Motion of a medium caused by an explosion. Under the influence of the high pressure of the gases formed during the explosion, the initiallyunperturbed medium undergoes abrupt compression and takes on high velocity. The state of motion is transmitted from one layer of themedium to another, so that the region enveloped by the blast wave expands. On the front of the expanding region, the medium passesabruptly from the initial, unperturbed state to a state of motion, with higher pressure, density, and temperature. The abrupt change in the stateof the medium (the shock wave) propagates with supersonic speed.
A blast wave is characterized by a change with time in the pressure, density, and velocity of the medium at various points in space or by thedistribution of these quantities in space at fixed moments of time.
One of the important parameters that determines the mechanical action of a blast wave is the maximum pressure created by the wave.During explosions in gaseous and liquid mediums, the maximum pressure is attained at the moment of compression of the medium in theshock wave. Another important parameter is the time interval of the action of the blast wave. The maximum pressure decreases and the timeof action increases with increasing distance from the place of the explosion. (See Figure 1.)
During the propagation of a blast wave in solid mediums, the shock front dissipates comparatively quickly, and the blast wave is convertedinto a number of consecutive, rapidly decaying oscillations, which are propagated with the velocity of elastic waves.
Blast waves have the property of similarity. In accordance with this property, the distances at which the blast waves for explosions ofchemical charges of identical shape but different masses have the same maximum pressure value are related as the cube root of the chargemasses. The time interval of the action of the blast wave varies in the same way. For example, if the distances and time interval given inFigure 1 increased by a factor of 10, then such a blast wave will correspond to the wave produced by the explosion of 1 ton rather than 1 kgof trinitrotoluene (TNT).
A blast wave tends toward the rapid loss of the characteristics determined by the nature of the explosion; therefore its subsequent motion ismainly determined only by the amount of energy transmitted to the surrounding medium. Because of this circumstance, blast wavesproduced in the same medium by explosions of different types prove to be basically similar; this makes possible the introduction of the so-called TNT equivalent for explosion characteristics.
A propagating blast wave expends a considerable part of its mechanical energy in heating the medium near the center of the explosion. Forexample, at a distance of 10 km, a blast wave in air caused by a 1,000-ton chemical explosive charge contains about 10 percent of itsoriginal explosive energy; in a nuclear explosion of the same energy, the wave has half as much energy (5 percent) owing to greater losses inheating the air. The maximum pressure increase in the wave for the abovementioned values for distance and explosion energy is