Evaluation of energy contributions using inter-particle forces in granular materials under impact loading

Abstract

A drop-tower based experimental setup was developed for the impact testing of 2D assembly of cylinders with impactor velocity of around 6 m/s. This drop tower setup was used to load 2D granular assemblies of polyurethane and polycarbonate cylinders of 1\(^{\prime \prime }\)–1.25\(^{\prime \prime }\) length with three different diameters of 1/4\(^{\prime \prime }\), 3/8\(^{\prime \prime }\) and 1/2\(^{\prime \prime }\). A high speed camera was used for recording the images at speeds between 10,000 and 55,000 fps to monitor the deformation of the cylinders. Kinematic and strain fields in individual grains during each experiment were measured using digital image correlation. These experimentally measured strain and kinematic fields were used as inputs for the granular element method (GEM) based force inference technique and the inter-particle forces in normal and tangential direction were determined at every contact in each experiment. The inter-particle forces at each contact can facilitate the calculation of frictional work done at each contact. The GEM based inter-particle forces for a simple 2 particle granular assembly were found to be in good agreement with predictions from ABAQUS explicit based FEM simulation. The influence of different model parameters was also characterized such as grain stiffness, frictional co-efficient was investigated qualitatively. The impact response of the various ordered granular assemblies was also investigated using the GEM approach and the effect of local defects such as voids or layering of granular materials on the wave propagation phenomena is also studied. The presence of the point or line defects have significant effects on the wave propagation in the granular assemblies due to wave scattering and attenuation.