3.1 Description of Explosion Forces

An explosion is an extremely rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave consists of highly compressed air that wave-reflects off the ground surface to produce a hemispherical propagation of the wave that travels outward from the source at supersonic velocities (see Figure 2-1). As the shock wave expands, the incident or over-pressures decrease. When it encounters a surface that is in line-of-sight of the explosion, the wave is reflected, resulting in a tremendous amplification of pressure. Unlike acoustical waves, which reflect with an amplification factor of two, shock waves can reflect with an amplification factor of up to thirteen, due to the supersonic velocity of the shock wave at impact. The magnitude of the reflection factor is a function of the proximity of the explosion and the angle of incidence of the shock wave on the surface.

The peak pressure is a function of the weapon size or yield, and the cube of the distance (see Figure 3-2). For an explosive threat defined by its charge weight and standoff, the peak incident and reflected pressures of the shock wave and other useful parameters such as the incident and reflected impulse, shock velocity, and time of arrival are evaluated using charts available in military handbooks.