Pattern transitions of localized deformation in high-porosity sandstones: Insights from multiscale analysis

Abstract

We employ a hierarchical multiscale modeling approach to investigate the transitions of localized deformation patterns in high-porosity sandstone subjected to sustained shear to understand their underlying physics. The multiscale approach is based on hierarchical coupling between finite element method (FEM) with discrete element method (DEM) to offer cross-scale predictions for granular rocks without assuming phenomenological constitutive relations. Our simulations show that when a high-porosity sandstone specimen is subjected to continuous deviatoric loading, compaction bands may occur and evolve, featuring a steady movement of the compaction front (i.e. the boundary between the compaction band and the rest uncompacted zone). The specimen reaches a homogeneous state of reduced porosity when the compaction fronts traverse the entire specimen. A re-hardening response is initiated in the specimen under further shear, which is followed by a shearing dominating stage with the emergence of shear bands. The material responses inside the ultimate shear bands approach a “steady state” of constant porosity and stress ratio. Cross-scale analyses reveal that debonding and pore collapse are dominant mechanisms for the compaction stage of the specimen, and debonding and particle rotation dictate the physics for the shear banding stage. The transitions from compaction to shear banding occurs due to the degradation of the cohesive contact network and significant reduction in porosity. There are limited number of interparticle bonds remaining at the “steady state” under sustained shear, with a preferential direction perpendicular to the loading direction, leading to a higher steady void ratio than the critical state void ratio of non-cohesive sand.