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

Abstract

This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material.Micro-scale specimens, representative of stent struts, are manufactured via laser micromachining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterization of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments.

Fatigue of Metallic Stents: From Clinical Evidence to Computational Analysis

“…On the contrary, the techniques based on a microstructural analysis of the inhomogeneities, coupled with crystal plasticity and stress-gradient models, have successfully captured the fatigue scatter. [79][80][81] Another difficulty is the presence of small curvature radii in stent struts, which are responsible for high stress concentrations in the presence of a heterogeneous microstructure. This has motivated the adoption of volumetric approaches (see Tables 5 and 7), the theory of critical distance, 10 as well as stress-gradient crystal plasticity models.…”

“…This has motivated the adoption of volumetric approaches (see Tables 5 and 7), the theory of critical distance, 10 as well as stress-gradient crystal plasticity models. 80, 81 To overcome such difficulties, the presence of heterogeneous microstructures has been discussed in Ref. 14 for the Dang Van criterion and extensions including a probabilistic setting have been proposed in Ref.…”

“…19 Most Nitinol testing are strainbased, 39,66,67,74 while most 316L stainless steel and cobalt chromium testing are stress-based. 23,55,82,88 Some limitations of these approaches have also to be considered: (i) due to size effects, specimens need to have dimensions comparable to those of stent components; 81 (ii) not all the manufacturing process can be reproduced; (iii) complex in-vivo loading conditions (e.g., bending) are difficult to simulate. …”

“…However, these data are difficult to procure and this approach may be less applicable to the initial stages of stent fatigue design, prior to approval for in-vivo operation. 81 …”

High-cycle fatigue behavior of thin-walled CoCr tubes

“…The experimental lifetime data are based on using bulk samples [7,8], thin wires [9] and foils [10] from which a brief overview is given. Ritchie and Lubock [7] suggested a procedure to estimate the useful fatigue life of the struts of cardiac valve prosthesis by means of stress life and damage-tolerant analysis.…”