Particle fragmentation and shear localization are vital to understanding the dynamics of catastrophic mass flows such as rock avalanches and landslides. This study systematically investigates the dynamic responses of fragmentable quartz sand, fragile halite, and unfragmentable glass beads under varying normal stresses (50–400 kPa) and displacements (0.1–100 m). Utilizing high-speed ring shear tests and X-ray computed tomography (X-CT), we analyze particle comminution and morphological evolution across multiple scales.
Results show that quartz sand and halite exhibit rapid grain fragmentation and pronounced frictional weakening, whereas glass beads maintain stable rolling friction. For fragmentable materials, higher normal stress accelerates particle fragmentation, driving an earlier peak in shear resistance. This frictional weakening is primarily caused by fine-particle (<0.075 mm) enrichment within the shear band, which reduces particle interlocking and enhances lubrication. 3D X-CT reconstructions reveal highly localized fragmentation within a distinct shear zone. Based on solid fraction (η) fluctuations, we propose a novel method to quantify the shear zone thickness, which progressively narrows with displacement to form a coarse grain-fine matrix dual-skeleton structure. Within the shear zone, the relative fragmentation index (B_r) grows logarithmically, slowing after 10 m. Microstructural analysis indicates that the generated fines possess higher sphericity (mean sphericity increasing from 0.73 to 0.84), further reducing sliding resistance.
Synthesizing macro-shear behavior and micro-morphology, we propose a synergistic mechanism of particle crushing and rounding. Granular friction evolves through three distinct stages: an initial stage governed by particle interlocking and force chain strengthening; a crushing-driven transition stage; and a final weakening stage controlled by fine-particle rolling and lubrication. This dynamic balance dictates the overall mobility of geomaterials, providing a physical framework for modeling large-strain, dense granular flows in natural hazards.
 

Granular materials;,Particle fragmentation,Shear localization,Frictional weakening,Visual analysis