Studies of the TNT equivalence of silver azide charges

Schlieren photographs and shadowgrams have been evaluated to provide the radius-time histories of the shocks generated by explosions of silver azide charges with masses in the milligram range. Series of photographs of the flow field produced by a single explosion were obtained with an image converter camera, to yield a time history of the shock trajectory. These data were used to determine the shock Mach number and subsequently the peak hydrostatic overpressure as a function of radius. The pressure-radius profile was compared with that from a unit charge of TNT to determine the TNT equivalence of silver azide as a function of peak pressure and of distance from the centre of a 1 mg silver azide charge. The shock radius and time were also determined from single-shot visualizations of silver azide explosions. A new method of analysis was developed so that the charge mass and the TNT equivalence of an explosion could be derived from such a single radius-time measurement. The TNT equivalence factor for silver azide was shown to vary from 0.3 to 1.4, depending on the distance from the centre of the explosion. This agrees well with the generally reported equivalence factor of about 0.4 (Baker et al. 1983), which was probably based on the chemical composition of the material. Procedures are described by which the TNT equivalence factor can be calculated at any specified distance from a silver azide charge of known mass, or from a measurement of the shock radius and time, even when the charge mass is not known. This factor can then be used to determine the physical properties of the TNT blast wave which best describe those from the silver azide charge at the specified distance. A further result of these studies was that the scaling laws for blast waves, whose validity has been verified for charge masses ranging from a few grams to hundreds of tons, i.e., for charge masses varying over more than nine orders of magnitude, also apply for charges with masses of the order of one milligram.