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Hétérogénéités de déformation au cours du fluage transitoire de la glace polycristalline. Mesures par corrélation d'images numériques et modélisation

Abstract : A Digital Image Correlation (DIC) technique has been adapted to polycrystalline icespecimens in order to characterize the development of strain heterogeneities at an intra-granular scale during transient creep deformation. These heterogeneities are due to thestrong viscoplastic anisotropy of the ice single crystal (with only one easy glide planefor dislocations), inducing strong mechanical interactions between adjacent grains duringdeformation, and the establishment of strong internal stresses. Specimens exhibit a co-lumnar microstructure so that plastic deformation is essentially 2-D with few in-depthgradients, and therefore surface DIC analyses are representative for the whole specimenvolume. Local misorientations at the intragranular scale were also extracted from mi-crostructure analyses carried out with an automatic texture analyzer before and afterdeformation. Highly localized strain patterns are evidenced by the DIC technique. Localequivalent strain can reach values as high as one order of magnitude larger than the ma-croscopic average. The structure of the strain pattern does not evolve signicantly withstrain during the transient creep regime. Almost no correlation was measured betweenthe local strain and the Schmid factor of the slip plane of the underlying grain ; thishighlights the importance of the mechanical interactions between neighboring grains. Atthe grain scale, the strong lattice misorientations have been associated to the local com-ponents of the displacement. In particular, kink band distortions could be correlated to adisplacement along the c-axis measured by DIC. The experimental microstructures wereintroduced into a full-eld FFT model that simulates the behaviour of elasto-viscoplasticpolycrystals (CraFT code). The simulated deformation elds present globally the samecharacteristics as the experimental ones. Most of the localization bands are well repro-duced, spatially and quantitatively. However, we could not reproduce the macroscopicbehaviour of the specimens upon unloading. The predicted recovery eective strain is oneorder of magnitude lower than the measured one. This limitation might be linked withthe internal stress eld induced by the dislocation structure, which is not explicitly takeninto account in the local constitutive relation.
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Fanny Grennerat. Hétérogénéités de déformation au cours du fluage transitoire de la glace polycristalline. Mesures par corrélation d'images numériques et modélisation. Autre. Université de Grenoble, 2011. Français. ⟨NNT : 2011GRENI091⟩. ⟨tel-00728174⟩

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