Sand Penetration: A Near Nose Investigation of a Sand Penetration Event

Abstract

This paper presents experimental and computational results of a long-rod penetrating dry granular sand at velocities near 100 m/s. The objective of this work is to develop a fundamental understanding of the formation and transmission of dynamic force chains, and the motion and fracture of the individual sand grains as the projectile passes. This is accomplished by launching a projectile along a view window, backed by sand, in order to directly view and photograph the projectile/sand interactions. Within the sand system, a two-wave structure was observed, composed of a compaction wave (bow shock) that detaches from the dart and moves through the sand at a wave speed near 100 m/s and a damage wave, which remains near the leading edge of the dart. The compaction wave removes porosity and the damage wave fractures grains in the region near the projectile nose. Grain fracture is not observed at dart speeds below 35 m/s. In addition the axial strain to failure of individual sand grains was measured in a quasi-static configuration. These results were used in conjunction with a simple analytic force balance model to predict the depth of penetration. The analytic results compare favourably with experiments until the dart slows below 35 m/s.