Anthony D. Severson, CPT, USA

Shock propagation in particulate materials is governed by the nonlinear interactions between particles. Spherical elastic particles have Hertzian contacts, meaning that they effectively interact via nonlinear springs. Hertzian interactions give rise to nonlinear wave behavior, which has been studied extensively for 1D chains of spheres (e.g., Newtonā€™s cradle), where energy is transmitted perfectly from one particle to the next and the internal sound speed of a particle is assumed to be quasi-infinite. In more realistic situations, forces do not propagate along a single network, but along force chains, which are spatially anistotropic structures that transmit stress. Additionally, shocks can propagate at speeds similar to the internal sound speed. Through shock experiments on 2D assemblies of frictional, photoelastic disks, we show three key results: (1) Two distinct wave speeds emerge after impact, one of which represents the primary force propagation wave; (2) This force propagation speed scales (as a power law) with both the confining pressure and intruder velocity; (3) These propagation speeds can indeed approach the characteristic sound speed inside a grain, and as they do, the power law scaling behavior changes.

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Jun 22, 2019

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