Recent studies have shown the viability of using granular matter for impact mitigation due to energy dissipation benefits from the discrete and nonlinear contact behavior between granules. One such approach employs granules made from ductile metals which yield at low loads, as compared to an equivalent bulk, because of the presence of stress concentrations in the vicinity of the contact points, thus promoting plastic dissipation.The goal of this work is to use discrete element method simulations to compare elasto-plastic shock propagation with its elastic counterpart in frictionless jammed granular media, and to determine if plastic dissipation during impact is enhanced with jamming pressure. Jamming is achieved by confining the granules beyond a critical volume fraction such that there is a non-zero pressure in the granular ensemble and an underlying force-chain network sustains the externally applied stresses. Our findings show that elasto-plastic shock propagation is confined between two limits of linear sound propagation dependent on the intensity of the shock and the confining pressure. Weak shocks, which define the lower limit of linear sound propagation, are common to both elastic and elasto-plastic systems and result from the linearization of the granular network with confining pressure. Strong shocks can be observed when the impact velocity is high enough to produce shocks which propagate independent of the confining pressure. However, unlike for strong shocks in elastic systems where the shock front velocity increases nonlinearly with the pressure behind the shock, elasto-plastic systems exhibit an upper limit governed by the maximal intergranular contact stiffness. Visualizing the shock structure reveals that elasto-plastic systems show more spreading of the shock front than in elastic Hertzian systems. However, similar trends such as a more compact and steady shock structure are noticed for strong shocks in both systems.The distinction in the wave propagation characteristics between the weak and strong shock domains is reflected in the amount of plastic dissipation as well. For weak shocks, we observe that the jamming pressure increases dissipation, but strong shocks do not show any variation in the amount of dissipation with jamming pressure. Weak shocks do not show an increase in the formation of contacts and the underlying initial force chain network sustains the dynamic stresses. Contrarily, the larger granular motion behind strong shocks causes a distortion of the granular network and generates dynamic pressures which are significantly higher than the jamming pressure, thereby removing signatures of the initial jammed state and in turn dissipating more energy.&nbsp;

Elasto-Plastic Shockwave Propagation in Jammed Granular Media

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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