Percolation of gas and attenuation of shock waves through granular beds and perforated sheets

The use of improvised explosive devices (IEDs) has become more widespread in recent decades causing significant injury from blast effects, not only to military personnel, but also civilians, including formerzones of conflict. In order to study such injuries of high complexity, it is essential to isolate the different time-dependent effects such as initial accelerations by shock waves and impacts due to loading geometry. The shock tube is a versatile experimental platform that is capable of reproducing those elements of blast waves on a laboratory scale. In this paper, investigations of the percolation of gas and attenuation of shock waves through inserted granular beds and perforated steel sheets using an air-driven shock tube are presented. Experiments were performed with blast waves of Mach number 1.31 ± 0.01, 1.26 ± 0.01 and 1.20 ± 0.01 produced with 50µm Mylar , 23µm Mylar and 40µm aluminium diaphragms respectively. These output blasts were equivalent to those from denotations of 20 kg TNT at 9.0 m, 10.8 m, and 12.6 m away from the source [1]. The evolutions of shock pressures were measured before and after the insertions. Overall, results are highly reproducible and show that the peak shock pressure can be reduced linearly with the open area of the perforated sheet, the Carman-Kozeny permeability of the granular bed and the bed depth. After attenuating the shock front, the granular bed also caused a gradual rise in pressure due to the filtration of gas through the granular medium. The impulse of the positive shock region was calculated for both transmitted and reflected shock waves from the perforated sheet, showing linear relationships with the percentage of open area. About 5% of impulse was found to be absorbed by the perforated sheets.