Dynamic behaviour of granular materials at impact

Abstract
We report on a series of experiments and Discrete Element Method computations on penetration into granular media at quasi-static (mm per min) and dynamic (200 m/s) rates. A significant dependence of the dynamics of the granular particles on the penetrator's velocity is observed in the experiments. Experimentally the material response was followed using digital speckle radiography, allowing the displacement fields within an internal plane of a deforming material to be determined. At dynamic rates long rod penetrators are seen to set a body of sand moving, leading to a rapid initial decrease in velocity during the early stages of impact. Subsequently the motion seems to enter a steady-state phase where the velocity varies little for a period of time. The response of the material at quasi-static rates is significantly different; little deformation is observed occurring in front of the projectile and large sections are seen to be moving towards the face at which penetration is occurring. Computationally, significant differences in the dynamics of the granular particles are observed between penetration of close packed and random packed systems. For mono-size systems the contact forces are transmitted upon impact via arching and single branching. The velocity profile for the mono-dispersed system has a similar structure to that of the force-chains and strongest motion is noted along the “shear band” directions. In contrast, the mechanism of transmitting forces in a random-packed system (mono or poly-dispersed) is via a tree root-like network of force chains. Interestingly in this case, at impact velocities of ~100 m/s, the particle velocities do not follow the structure of the normal-force chains. This indicates that significant reorganization, sliding, and void-filling is taking place in the system.