Micromechanical investigation into the effect of texture on the fatigue behaviour of superelastic nitinol

Abstract

One material that has found particular favour for use in the manufacture of biomedical endovascular stents is the near equiatomic NiTi alloy, Nitinol. Stents are typically laser-cut from textured micro-tubing; texture is the distribution of crystallographic grain orientations in a polycrystalline material. Despite the well documented dependence of mechanical behaviour on crystallographic texture, the standard computational design practice for such stents simply calls for the use of uniaxial homogenous material properties which assumes continuum material behaviour. This study offers a computational examination into the effect of crystallographic texture on NiTi’s fatigue behaviour in order to highlight concerns with this current design practice. The computational methodology first developed by Bruzzi and McHugh (2002), and modified for use with superelastic NiTi by Weafer and Bruzzi (2016), is adapted to include crystallographic texture. This defect tolerant approach correlates local crack-tip driving force conditions of an initial small crack with an experimental long crack growth rate curve, using crack closure. Computationally derived predictions of fatigue life are compared for superelastic NiTi specimens considering the material as (1) a continuum material with homogenous behaviour, and (2) a textured material with anisotropic granular behaviour, presented using both realistic and idealised microstructural features. In this manner, further insight is achieved into the effect of crystallographic texture on the fatigue behaviour of superelastic NiTi and offers a quantitative explanation towards the observed scatter in experimental fatigue data.