Thermo-hydro-mechanical couplings and strain localization in Cosserat continua : application to stability analysis of rapid shear in faults

Abstract : When materials are subjected to large deformations, most of them experience inelastic deformations. It is often accompanied by a localization of these deformations into a narrow zone leading to failure. One particular case of strain localization is the formation of shear bands which are the most common patterns observed in geomaterials. In geological structures, they appear at very different scales, from kilometer scale for subduction zones, to micrometric scale inside fault cores. Studying their occurrence and evolution is of key importance to describe the failure of geomaterials and model seismic slip for mature crustal faults. The pressure and temperature conditions in these faults and the interaction with the pore water inside a highly fractured materials highlight the importance of different physical processes involved in the nucleation of earthquakes. In this thesis, we study the occurrence and evolution of shear bands inside fault gouges taking into account the material microstructure by resorting to elastoplastic Cosserat continua and also the effect of thermo-hydro mechanical couplings. The use of Cosserat theory introduces information about the gouge microstructure, namely the grain size, and permits to regularize the mathematical problem of in the post-localization regime by introducing an internal length into the constitutive equations. Two approaches are used to study the coupled non-linear partial differential set of equations: linear stability analysis and finite element simulations. Linear stability analysis allows to study the occurrence of localized deformation in a mechanical system with multi-physical couplings. Considerations on the dominant wave length of the perturbations permit also to determine the width of the localized zone. This shear band thickness is confirmed by numerical integration in the post-localization regime for a certain range of deformation. The obtained widths of the localized zone are key parameters for understanding fault behavior, are in agreement with experimental and field observations. Moreover, numerical finite element computations enable to model the mechanical response of a fault gouge during seismic slip and give insights into the influence of various physical couplings on the energy budget
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Hadrien Rattez. Thermo-hydro-mechanical couplings and strain localization in Cosserat continua : application to stability analysis of rapid shear in faults. Géotechnique. Université Paris-Est, 2017. English. ⟨NNT : 2017PESC1181⟩. ⟨tel-01916108⟩

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