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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.

SYNOPSISGel permeation chromatography has been used to monitor the molecular weight changes that occur when polymers are degraded by photooxidation. Methods of sample preparation and procedures for conducting the chromatography measurements and for analyzing the results are discussed. Examples are given of applications to studies of polystyrene (PS) , polypropylene (PP) , and glass fiber-reinforced polypropylene (GFPP) exposed to ultraviolet (UV) irradiation in the laboratory. The results show that the degradation rate is fastest near the exposed surface, but in PS and PP, degradation is much faster at the unexposed surface than in the center of the molding, where the UV intensity is greater than at the unexposed surface. It is deduced that degradation at the center is slowed by a shortage of oxygen.

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