Accelerated Fatigue Behavior of Commercial Nitinol Tubes – Correlations between Standard and New Testing Methods

Despite the current understanding of the relationship between deformation modes and transformations in Nitinol materials, there is still not a clear understanding of the fracture mechanisms in medical devices manufactured from commercial precision Nitinol tubes. Complex mechanical load conditions of Nitinol components require a better understanding of the correlations between material characteristics, micro structure and fatigue behavior of the final processed materials.

The cause for ‘fast’ and ‘slow’ crack initiations and propagations in low and high cycle fatigue testing was investigated in more than 800 micro-fatigue samples. The samples were broken into groups of very small, medium and large diameter Nitinol tubes of various wall thicknesses from different suppliers. Micro test samples were laser-cut from tube materials and processed similar to final Nitinol medical implants. Correlations will be presented between tensile, micro hardness, differential scanning calorimetry measurements and accelerated fatigue testing - a combination from rolling contact and rotating bending fatigue. Data which shows the impact between ingots and tube drawing processes from major tube suppliers will be discussed. Finally, a correlation matrix within these different material tests will be provided that indicates fatigue resistance and major causes for failures.