Experimental characterisation for micromechanical modelling of CoCr stent fatigue

Micro-scale testing and micromechanical modelling for high cycle fatigue of CoCr stent material

“…Optical microscopy was first carried out to determine the grain structure in the foil. An etching solution of 100 ml HCl and 5 ml 30% H 2 O 2 was used, as described for the macro-scale CoCr material in [10,28]. Optical micrographs of the foil are shown in Fig.…”

“…The authors have previously applied a crystal plasticity (CP) approach (i.e. modelling of slip on individual slips systems in the crystal lattice of discrete grains) to fatigue assessment of 316L stainless steel [9] and L605 CoCr alloy stents [10], demonstrating the ability to capture the effect of microstructural inhomogeneity and illustrating scope for the CP framework as a tool for fatigue design of stent geometries. Patient-specific modelling, such as presented by Schievano et al [11], can offer a means of validation via direct comparison between models and deformed stents, extracted from real patient vessel geometries.…”

“…L605 foil micro-specimens are manufactured and tested under HCF loading conditions, and microscopy is carried out on the foil to aid in generation of realistic crosssection polycrystalline models for the foil specimens. The micromechanical framework used for HCF prediction has been developed and calibrated in previous works [10,28] for the LCF fatigue behaviour of the L605 alloy, and is hypothesised to predict fatigue behaviour across both LCF and HCF regimes. Two CP models are investigated and compared within the micromechanical framework for describing the material constitutive behaviour.…”

“…The first is a physically-based strain-gradient CP model, which has been calibrated against cyclic plasticity data for as-received, fine-grain (d = 32 µm) L605 material in previous work [28] and, with the same calibrated constants, has been shown to also capture the behaviour of heat-treated, coarse grain (d = 243 µm) material. A phenomenological power-law constitutive model, previously calibrated against both monotonic and cyclic plasticity data, and applied to the assessment of a stent fatigue design for the L605 alloy [10], is also investigated. Two microstructure-sensitive FIPs, effective plastic strain, p, and crystallographic work, W, are implemented for the prediction of the number of cycles to crack initiation, N i , for HCF behaviour, based on critical FIP values calibrated and validated from macro-scale LCF test data [10,28], and the predictions are compared with the experimental stress-life data.…”

“…A phenomenological power-law constitutive model, previously calibrated against both monotonic and cyclic plasticity data, and applied to the assessment of a stent fatigue design for the L605 alloy [10], is also investigated. Two microstructure-sensitive FIPs, effective plastic strain, p, and crystallographic work, W, are implemented for the prediction of the number of cycles to crack initiation, N i , for HCF behaviour, based on critical FIP values calibrated and validated from macro-scale LCF test data [10,28], and the predictions are compared with the experimental stress-life data. The micromechanical framework presented and validated here for stent fatigue design addresses an outstanding predictive need, highlighted by Marrey et al [6], for HCF of CoCr alloy stents.…”