Thermal and chemical effects in shear and compaction bands

Abstract

to appearInternational audienceStrain localization zones in the form of shear bands or compaction bands in geomaterials are observed across scales from sub-millimetric (grain size) to kilometric scale (geological structures). Triggering and evolution of such narrow zones of localized deformation depends on many factors. The mechanical behavior of geomaterials is central for the formation of such zones. However, thermal, pore-pressure and chemical effects play a crucial role in shear and compaction banding. Temperature increase and activation of chemical reactions such as mineral dehydration, carbonate decomposition, as well as dissolution/precipitation control the triggering and the evolution of localized deformation zones. Moreover, the inherent heterogeneous microstructure of geomaterials plays a significant role during strain localization. The purpose of this paper is to provide a review of recent research regarding the effects of temperature, pore-pressure, chemical reactions and microstructure on strain localization in geomaterials. Examples have been taken in relation with seismic slip and with compaction banding. Strain localization is treated as an instability from a homogeneous deformation state. Different types of instabilities may (co-) exist depending on different multi-physical couplings and micro-mechanisms. Finally, a comparison of rate dependent Cauchy continuum and rate independent generalized continua (Cosserat continuum) is made, which leads to an analog expression for the critical perturbation wave length, which scales the thickness of the localized zone and defines the region that non-homogeneous deformations are possible

Localisation of Deformation for Shearing of a Fault Gouge with Cosserat Microstructure and Different Couplings

“…Both rate dependent effects and the size of the microstructure (e.g. grain size) determine the shear band thickness [7]. Here rate effects are taken into account indirectly through the THM couplings, i.e.…”

Vaporization of fault water during seismic slip

“…Recent modeling studies are now leading to an improved understanding of the coseismic frictional heating and fluid pressurization processes and of their influence on fault rupture propagation [e.g., Noda and Lapusta , ]. Other recent studies also show that the operation of a thermal or chemical (weakening) process plays an important role in the generation of localized shear bands [ Platt et al ., , ; Sulem and Stefanou , ], compared with distributed slip, which can lead to slip acceleration and in turn enhance frictional heating. It is important for these models to also include the shear‐induced vaporization process and the associated strain localization effect.…”

Cosserat Approach to Localization in Geomaterials

“…The chemical pressurization coefficient may be insignificant in most of the cases of dissolution/precipitation reactions in rocks [30]. However, this is not the case for dehydration or decarbonation reactions [31], [32].…”

“…Under these assumptions, the heat equation is written in indicial notation as follows (see Eq. 14 (30) where the infinite layer hypothesis was considered and consequently the derivatives in the 1 x and 3…”

“…Finally, in section 6, a simple example of the adiabatic shearing of a rock layer under constant shear stress is presented in order to highlight the similarities of a rate independent Cosserat and a rate dependent Cauchy continuum as far it concerns the conditions for which shear band localization takes place. It is worth mentioning that Cosserat continuum is a promising framework in fault mechanics as it enables to compute the thickness of the principal slip zone inside a fault core, its dependence upon the microstructure of the fault gouge, [8], [21]- [24] and its impact on the seismic energy budget.…”