High-Durability Nitinol for Medical Devices: A 100 Million-Cycle Bending Fatigue Characterization Study

This study investigates the bending fatigue performance of Vacuum Arc Remelted/Electron Beam Refined (VAR/EBR) Nitinol for cardiovascular applications. Diamond-shaped fatigue specimens were manufactured from ultra-clean VAR/EBR Nitinol tubing with inclusion sizes below 10 μm and tested under physiologically relevant conditions to 100 million cycles. Testing included multiple combinations of mean strains (0–7%) and strain amplitudes (0.75–2.50%) to simulate in vivo conditions for cardiovascular devices. Results demonstrate that VAR/EBR Nitinol exhibits as high as 275% improvement in 10 8 -cycle Fatigue Strain Limit (FSL) compared to conventional VAR Nitinol at mean strains between 3 and 5%. The FSL behavior shows three distinct strain regimes: increasing FSL with mean strains from 0 to 3%, plateau between 3 and 5%, and decreasing FSL beyond 5%. Analysis confirms the critical impact of limiting inclusion size below the theoretical critical small crack length of 10 μm, where fatigue performance becomes dictated by cyclic stress/strain rather than flaw size. Advanced (S)TEM imaging of the fatigue specimens provides evidence that advanced thermomechanical processing leads to stable microstructures to enhance fatigue response up to 10 8 fatigue cycles.