The Basics of Blast Mitigation

The primary cause of glazing damage is the shock wave that expands in all directions from the explosive charge. A secondary cause of damage is debris. The two main sources of debris are the intentional missiles in the composition of the weapon and fragments from the surrounding environment. To resist these damaging effects, the window system must be able to withstand two specific pressure wave assaults.

Peak pressure
An enormous amount of energy is released when a bomb is detonated. After the detonation occurs, a shock wave travels out from the point of detonation. This shock wave instantaneously increases the pressure as it radiates out from the bomb and promptly begins to decay. The initial impact of the shock against a static object creates a huge pressure against that static object known as Peak Pressure and this is measured in psi.

Impulse (positive duration phase)
The shock wave immediately starts to decay as soon as it hits. The rate and duration of decay however is very significant. A fast rate of decay is associated with commercial and military explosives. (TNT has a very fast rate of combustion). A slow rate of decay is associated more with a gas explosion. (Gas has a slower rate of combustion). This decay from peak pressure back down to ambient pressure is known as the Impulse and is measured in psi/msec.

It should also be noted that peak and impulse values are not only affected by the size and type of charge, but also the distance from the explosion.