Effect of Confinement on the Impact Response of a Granular Array

Background: Plastic dissipation at inter-granular contacts during elasto-plastic wave propagation plays a significant role in wave attenuation. However, it is unknown if plastic dissipation during impact is enhanced if the granular medium is initially in an unconfined ’fluid-like’ state or that of a more rigid ‘solid-like’ state caused by applying a confining pressure. Objective: The goal of this work is to investigate both experimentally and numerically the impact response of a two-dimensional hexagonal granular array consisting of metallic spheres enclosed in a polymeric membrane subjected to different levels of confining pressure. We seek to quantify the granular trajectories, the effect of the membrane, and the ratio of the dissipated plastic energy to the net input energy between the unconfined and confined states. Methods: We perform experiments using a modified split Hopkinson pressure bar on a specimen of monodisperse brass spheres confined by a polymeric membrane and record the impact event using high-speed photography so that particle tracking can be used to track granular motion. After impact, the sphere surfaces are examined to measure plastic contact areas, allowing the dissipated plastic energy to be estimated. To support the experiments, capture the lateral confining effect of the membrane and applied pressure, and investigate larger arrays, discrete element simulations are conducted. Results: When the granular array is confined, we observed shorter and consistent granular trajectories between trials, a greater dissipated energy to net input energy ratio, and a stiffer membrane response. Conclusion: Experimental and numerical results indicate that the external confining pressure increases plastic dissipation.