Dynamic characteristics of granular beds subjected to projectile impact

Abstract

The study aims to investigate the internal dynamic characteristics of granular beds subjected to projectile impact. To facilitate this, a novel, cost-effective, and easily implementable experimental setup was designed for drop tests involving a spherical projectile impacting a 3D granular bed. This setup enables precise measurement of both translational and angular motion of the projectile within 3D granular systems. A 3D discrete element method (DEM) model was employed to simulate these impact events and validated through comparison with physical experiments. Key physical properties included the penetration depth, translational and angular velocities, translational and angular accelerations of the projectile, and the surface velocity field of the granular bed. The adopted DEM model demonstrated good agreement with the experimental observation. The validated DEM model was then used to further explore the internal dynamic characteristics of the granular bed during impact. The solid volume fraction of the granular bed is partially affected by the impact process, particularly in the region surrounding the projectile. However, the coordination number and mobilized friction are influenced throughout the entire granular bed. Vertical normal stresses dominate during impact, with contact forces displaying isotropic distribution in the horizontal plane, but anisotropic distribution in the vertical plane. The impact of the projectile significantly enhances the mobilization of the inter-bead and bead-wall friction. The probability distributions of inter-bead normal and tangential contact forces exhibit distinct patterns between the end of filling and the projectile impact: a linear curve and a decaying exponential curve on a semi-logarithmic plot, respectively. The velocity of the longitudinal wave is dependent on both bead size and granular porosity, whereas the shear wave velocity is only influenced by the bead size. Interestingly, the ratio of longitudinal wave velocity to shear wave velocity remains approximately constant during impact.