Modelling the mechanical and fatigue behaviour of superelastic NiTi: analysing the role of microstructural phase and crystallographic texture
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Superelastic NiTi self-expanding stents push the boundaries of our current understanding of material behaviour which ultimately influences the design of biomedical devices. In particular, the influence of microstructure on NiTi’s behaviour is not well understood. Therefore, the objective of this thesis is to computationally investigate the impact of the two major influences on NiTi’s fundamental mechanical and fatigue behaviour, namely, microstructural phase and crystallographic texture. Employing the Finite Element Analysis (FEA) platform ABAQUS™, the influence of microstructural phase and crystallographic texture is first examined under quasi-static tension-compression conditions as a potential indicator of superelastic NiTi’s fatigue performance. Subsequently, the fatigue behaviour of superelastic NiTi’s is investigated using fatigue modelling methods incorporating the microstructural effects of both microstructural phase and crystallographic texture, respectively. Firstly, through the individual examination of the mechanical behaviour of both the austenitic and martensitic microstructural phases of NiTi, it is confirmed there exists a strong association between the stress-induced martensitic (SIM) phase and the observed unusual fatigue behaviour of superelastic NiTi. It is proposed that superelastic NiTi’s unique trait of increasing fatigue performance with increasing mean strain can be attributed to the shift from austenitic to martensitic NiTi fatigue behaviour, i.e. through stress-induced martensite transformation (SIMT). Secondly, grain orientation distribution (crystallographic texture) is determined to cause significant deviation of the fundamental mechanical and fatigue behaviour of polycrystalline superelastic NiTi specimens from that of their predicted continuum behaviour. It is suggested that the use of homogenous bulk material properties, as is standard practice in the computational design process of biomedical devices, are inadequate to fully describe the complex material behaviour of superelastic NiTi. It is proposed current material models should be amended to include such microstructural effects for the continued safe use of NiTi for commercial applications and products.