Crack Formation and Pathways in Nitinol Biomedical Devices

Abstract

Nitinol is a near equiatomic intermetallic that is increasingly being used in medical devices due to its unique shape memory behaviors of both superelasticity and shape memory effect, as well as its ability to be heat set into complex shapes. The metallurgical characteristics and properties rely on a diffusionless solid-state phase transformation between cubic Austenite and monoclinic Martensite. Such implanted Nitinol devices may experience millions to billions of in vivo cyclic deformations; these cycles may result in microstructural damage accumulation with the result of functional fatigue (e.g., change in strain recovery, transformation temperature, displacements and/or forces) and/or structural fatigue (e.g., cracks and fractures). Consequently, lifetime predictions of components are critical for the design and optimization of devices manufactured from Nitinol. Although most medical device companies conduct total life fatigue tests on their devices, damage-tolerant fatigue assessment is also important in order to understand safe-use conditions. This paper reviews the literature on investigations of crack formation and propagation in Nitinol materials under a variety of conditions.