Contact network evolution and strength in DEM-based UCS simulations: Effect of aspect ratio and strain rate

Unconfined compressive strength (UCS) of a rock is widely used to characterize its strength, deformation, and failure mechanisms; however, the experiment used to obtain this property provides limited access to particle-scale processes. In this study, Discrete Element Method (DEM) simulations of bonded granular assemblies were employed to investigate the coupled effects of specimen aspect ratio ( l/d = 1–4) and strain rate (0.1–5 mm/s) on strength, damage evolution, and micromechanical behavior during unconfined compression testing. The analyses systematically examined the stress–strain response, crack development, porosity evolution, coordination number, ball displacement magnitude, and contact force chain characteristics. The outcomes of the study clearly show that UCS is primarily governed by specimen geometry, decreasing by approximately 14 MPa as l/d increases from 1 to 3, while strain rate has a secondary effect. The Crack number, displacement localization, porosity dilation, coordination number reduction, and force-chain disruption are strongly interrelated and control failure localization. Higher strain rates preserve contact networks and promote abrupt shear localization, whereas higher l/d ratios enhance distributed damage and reduce strength. The study provides a unified micromechanical framework linking contact network evolution to macroscopic UCS behavior.