Visualizing kinematics of dynamic penetration in granular media using transparent soils

This paper describes a new method for quantitative analysis of dynamic penetration into granular media. The method is based on refractive index matching used to produce transparent synthetic soils. First, a procedure referred to as the embedded plane technique is introduced, to overcome limitations with laser illumination in high-speed photography. The technique consists of seeding an embedded plane with opaque particles sandwiched within the transparent soil to visualize in-plane granular kinematics. Details of sample preparation are presented, and other issues related to rapid penetration into index-matched media are discussed. High-speed image acquisition is introduced for transparent soils, and relevant target illumination techniques are described. Finally, improved digital image correlation methods are introduced and used to derive displacement fields. Shear and volumetric strains are then calculated. Analyses performed on acquired images are used to illustrate the applicability of these novel index-matching methods to the study of low-velocity penetration into granular media. The data illustrates that, for impact velocities considered in this study, the majority of lateral displacements during penetration are contained to approximately four projectile diameters from the penetration axis. Vertical displacements extend several penetrator diameters ahead of the penetrator. Moreover, penetration is accompanied by significant vertical afterflow. Finally, a region of dense sand forms ahead of the penetrator with a conical shape, which travels with the penetrator during penetration.

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