Shear-induced pressure anisotropy in granular materials of nonspherical particles

When a granular material composed of elongated grains is sheared in a split-bottom shear cell, a pressure difference develops within the material. This pressure difference depends on the interparticle friction ($μ$), which affects shear localization and particle alignment. For large $μ$, alignment is confined to a narrow shear band, leading to localized increases in packing density and pressure. For small $μ$, particles align over a wider region, leading to a nearly uniform packing density and pressure throughout the material. In contrast, spherical particles, regardless of $μ$, maintain a uniform packing density and pressure throughout the material. We observe a phenomenological similarity to the Weissenberg effect in non-Newtonian fluids, where normal stress differences induce radial pressure gradients, unlike the uniform pressure in Newtonian fluids.

💡 Research Summary

This paper presents a detailed numerical investigation into the emergence of shear-induced pressure anisotropy in granular materials composed of non-spherical particles. Using Discrete Element Method (DEM) simulations within a linear split-bottom shear cell (LSC), the study systematically explores how particle shape, characterized by aspect ratio (AR), and interparticle friction coefficient (μ) govern the spatial distribution of pressure under steady shear.

The core finding is that elongated particles (AR=5) develop a significant pressure difference between the center of the shear band and the surrounding bulk material, whereas spherical particles (AR=1) maintain a nearly uniform pressure distribution regardless of friction. For elongated particles, the key mechanism is shear-induced alignment along the flow direction. The degree of friction critically modulates this process: at high μ, alignment is confined to a narrow shear band, leading to a localized increase in packing density and, consequently, a pronounced pressure peak within the band. At low μ, particles align over a broader region, resulting in a more uniform packing density and pressure profile. Spherical particles, due to their symmetric shape, do not exhibit such directional alignment under shear, thus preserving an isotropic contact network and a homogeneous stress state.

The analysis employs coarse-graining techniques to derive macroscopic fields from particle-scale data. The pressure, defined as the mean normal stress, shows a consistent elevation inside the shear band for elongated particles. Examination of individual stress tensor components (σ_xx, σ_yy, σ_zz) reveals that the pressure increase is primarily driven by a rise in the lateral normal stress (σ_yy), attributed to enhanced confinement effects due to particle alignment. The time evolution of these fields confirms that the pressure anisotropy is a steady-state phenomenon developed under continuous shear.

The authors draw a compelling phenomenological analogy between this granular behavior and the Weissenberg effect in non-Newtonian fluids. In both cases, an inherent material anisotropy (particle orientation in granules, polymer chain orientation in fluids) gives rise to normal stress differences under shear, which in turn generate non-uniform pressure fields—unlike the hydrostatic pressure in Newtonian fluids or sheared spherical granules. This work underscores particle shape as a fundamental design parameter controlling the rheology and stress transmission in granular materials, with implications for industrial processes involving non-spherical grains.