An explosive material is a substance that contains a great amount of stored energy that can produce a sudden expansion of the material after initiation, usually accompanied by the production of light, heat, and pressure. This event is called the explosion. An explosive charge is a measured quantity of explosive material.
The energy stored in an explosive material may be
- chemical energy, such as nitroglycerine or grain dust
- pressurized compressed gas, such as a gas cylinder or aerosol can
- nuclear, such as fissile isotopes of uranium-235 and plutonium-239
Classification
Chemical Explosives
Explosives are classified as low or high explosives according to their rates of burn: 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.
Explosives usually have less potential energy than petroleum fuels, but their high rate of energy release produces a great blast pressure. TNT has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the detonation of a pentane-air mixture, and the 0.34-m/s stoichiometric flame speed of gasoline combustion in air.
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.
The properties of the explosive indicate the class into which it falls. In some cases explosives can be made to fall into either class by the conditions under which they are initiated. In sufficiently large quantities, almost all low explosives can undergo a Deflagration to Detonation Transition (DDT). For convenience, low and high explosives may be differentiated by the shipping and storage classes.
Exotic Explosives
In addition to chemical explosives, there exist varieties of more exotic explosive material, and theoretical methods of causing explosions. Examples include nuclear explosives, antimatter, and abruptly heating a substance with a high-intensity laser or electric arc.
Low explosives
A low explosive is usually a mixture of a combustible substance and an oxidant that decomposes rapidly (deflagration), however they burn slower than a high explosive which has an extremly fast burn rate, as opposed to 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 .
High explosives
High explosives normally are employed in mining, demolition, and military warheads. High explosive compounds detonate at rates ranging from 3,000 to 9,000 meters per second, and are, conventionally, subdivided into two explosives classes, differentiated by sensitivity:
Primary Explosives
- Primary explosives are extremely sensitive to mechanical shock, friction, and heat, to which they will respond by burning rapidly or detonating. Examples include mercury fulminate, lead styphnate and lead azide.
Secondary Explosives
- Secondary explosives , also called base explosives , are relatively insensitive to shock, friction, and heat. They may burn when exposed to heat or flame in small, unconfined quantities, but detonation can occur. These are sometimes added in small amounts to blasting caps to boost their power. Dynamite, TNT, RDX, PETN, HMX, and others are secondary explosives. PETN is the benchmark compound; compounds more sensitive than PETN are classed as primary explosives.
Tertiary Explosives
Some definitions add a third category:
- Tertiary explosives or blasting agents , are insensitive to shock. They cannot be reliably detonated with practical quantities of primary explosive, and, instead, require an intermediate explosive booster, of secondary explosive, e.g. ammonium nitrate/fuel oil mixture (ANFO) and slurry (wet bag) explosives that are primarily used in large-scale mining and construction.
Note that many, if not most, explosive chemical compounds may usefully deflagrate and detonate, and are used in high- and low-explosive compounds. Thus, under the correct conditions, a propellant (for example nitrocellulose) might deflagrate if ignited, or may detonate if initiated with a detonator.
Explosive compatibility groupings
Shipping labels and tags will include UN and national, e.g. USDOT, hazardous material Class with Compatibility Letter, as follows:
- 1.1 Mass Explosion Hazard
- 1.2 Non-mass explosion, fragment-producing
- 1.3 Mass fire, minor blast or fragment hazard
- 1.4 Moderate fire, no blast or fragment: a consumer firework is 1.4G or 1.4S
- 1.5 Explosive substance, very insensitive (with a mass explosion hazard)
- 1.6 Explosive article, extremely insensitive
A Primary explosive substance (1.1A)
B An article containing a primary explosive substance and not containing two or more effective protective features. Some articles, such as detonator assemblies for blasting and primers, cap-type, are included. (1.1B, 1.2B, 1.4B)
C Propellant explosive substance or other deflagrating explosive substance or article containing such explosive substance (1.1C, 1.2C, 1.3C, 1.4C)
D Secondary detonating explosive substance or black powder or article containing a secondary detonating explosive substance, in each case without means of initiation and without a propelling charge, or article containing a primary explosive substance and containing two or more effective protective features. (1.1D, 1.2D, 1.4D, 1.5D)
E Article containing a secondary detonating explosive substance without means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) (1.1E, 1.2E, 1.4E)
F containing a secondary detonating explosive substance with its means of initiation, with a propelling charge (other than one containing flammable liquid, gel or hypergolic liquid) or without a propelling charge (1.1F, 1.2F, 1.3F, 1.4F)
G Pyrotechnic substance or article containing a pyrotechnic substance, or article containing both an explosive substance and an illuminating, incendiary, tear-producing or smoke-producing substance (other than a water-activated article or one containing white phosphorus, phosphide or flammable liquid or gel or hypergolic liquid) (1.1G, 1.2G, 1.3G, 1.4G)
H Article containing both an explosive substance and white phosphorus (1.2H, 1.3H)
J Article containing both an explosive substance and flammable liquid or gel (1.1J, 1.2J, 1.3J)
K Article containing both an explosive substance and a toxic chemical agent (1.2K, 1.3K)
L Explosive substance or article containing an explosive substance and presenting a special risk (e.g., due to water-activation or presence of hypergolic liquids, phosphides or pyrophoric substances) needing isolation of each type (1.1L, 1.2L, 1.3L)
N Articles containing only extremely insensitive detonating substances (1.6N)
S Substance or article so packed or designed that any hazardous effects arising from accidental functioning are limited to the extent that they do not significantly hinder or prohibit fire fighting or other emergency response efforts in the immediate vicinity of the package (1.4S)
Explosive train
The explosive train , also called an initiation sequence or firing train , is the sequence of charges that progresses from relatively low levels
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