Microstructure-sensitive fatigue modeling of medical-grade fine wire

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Clark, Brian
Neu, Richard W.
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This work presents a model to assess the microstructure-sensitive high-cycle fatigue (HCF) performance of thin MP35N alloy wires used as conductors in cardiac leads. The major components of this model consist of a microstructure generator which constructs a statistically representative microstructure, a crystal plasticity finite-element analysis to determine the local response behavior of the microstructure, and a postscript employing fatigue indicating parameters (FIPs) to assess the fatigue crack incubation potency at fatigue hotspots. The crystal structure of the MP35N alloy, which contains major elements (wt %) 35Ni-35Co-20Cr-10Mo, is modeled as single-phase, face-centered cubic material, and the constitutive behavior is calibrated based on monotonic tensile and cyclic ratcheting stress-strain response data. Non-metallic inclusions (NMIs) have been shown to be detrimental in fatigue of MP35N wires by serving as fatigue crack nucleation sites. The detrimental effects of NMIs are modeled within a stochastic framework. By evaluating multiple statistical volume elements, the fatigue potency and inherent variability of inclusion-grain and grain-grain interactions at the NMI-matrix interface can be assessed. The extreme-value distributions of the Fatemi-Socie FIP were successfully correlated to rotating beam bending fatigue (RBBF) life data collected for MP35N fine wire. The correlation indicates that the fatigue potency in RBBF is strongly influenced by the NMI proximity to the wire surface with the most severe case occurring when the NMI intersects the surface. A significant drop in fatigue potency is observed when the NMI is fully embedded in the wire. Fatigue-life correlations to a second set of RBBF data were performed in order to identify a transition life value between crack incubation and microcrack growth fatigue mechanisms. The model has applications in numerous additional aspects of microstructure-sensitive HCF which can be explored in a future work.
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