Micromechanical modelling of fatigue crack nucleation in stents

Abstract

A finite element based micro-mechanical methodology for cyclic plasticity and fatigue crack initiation in cardiovascular stents is presented. The methodology is based on the combined use of a (global) three-dimensional continuum stent-artery model, a local micromechanical stent model, the development of a combined kinematic- isotropic hardening crystal plasticity constitutive formulation, and the application of micro- structure sensitive crack initiation parameters. The methodology is applied to 316L stainless steel stents with random polycrystalline microstructures, based on scanning electron microscopy images of the grain morphology, under realistic elastic-plastic loading histories, including crimp, deployment and in-vivo systolic-diastolic cyclic pressurisation. Identification of the micro- mechanical cyclic plasticity and failure constants is achieved via application of an objective function and a unit cell representative volume element for 316L stainless steel. Cyclic stent deformations are compared with the J2 -predicted response and conventional fatigue life prediction techniques. It is shown that micro- mechanical fatigue analysis of stents is necessary due to the significant predicted effects of material inhomogeneity on micro-plasticity and micro-crack initiation.