Influence of Microstructure on the Fatigue Performance of Nitinol: A Computational Analysis

One material that has found particular favour within the biomedical industry is the near equi-atomic superelastic NiTi alloy, Nitinol. Superelasticity can be attributed to a stress-induced martensite transformation (SIMT) from a parent austenite to the daughter martensite phase. By this microscopic process, Nitinol can withstand approx. 8-10% strain without permanent deformation. This is a highly desirable attribute in biomedical stent design for stent flexibility, durability and conformance. Upon implantation, stents will experience cyclic loading (strain amplitudes) due to the physiological loading conditions within a pulsating artery. In addition, the tortuous anatomy produces conditions of a constant mean strain on the structural device. Typical engineering materials will experience a decrease in fatigue life with increasing mean strain; however, Nitinol has been shown to exhibit improved fatigue performance [1-6].

A considerable amount of data has been gathered in relation to the remarkable mechanical properties of Nitinol. However, clarification into the observed phenomenon of Nitinol’s increasing fatigue life with mean strain still remains incomplete in literature. This study offers unique fatigue data spanning all experimentally-achievable mean strains and strain amplitudes. Rigorous fatigue testing of Nitinol diamond shaped specimens (Af = 22^oC) is completed using the EnduraTEC ELF/3200 at varying test temperatures ranging from -100^oC to +100^oC; the fatigue behaviour of fully austenitic, martensitic as well as transforming microstructures is therefore investigated. In conjunction with the experimental investigation, Finite Element Analysis is employed to explore the effect of texture on Nitinol’s macroscopic behaviour. An in-built subroutine, UMAT/Nitinol, is employed to simulate the behaviour of Nitinol on a micro-scale level; each idealised hexagonal unit represents an individual grain. Single crystal properties [7] were randomly assigned to grains to represent various textural possibilities. This study highlights the importance of microstructure, in particular the volume fraction of SIM, when discussing fatigue endurance limits of Nitinol.