Before the effects and hazards of an explosion can be described the reader must understand the difference between deflagration and detonation.
Deflagration (Rapid Combustion)
Deflagration is the rapid combustion or rapid burning of an explosive. The speed of the burning action actually constitutes the difference between combustion, deflagration, and detonation.
Detonation (Instantaneous Combustion)
Detonation can be defined as most rapid form of combustion and is often times referred to as "instantaneous combustion." However, detonation is not exactly instantaneous there is a short time interval (microseconds) for the combustion action to transfer from one particle of the explosive compound to the next. The velocity of this "instantaneous combustion" has been measured for most explosives and is referred to as the detonation velocity of the explosive. Detonation velocities of high explosives range from approximately 9,000 feet per second to more than 27,000 feet per second. A high-order detonation is a complete detonation of the explosive at its highest possible velocity. A low-order detonation is either incomplete detonation or complete detonation at lower than maximum velocity. Low-order detonations may be caused by any one or a combination of the following factors: (1) initiator (blasting cap) of inadequate power, (2) deterioration of the explosive, (3) poor contact between the initiator and the explosive, and (4) lack of continuity in the explosive (cracks or air space).Effects and Hazards of an Explosion
When an explosive is detonated, the explosive material is instantaneously converted from a solid into a rapidly expanding mass of gases. The detonation of the explosive will produce three primary effects, and several associated secondary effects that create great damage in the area surrounding the explosion. The three primary effects produced are blast pressure, fragmentation, and incendiary or thermal effects.
Blast Pressure Effect Hazard
When an explosive charge is detonated, very hot, expanding gases are formed in a period of microseconds. These gases exert pressures of about 700 tons per square inch on the atmosphere surrounding the point of detonation and rush away from the point of detonation at velocities of up to 7,000 miles per hour, thus compressing the surrounding air. This mass of expanding gas rolls outward in a circular pattern from the point of detonation like a giant wave, weighing tons, smashing and shattering any object in its path. The further the pressure wave travels from the point of detonation, the less power it possesses until, at a great distance from its creation, it dwindles to nothing. The exact distance depends upon the amount and type of explosives involved and the surrounding environment. The wave of pressure is commonly referred to as the blast pressure wave. The blast pressure wave has two distinct phases, which will exert two different types of pressures on any object in its path. These phases are the positive pressure phase and the negative or suction phase. The negative phase is less powerful, but lasts about three times longer than the positive phase. The entire blast pressure wave, because of its two distinct phases, actually delivers a one-two punch to any object in its path. The blast pressure effect is the most powerful and destructive of the explosive effects produced by the detonation of high explosives.The surrounding environment has a dramatic effect on the blast pressure wave. A blast pressure from a detonation in the open environment will dissipate at a significantly lesser distance that a blast pressure wave that is reflected and focused off of the surroundings like building or walls.
When an explosive charge is buried in the earth or placed underwater and detonated, the same violent expansion of gases, heat, and shock results. Since earth is more difficult to compress than air, and water is uncompressible, the detonation will seem less violent from the surface. Nevertheless, the same energy released as compared to the same detonation in the open air. The effect however, is manifested in a different manner. The blast wave is transmitted through the earth or water in the form of a shock wave, is comparable to a short, powerful earthquake. This shock wave will pass through earth and water just as it does through air, and when it strikes an object such as a foundation, the shock wave will, impart it's energy into the structure which may cause damage much like an earthquake would. An explosive charge detonated underwater will produce damage at greater distances because water is uncompressible and cannot absorb energy. As a result, the blast transmits the shock wave much faster and farther, and consequently produces greater damage within a larger area.
Fragmentation Effect Hazard
When an encased explosive such as a bomb detonates, the rapidly expanding gases produced by the explosion cause the casing to enlarge to about one and one-half times its original diameter before it ruptures and breaks into fragments that are propelled away from the center of detonation at velocities of approximately 2,500 feet per second. About half the total energy released by the explosion is expended in rupturing the case and propelling the broken pieces of the casing outward in the form of fragments. These fragments usually travel in a straight line of flight until they either lose velocity and fall to earth or strike an object and ricochet or become imbedded. Certain hand grenades have perforated bodies that break apart easier thus allowing the saved energy to be imparted into the fragments. Other ordnance designs have precut or preformed objects such as ball bearings placed inside. These objects are referred to as shrapnel. Shrapnel serves the same purpose and has the same effect on personnel, material, and structures as fragmentation. The advantage of using shrapnel is that part of the energy released by the explosion, which would normally have been expended in fracturing the ordnance casing into fragments, is instead used in propelling the preformed, separate pieces of shrapnel.
Fragments resulting from the detonation of high-explosive fillers have a stretched, torn, and thinned configuration due to the tremendous heat and pressure produced by the explosion. Fragments can be used by EOD technicians and UXO specialists for identification purposes. The size, shape, color, thickness and amount of fragmentation in an area can be used to identify the types of ordnance that may have been used on a particular range.
Incendiary Thermal Effect Hazard
The incendiary thermal effect produced by the detonation of high explosives varies greatly from one explosive to another. In general, a high explosive will produce a short (fractions of a second) bright flash or fireball at the instant of detonation. Unless highly combustible materials are involved, the thermal effect plays an insignificant part in an explosion. Should highly combustible materials be present and a fire started, the debris resulting from the explosion may provide additional fuel and contribute to spreading the fire. Incendiary thermal effects are generally the least damaging of the three primary detonation effects.
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