J.P. McGarry

Computational Examination of the Effect of Material Inhomogeneity on the Necking of Stent Struts Under Tensile Loading

This study presents a computational investigation of tensile behavior and, in particular, necking due to material inhomogeniety of cardiovascular stent struts under conditions of tensile loading. Polycrystalline strut microstructures are modelled using crystal plasticity theory. Two different idealized morphologies are considered for three-dimensional models, with cylindrical grains and with rhombic-dodecahedron grains. Results are compared to two-dimensional models with hexagonal grains. For all cases, it is found that necking initiates at a significantly higher strain than that at UTS (ultimate tensile stress). Two-dimensional models are shown to exhibit an unrealistically high dependence of necking strain on randomly generated grain orientations. Three-dimensional models with cylindrical grains yield a significantly higher necking strain than models with rhombic-dodecahedron grains. It is shown that necking is characterized by a dramatic increase in stress triaxiality at the center of the neck. Finally, the ratios of UTS to necking stress computed in this study are found to compare well to values predicted by existing bifurcation models.

Computational modelling of the extrusion of an Al-SiC metal matrix composite using macroscale and microscale methods

This paper explores the use of both macroscale and microscale modelling for the analysis of extrusion of an AA2009 + 25%SiCp metal matrix composite (MMC). The performance of a micromechanical model, where the heterogeneous microstructure of the MMC is explicitly modelled, in predicting the tensile stress—strain behaviour of an AA2009 + 25% SiCp MMC is examined. A macroscale modelling approach is used to simulate extrusion of the MMC through two different die designs, where the MMC is modelled as a homogeneous continuum. Firstly, the extrusion results are used to compare the two die designs, to determine which is the more favourable. Secondly, the predicted macroscale plastic strain distributions and pressures are used with the micromechanical model to assess microscale stress states in the material during extrusion with a view to gaining insights into the risk of damage in the material. In this context, pressure is shown to be hugely important in controlling tensile stress magnitude and in reducing microscale damage risk, and essentially ensuring that extrusion can be achieved in practice. However, the results reveal that damage risk is not totally eliminated and that there may still be locations where the material may rupture.

Webbing and Delamination of Drug Eluting Stent Coatings.

The advancement of the drug-eluting stent technology raises the significant challenge of safe mechanical design of polymer coated stent systems. Experimental images of stent coatings undergoing significant damage during deployment have been reported; such coating damage and delamination can lead to complications such as restenosis and increased thrombogenicity. In the current study a cohesive zone modeling framework is developed to predict coating delamination and buckling due to hinge deformation during stent deployment. Models are then extended to analyze, for the first time, stent-coating damage due to webbing defects. Webbing defects occur when a bond forms between coating layers on adjacent struts, resulting in extensive delamination of the coating from the strut surfaces. The analyzes presented in this paper uncover the mechanical factors that govern webbing induced coating damage. Finally, an experimental fracture test of a commercially available stent coating material is performed and results demonstrate that the high cohesive strength of the coating material will prevent web fracture, resulting in significant coating delamination during stent deployment.