Experimental investigation of the interaction between weak shock waves and granular layers

The paper describes new experimental results regarding the pressure fields in front of and inside granular layers of different materials during their collision with weak shock waves. A variety of waves result from the shock wave-granular layer interaction. The pressure behind the reflected wave from the material interface approaches the equilibrium value, P-5, which would have been reached had the shock wave reflected from a solid end-wall. The wave succession inside the layer depends solely on two processes: the complex interaction of the compaction wave with the granular material and the gas filtration, which affects the particles by the drag forces between the two phases. Inside a material with a permeability coefficient f> 0.001 mm(2) the transmitted wave moves with a constant velocity which is largely governed by the gas filtration. For low permeability materials (f< 0.0003 mm(2)) the transmitted wave trajectory strongly depends on the compaction wave propagation. In such cases the compaction wave was found to be unsteady and its acceleration was higher in material having low material densities. The maximum compressive stress values, P-5, reached at the shock tube end-wall, covered by the materials under investigation, manifested as an unsteady pressure peak twice as large as the gas pressure P-5, measured ahead of the layer. Comparing the present data with those available in the literature showed that the amplitude of the unsteady pressure peak was higher in materials having low effective densities, gamma, and small permeability coefficients f. Contrary to flexible foams where the available experimental data indicated that the compressive stress in the post peak period converges to P-5=P-g, the results obtained in the present study indicated that during the test time the compressive stress, P-5, was well preserved in the material and for most of the sample length its value was within the range P-s>P-5>P-g.