Material Properties of Thermo-Mechanical Processed Miniaturized Nitinol Specimen - Correlations Between Fatigue and Mechanical Properties Obtained On Different Nitinol Tube Materials

Nitinol is widely used for minimal invasive vascular implants due to its superelastic material behaviour. To fully utilize the material properties for vascular implants such as stents for neurology, superficial femoral artery (SFA) or heart valve frames, significant knowledge about the mechanical characteristics of the material in the fully processed state is required. A better understanding of the material characteristics in the as-processed state, such as elastic modulus, plateau stresses as well as fatigue life could improve future material models for FEA calculations and hence better utilize Nitinol for implants.

In this study Nitinol micro tensile test samples laser cut from tubing underwent different thermo-mechanical processing similar to stent processing. We determined all major mechanical properties, such as elastic modulus, tensile strength, plateau strengths and residual stresses in tensile tests performed at 37°C. Other samples were subjected to accelerated fatigue tests using a 3-point bending model. Differential scanning calorimetry, metallographical and SEM investigations were performed to assess the impact of nucleated precipitations, inclusion sizes and densities as well as characterize fracture surfaces of fatigue samples. Finally the different results were put into a matrix in order to correlate the different results with the thermo-mechanical treatment of the Nitinol materials.