Deformation and cavitation at the spherulite scale of semi-crystalline polymers
Résumé
Engineering stress-strain curves have been generated from tensile tests on semi-crystalline thermoplastics, which
may exhibit non linearity and/or peak stress associated with striction/necking phenomenon of the specimen at
the macroscopic scale. This work addresses this state of deformed specimen, on an isotactic polypropylene, where
irreversible strains have led to a variable cross sectional area along the necked region. 3D images in this region,
obtained by using Synchrotron Radiation Computed Tomography with two high resolutions were exploited. The best
resolution (1 pixel length = 0.05μm) allowed a better understanding of the morphology of several deformed spherulites
within which polar fan arrangements were clearly detailed. Thanks to the identification of the boundaries of spherulite
patterns, with a 0.7μm resolution, the longitudinal and transverse elongations of larger numbers of spherulites were
measured. The evolution of the volumetric plastic strains due to cavitation at the spherulitic scale along the necked
regions, was comprehensively analysed. Volume changes at this scale were highlighted, consisting of increases in the
case of void growth followed by a decrease (compaction) at large strains due to the collapse of elongated voids. The
effects of these results on the establishment of reliable constitutive model are discussed.
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