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

Abstract

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 108-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 108 fatigue cycles.