Identifying and modelling the formation of compaction bands in high-porosity carbonate rocks

Abstract:

The formation and propagation of compaction bands in high-porosity carbonate rocks are investigated. Axisymmetric triaxial tests at different levels of confining pressure are performed on cylindrical samples of Saint-Maximin limestone, where deviatoric loading is applied in several stages. X-Ray computed tomography images are recorded before and after each loading stage and the digital volume correlation technique is applied to compute local strain field maps. Strain localization bands are observed just after the plasticity onset. A new post-processing method based on the analysis of the kinematics throughout the observed localization bands is proposed to identify their type. Compaction bands are identified at relatively high-confining pressures, while shear bands are observed at lower confinements. Compared to computed porosity maps, the deformation maps reveal the formation and propagation of compaction bands in the more porous zones, whereas shear bands can cross both high- and low-porosity zones.

The non-local aspect of the deformation, as identified in the local strain field maps, has led to develop a constitutive model within the frame of gradient-dependent plasticity. Higher order continuum models permit to predict deformation bands with finite thickness. However, since additional parameters are introduced, a standard calibration of the constitutive law using the macroscopic mechanical data measured at the level of the sample is not sufficient. We thus develop a full calibration procedure of the model parameters based on data at the microscopic scale provided by imaging techniques. Finally, the constitutive law is implemented in a finite element code and the numerical results are in good agreement with the experimental observations in terms of compaction bands thickness and strain amount inside the deformation bands.