Multiphysics Couplings and Stability in Geomechanics

Abstract:

Even under uniform loading conditions, a critical configuration of a deformed body can exist, where homogeneous deformation breaks down into deformation bands. Shear bands, dilation bands, compaction bands, as observed in nature and in laboratory experiments, are examples of such strain localization phenomena.

It is commonly observed that strain localization is favored by strain softening and most of the existing theoretical studies focus on pure mechanical reasons for deformation band formation (mechanical softening due to grain and matrix damage and/or pore collapse). The role of chemical reactions (e.g. dissolution, mineral dehydration) has been explored showing that such couplings may play an important role in shear band instabilities and strain localization. Coupling between progressive mechanical damage and dissolution rate of minerals has been considered for chemically induced compaction bands formation. In case of mineral dehydration, the chemical reaction rate depends upon the pore pressure and the temperature. Competing effects of fluid released by the chemical reaction and volume change by dehydration affect the pore pressure and can potentially lead to pore fluid run away.

Multiphysics couplings play a major role in earthquake nucleation and seismic slip. Earthquake nucleation is commonly modelled as a slip instability along a pre-existing fault or a plate interface. The effect of shear heating and thermal pressurization may be of prime importance in the control of the stability of shear deformation inside the slipping zone. Other mechanisms in relation with thermal decomposition of minerals affect the pore pressure and the temperature evolution of the system, and can play a significant role in the extreme strain localization observed in field. The actual width of the localized zone is a key parameter for understanding fault weakening mechanisms. Thickness and periodicity of localized structures is discussed considering the heterogeneous microstructure of the representative elementary volume and the wave length selection of the instability mode.