Blast (explosives)

Blast is the process by which explosives, in a millisecond or less, are converted to hot explosive gases. Depending on the construction of the container holding the explosive, as much as half the force may go into disrupting it rather than producing effects; the strongest cases are military warheads and gravity bombs.

Blast effects are most complex in air, although they are not trivial in air or in solids. Indeed, there is a complex discipline of forming solid structures with explosive waves, perhaps most complex in the compression systems of nuclear weapons.

Air

See also: Mach effect

At the leading edge of the detonation is a sharply delineated reagion called the shock front, in which the pressure rises dramatically. Its initial velocity is much greater than sound but drops dramatically. The gases of detonation follow it.

Pressure vs. time in air explosion

The inertia of these gases are prevented, by their inertia, from slowing abruptly at the shock front, but rarefaction still take place, and a low-pressure region may follow behind the leading edge of the gases. Indeed, there may even be a lower-than-atmospheric area further behind, and a vacuum area produces. "When the pressure becomes less than atmospheric, the wind reverses in direction and blows backward toward the point of detonation. Any light object, such as a leaf, when struck by the shock front, is rapidly carried away from the point of detonation and then is blown back almost to the original position when the wind reverses." In especially hot explosions, the heating of the air and its consequence reduction and density becomes important, especially in nuclear explosions.

"portion of the blast wave is about one-fourth that of the portion at less than atmospheric pressure, the amplitude of the pressure at the shock front is much greater than the amplitude of the decrease in pressure below atmospheric. The duration of the positive phase varies with the explosive charge and distance from the point of origin. In one case, the duration of the positive phase is of the order of 0.006 second at a distance of 15.5 meters from the bomb. A wall struck by such a blast wave is first pushed forward by the short, sharp, hammer-like blow of the positive pressure phase and then pulled backward by the much longer negative phase. Whether the wall falls forward or backward depends upon circumstances.

Effects

Blast effect is measured by two criteria, peak pressure and impulse. Peak pressure is the pressure increase at the shock front or the highest pressure in the shock wave minus atmospheric pressure. Impulse is mathematically equal to the area under the time pressure curve for the duration of the positive phase (figure 4-9). This is approximately half the peak pressure multiplied by the duration of the positive phase. Peak pressure represents a measure of the maximum force exerted against a surface by a blast wave, since force is equal to the product of pressure and area. Impulse represents a measure of the force multiplied by the duration."