This study deals with the influence of processing induced crystalline orientation on the macroscopic deformation and failure behavior of thin samples of polyethylene and polypropylene. Distribution and structure of flow-induced orientations were characterized by optical microscopy, X-ray diffraction techniques, and transmission electron microscopy. Hermans' orientation functions were either determined from the flat plate wide-angle X-ray diffraction patterns or calculated from full pole figures. The deformation behavior of the oriented samples was studied in both impact and tensile testing conditions and was found to be strongly anisotropic and related to the oriented structure. For all polymers studied, an increase of extended chains (shish) in the loading direction is proposed to cause an increase in the yield stress, and a lamellar structure oriented perpendicular to loading direction leads to an increase in strain hardening. In the extruded samples, where a low level of extended chains and a high level of oriented lamellae were found, the resulting combination of yield stress and strain hardening leads to homogeneous deformation. Brittle-ductile transitions in impact toughness of the molded samples could also be explained from differences in yield stress and strain hardening. Toughness enhancement was found to be most efficient with increasing strain hardening, and the effect was less pronounced in the polypropylene samples.

Time-to-failure of polymers, and the actual failure mode, are influenced by stress, temperature, processing history, and molecular weight. We show that long-term ductile failure under constant load is governed by the same process as short term ductile failure at constant rate of deformation. Failure proves to originate from the polymer's intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study, we developed a constitutive model that includes physical aging and is capable of numerically predicting plastic instabilities. Using this model the ductile failure of polycarbonates with different thermal histories, subjected to constant loads, is accurately predicted also for different loading geometries. Even the endurance limit, observed for quenched materials, is predicted and it is shown that it originates from the structural evolution due to physical aging that occurs during loading. For low molecular weight materials this same process causes a ductile-to-brittle transition. A quantitative prediction thereof is, however, outside the scope of this paper and requires a more detailed study.

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