Impulse Mitigation of Structures Impacted by Granular Media

Efforts to design impulse resistant structures has led to interest in understanding the mechanisms by which momentum is transferred to a structure following the detonation of a buried explosive. Detonation of an explosive material transfers momentum to the surrounding granular media causing granular compaction across an outward propagating shock front. Upon reflection of the shock at the air/granular medium interface, granular particles are accelerated away from the soil surface and can impact a nearby structure, transferring all or a part of their momentum to the structure. This granular impact process results in the development of a pressure load distribution on the structure that can be sufficient to initiate elastic, and in some cases, high rate plastic deformation which is sometimes sufficient to initiate dynamic fracture.

A particle based method implemented in the IMPETUS Afea Solver code allows fully coupled calculations with discrete particles and finite element models in which particle/structure interactions can be directly analyzed. This dissertation has investigated the validity of this approach by performing a simulation of an experiment in which a well-defined explosive/granular particle test charge is used to load an edge clamped square, ductile 304 stainless steel plate. The experimental measurement for the time dependent motion of the surface of the granular medium measured using high-speed video imaging, the impulse measured using a Kolsky bar technique, and the final permanent displacement profile of the test plate were all well predicted by the simulation approach.

A series of five experimental tests were then performed with suspended water-saturated sand charges of systematically increased explosive and granular media mass (varying from ~25 to 150 kg) using the same target plate system. The expansion velocity of the granular media, tracked by high-speed video imaging, varied from ~500-1200 m/s and was dependent upon the ratio of the granular particle to high explosive mass. The impulsive loading response for each test charge was characterized by the Kolsky bar technique and four regions of loading by granular impacts were identified. These corresponded directly to impacts by various regions of the granular media. As the sand expanded and suffered momentum transfer with surrounding air particles, density and velocity gradients developed within the expanding volume. Examination of the simulations helped to resolve these regions of loading and corresponding impacts by the different particle types (air, sand, and high explosive particles).

The deflection of solid plates impacted by these charges revealed an increasing linear relationship with the applied impulse. Recent studies of the impulsive loading of edge clamped panels has shown that some sandwich panel constructions have a higher deflection resistance than solid equivalent mass per unit area plates. Since reduction of the dynamic deflection is an important goal of many blast resistant target designs, this potential mitigation strategy was investigated by the use of high strength, square honeycomb sandwich panels. Experiments confirmed that permanent plastic deflections could be reduced by the use of sandwich panel constructions, and simulations were used to investigate the phenomena governing this dynamic response.

Motivated by recent laboratory scale studies at the University of Cambridge, the effect of target inclination upon momentum transfer was investigated using a vertical impulse test rig and shallow buried planar explosive charges. These experimental studies and subsequent simulations revealed that substantial momentum transfer reductions could be achieved when the granular medium was not stagnated against a test structure. However, the laboratory study showed that more momentum was transferred to the test structure than predicted by a simple resolution of momentum onto the inclined surface. This resulted from frictional interactions of the sand particles with the target surface. A novel impulse reduction concept was then investigated both experimentally and with validated simulations. This concept used a lubricated (low dynamic friction) sliding plate attached to the inclined target surface and was found to offer significant reductions in impulse transfer beyond that of surface inclination. The simulation solver was then used to explore the efficiency of the sliding plates through a parametric study, examining impulse reductions achieved by changes to the sliding plate mass, the explosive charge mass, and the number of attached (stacked) sliding plates.