In the past couple of decades, problems concerning the impact of aggregated particles, that is, a collection of many solid particles such as granular matter and powder, have become increasingly important in mechanical engineering, materials engineering and physics. The collision between two particles is now fairly well studied, but the collision mechanics of aggregated particles involving more than two particles has not yet been fully elucidated, because the fluidity and compressibility of aggregated particles are quite different from those of liquid and chunks of solid.

In journals dealing with granular matter and powder technology, many papers about linear or nonlinear pressure waves and shock waves through aggregated particles have been published since the 1980s. In some papers, it was assumed that aggregated particles are not discrete media, but a continuous body. On the other hand, some authors have treated aggregated particles as a collection of many discrete particles and the interaction between contacting particles is incorporated. In several reports, the large plastic deformation, temperature change and the transformation of material during the propagation of strong shock waves were considered.

At low impact velocities, the plastic deformation of particles and the propagation of a wave have strong influences on the dynamic response of aggregated particles. Thornton and Randall carried out discrete element method (DEM) simulations of a two-dimensional solid particle system subjected to projectile impact at low velocities. The present authors have investigated the dynamic behavior of two- and three-dimensional aggregated particles subjected to projectile impact through experiments and DEM simulations at velocities less than 20 m/s, in order to examine the relationship between the motion of projectiles and aggregated particles after impact and the propagation properties of contact forces in it. Rioual et al. have also carried out some experimental studies on the collision process of a sphere on a two-dimensional granular bed at velocities from 6 m/s to 22 m/s, in the context of aeolian sand transport.

In the present study, the dynamic phenomena of aggregated particles subjected to projectile impact were simultaneously recorded using two high-speed video cameras and investigated numerically using DEM. The aggregated particles consist of nylon-66 spheres arranged randomly in a rectangular parallelepiped container. Two types of aggregated particles with approximately 43% porosity, were used: 8000 spheres with a diameter of 6.35 mm (1/4 inch); and 5000 spheres each with diameters of 6.35 mm and 4.76 mm (3/16 inch), respectively. A steel sphere obliquely strikes a nylon sphere or several nylon spheres near the center in the top layer of aggregated particles.

The effects of the impact velocity (1-25 m/s), impact angle (0-65°), diameter of the steel projectile (6.35 mm, 9.52 mm, 12.0 mm), and diameter of aggregated particles (4.76 mm, 6.35 mm) on the movement of aggregated particles and steel projectile were examined along with the propagation properties of contact force in aggregated particles. Images from high-speed video cameras revealed that the movement of a steel projectile just after impact can be classified into four types: stopping at the surface of aggregate particles; penetration into aggregated particles; horizontal movement along the aggregated particle surface; and rebound from aggregated particles. These four types of movements depend on the impact velocity and impact angle. The boundaries demarcating the four types of movements became clearer as the size of the projectile increased. When the aggregated particles include nylon spheres with a diameter of 4.76 mm, the steel projectile easily penetrates the aggregated particles even at low velocities, and the nylon spheres' upward displacement intensifies.

These impact phenomena of steel projectile and aggregated particles in the experiment could be well simulated by DEM analysis using Thornton's simple theory for collinear collisions of elastic-perfectly plastic spheres. The movement of steel projectile and aggregated particles just after impact could be explained by the distribution and the propagation of the contact forces acting between nylon spheres and the velocity vector of each nylon sphere obtained by the DEM analysis.