Explosive

An explosive material is a material that either is chemically or otherwise energetically unstable or produces a sudden expansion of the material usually accompanied by the production of heat and large changes in pressure (and typically also a flash and/or loud noise) upon initiation; this is called the explosion. An explosive charge is a measured quantity of explosive material.

Explosives are classified as low or high explosives according to their rates of decomposition: low explosives burn rapidly (or deflagrate), while high explosives detonate. While these definitions are distinct, the problem of precisely measuring rapid decomposition makes practical classification of explosives difficult. The chemical decomposition of an explosive may take years, days, hours, or a fraction of a second. The slower processes of decomposition take place in storage and are of interest only from a stability standpoint. Of more interest are the two rapid forms of decomposition, deflagration and detonation.

The latter term is used to describe an explosive phenomenon whereby the decomposition is propagated by the explosive shockwave traversing the explosive material. The shockwave front is capable of passing through the high explosive material at great speeds, typically thousands of meters per second. Explosive force is released in a direction perpendicular to the surface of the explosive. If the surface is cut or shaped, the explosive forces can be focused to produce a greater local effect; this is known as a shaped charge. In a low explosive (which deflagrates), the decomposition is propagated by a flame front which travels much more slowly through the explosive material.

A low explosive is usually a mixture of a combustible substance and an oxidant that decomposes rapidly (deflagration); unlike most high explosives, which are compounds. Under normal conditions, low explosives undergo deflagration at rates that vary from a few centimeters per second to approximately 400 metres per second. It is possible for them to deflagrate very quickly, producing an effect similar to a detonation. This usually occurs when ignited in a confined space. Low explosives are normally employed as propellants. Included in this group are gun powders, pyrotechnics such as flares and illumination devices.