Effects of Tube Processing on the Fatigue Life of Nitinol

High-cycle fatigue life of Nitinol remains of interest as this unique material is used in an increasing number of Class 3 medical devices. With devices approved for more demanding cardiovascular applications as well as younger cohorts of patients with concomitant increased required duty cycles, long-term structural integrity of the substrate remains of paramount concern. With these considerations in mind, much attention has been paid to factors affecting the fatigue life of Nitinol in order to ensure long-term device efficacy. Initial fatigue studies using diamond samples, meant to represent the fundamental construct of laser-cut devices and capture in-vivo loading conditions, showed high cycle fatigue strain limits on the order of 0.4% - 0.8%. As with most wrought ductile structural metals the role of non-metallic inclusions in the form of stringers resulting from axisymmetric working operations act as fatigue crack nucleation sites. Despite recent efforts on the part of raw material producers to improve material cleanliness there has been little work investigating the effect of metal processing techniques on fatigue properties. In the current study, the effects of tube manufacturing methods on the high-cycle fatigue life of Nitinol were explored. Diamond samples, laser cut from tubing made using standard grade Nitinol interstitial contents viz., O₂, C, and N, were pre-strained to 6% unloaded to 3% mean strain and fatigued to 10⁷ cycles at a variety of strain amplitudes. Through a combination of enhanced metal working and heating conditions high-cycle fatigue limits to 10⁷ cycles of nearly 1.3% strain amplitude have been obtained.