USING IMAGE ANALYSIS TO QUANTIFY PROPAGATION SPEEDS AND STRESS ANISOTROPY IN GRANULAR ACOUSTIC EXPERIMENTS

Understanding how forces propagate through granular media is broadly relevant in acoustic and non-acoustic warfare applications. For example, sediment type can impact acoustic propagation during bottom bounce events that undersea assets use for navigation. There is not yet a complete theory of force propagation in granular media due to several nonlinearities, like friction and Hertzian contact interactions. Of particular interest is the crossover region between linear and nonlinear regimes, where the propagating stresses are comparable to the initial confining stress in the system, as well as the shock regime, where stresses greatly exceed confining prestress. Aiming to develop more complete theories of force propagation in these regimes, we perform experimental impact tests and analyze how forces propagate in two-dimensional, frictional granular media. Assemblies of photo elastic disks are subjected to a vertical confining stress and then impacted from the side by a pendulum. Both the confining stress and the impact speed can be varied, and high-speed video captures the forces propagating through the material. We use image processing to quantify the speed of the moving wave and the force network’s structure. In particular, we focus on the phenomenology of the traveling shock and the stress anisotropy of the force networks, which has never before been experimentally measured.http://archive.org/details/usingimageanalys1094564110Ensign, United States NavyApproved for public release; distribution is unlimited