Wayne Metals Altus^TM Nitinol. Raw material properties of the feedstock material (i.e. grain size, max inclusion size and area percent, and transformation temperatures) were also analyzed. Fort Wayne Metals Altus™ Nitinol performs favorably compared to other suppliers of Nitinol for demanding fatigue applications as analyzed by fully-reversed cycling.