Micromechanical methodology for fatigue in cardiovascular stents.

A finite element based micromechanical methodology for cyclic plasticity and fatigue crack initiation incardiovascular stents is presented. The methodology is based on the combined use of a (global) threedimensional continuum stent-artery model, a local micromechanical stent model, the development of a combined kinematicisotropic hardening crystal plasticity constitutive formulation, and the application of microstructure 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 elasticplastic loading histories, including crimp, deployment and in vivo systolicdiastolic cyclic pressurisation. Identification of the micromechanical 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 micromechanical fatigue analysis of stents is necessary due to the significant predicted effects of material inhomogeneity on micro-plasticity and micro-crack initiation.