Digital Volume Correlation Applied to X-ray Micro-Tomography Images in Uniaxial Creep Tests on Anisotropic Clayey Rock

Creep tests are commonly performed to characterize time-dependent deformation of geological materials. Classical measuring methods are not suitable for long term tests and not able to provide full three-dimensional strain fields. In this study, Digital Volume Correlation (DVC) is applied to X-ray micro-tomography (XRMT) images from creep tests on a hard clayey rock. In situ uniaxial compression creep tests are performed under different levels of stress and with different loading orientations with respect to the structural anisotropy of rock. Based on the XRMT images taken during the creep tests, DVC is applied to compute the full three dimensional strain fields and global averages strains of tested samples. The effects of bedding planes and hard inclusions on the non-uniform distribution of strains are analyzed.

A Coupled Elastoplastic Damage Model for Clayey Rock and Its Numerical Implementation and Validation

This paper presents a new constitutive model for describing the strain-hardening and strain-softening behaviors of clayey rock. As the conventional Mohr-Coulomb (CMC) criterion has its limitation in the tensile shear region, a modified Mohr-Coulomb (MMC) criterion is proposed for clayey rock by considering the maximal tensile stress criterion. Based on the results of triaxial tests, a coupled elastoplastic damage (EPD) model, in which the elastic and plastic damage laws are introduced to describe the nonlinear hardening and softening behaviors, respectively, is developed so as to fully describe the mechanical behavior of clayey rock. Starting from the implicit Euler integration algorithm, the stress-strain constitutive relationships and their numerical formulations are deduced for finite element implementation in the commercial package ABAQUS where a user-defined material subroutine (UMAT) is provided for clayey rock. Finally, the proposed model is used to simulate the triaxial tests and the results validate the proposed model and numerical implementation.