Coupled grain-pore fabric evolutions in sheared granular materials: Anisotropy lagging and geometric emergence

Abstract

The morphological evolution of pore spaces is a critical yet poorly quantified microstructural determinant of the macroscopic mechanical and hydraulic behavior of granular materials. While the anisotropy of the grain contact network (Fc) is known to dictate material response, the concurrent evolution of pore space anisotropy (Fp) and its coupling with Fc remains inadequately understood. This study employs Minkowski moment tensor analysis within a Discrete Element Method (DEM) framework to bridge this gap. We systematically investigate dense and loose, monodisperse and polydisperse assemblies under cyclic triaxial loading to quantify the dynamic coupling between Fc and Fp. We demonstrate a moderate to strong correlation between Fc and Fp, with a systematic lag in the response of Fp attributed to hierarchical geometric emergence across scales. This lag is constrained by particle-scale free-volume reorganization and its kinematic compatibility with particle motion. Additionally, key pore-scale metrics, including inverse Voronoi cell fractions (ϕv−1), pore-scale porosity (ϕp), and pore shape anisotropy β^, are well described by gamma distributions across all packing densities and strain levels. Notably, the scaled ϕv−1 follows a k-gamma distribution, providing a statistically consistent descriptor for volume fluctuations. A strong correlation is also observed between the average pore shape factor (|β|avg) and global porosity, suggesting that |β|avg serves as a geometry-based descriptor linking collective pore deformation to packing density. These findings underscore the utility of the Minkowski tensor approach in capturing 3D fabric evolution and explicitly linking pore- and grain-scale interactions. The quantitative relationships and statistical descriptors presented here provide a new foundation for enhancing constitutive models in geotechnics and powder technology, offering insights relevant to future investigations into permeability evolution and shear band formation.