Mechanism of compressive stress formation during weak shock waves impact with granular materials

Certain aspects of wave propagation and the dynamic reaction of a granular material when subjected to a long-duration impulse load are studied. In the majority of studies published on this subject the unsteady pressure behavior at the end-wall covered by a layer of granular material was observed and documented. However, up to now little attention was given to explaining the physical mechanism of this process. Experimental results, obtained in the course of this study, regarding the pressure fields inside granular layers of different materials, clearly show that the compaction effect strongly depends on the characteristics of the medium. This phenomenon manifests itself by changing the gas-particle interaction in the course of the gas filtration, and by variation in the contribution of the different forces and effective stress, a, to the energy exchange between the gas, the particles and the shock-tube wall. The material permeability, f, the relative density, v, and the particle response time, tau(p), are the most important parameters affecting the stress formation at the end-wall covered by the granular layer. In addition to the effect of the material parameters, the effective stress, sigma, was found to strongly depend on the granular layer height, h. Based on detailed pressure measurements a qualitative analysis regarding the role of the particle rearrangement in the formation of the unsteady peak at the end-wall was performed. The phenomenology of the particle-particle interaction includes rotation and consolidation of the granules and movement or sliding of particle planes within the layer over each other. Most of these processes are frictional in their nature. They are related to the energy losses and affect the profile and magnitude of the compressive stress as measured at the shock-tube end-wall covered by the granular layer.