Effects of Tube Processing on the Fatigue Life of Nitinol - Part 2

Our initial work compared probabilistic fatigue lives of diamond samples lasercut from a standard grade of Nitinol tubing manufactured using TM-1 tube processing techniques, to published data in which similar diamonds were lasercut from the same standard grade of Nitinol, two other standard grades of Nitinol and two high-purity grades of Nitinol expressly designed to improve fatigue life. All tubes in the published work were fabricated using TM-2 tube manufacturing techniques. Initial results indicated an effect of tube processing method on fatigue life especially comparing behaviors of the standard grades of Nitinol. To compare effects of tube manufacturing methods TM-1 and TM-2 on fatigue lives of samples made from the cleaner grades of Nitinol, 25mm diameter hollows made from the cleaner materials were drawn to final size using TM-1 methods. Diamonds were fabricated then fatigued to 10⁷ cycles. As well, initial correlation of probabilistic fatigue lives with final microstructures for the standard grades of material showed an inverse power-law relationship with a negative exponent near unity between median fatigue life and transverse median NMI particle diameter. This result was shown to be qualitatively consistent with the nature of the relationship between fatigue life and transverse particle diameter found for steels. Such relationships for the two cleaner materials are presented and discussed in terms of models describing the effects of NMI particles in Nitinol acting as primary nucleants.