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Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. Supersonic explosions created by high explosives are known as detonations and travel via supersonic shock waves. Subsonic explosions are created by low explosives through a slower burning process known as deflagration.
Natural
Explosions can occur in nature. Most natural explosions arise from volcanic processes of various sorts. Explosive volcanic eruptions occur when magma rising from below has much dissolved gas in it; the reduction of pressure as the magma rises causes the gas to bubble out of solution, resulting in a rapid increase in volume. Explosions also occur as a result of impact events and in phenomena such as hydrothermal explosions (also due to volcanic processes). Explosions can also occur outside of Earth in the universe in events such as supernovae. Explosions frequently occur during bushfires in eucalyptus forests where the volatile oils in the tree tops suddenly combust.[1]
Animal bodies can also be explosive, as some animals hold a large amount of flammable material such as animal fat. This, in rare cases, results in naturally exploding animals.
Astronomical
Among the largest known explosions in the universe are supernovae, which result when a star explodes from the sudden starting or stopping of nuclear fusion, and gamma ray bursts, whose nature is still in some dispute. Solar flares are an example of explosion common on the Sun, and presumably on most other stars as well. The energy source for solar flare activity comes from the tangling of magnetic field lines resulting from the rotation of the Sun's conductive plasma. Another type of large astronomical explosion occurs when a very large meteoroid or an asteroid impacts the surface of another object, such as a planet.
Chemical
The most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation reaction that produces large amounts of hot gas. Gunpowder was the first explosive to be discovered and put to use. Other notable early developments in chemical explosive technology were Frederick Augustus Abel's development of nitrocellulose in 1865 and Alfred Nobel's invention of dynamite in 1866. Chemical explosions (both intentional and accidental) are often initiated by an electric spark or flame. Accidental explosions may occur in fuel tanks, rocket engines, etc.
Electrical and magnetic
A high current electrical fault can create an electrical explosion by forming a high energy electrical arc which rapidly vaporizes metal and insulation material. This arc flash hazard is a danger to persons working on energized switchgear. Also, excessive magnetic pressure within an ultra-strong electromagnet can cause a magnetic explosion.
Mechanical and vapor
Strictly a physical process, as opposed to chemical or nuclear, e.g., the bursting of a sealed or partially sealed container under internal pressure is often referred to as a 'mechanical explosion'. Examples include an overheated boiler or a simple tin can of beans tossed into a fire.
Boiling liquid expanding vapor explosions are one type of mechanical explosion that can occur when a vessel containing a pressurized liquid is ruptured, causing a rapid increase in volume as the liquid evaporates. Note that the contents of the container may cause a subsequent chemical explosion, the effects of which can be dramatically more serious, such as a propane tank in the midst of a fire. In such a case, to the effects of the mechanical explosion when the tank fails are added the effects from the explosion resulting from the released (initially liquid and then almost instantaneously gaseous) propane in the presence of an ignition source. For this reason, emergency workers often differentiate between the two events.
Nuclear
In addition to stellar nuclear explosions, a man-made nuclear weapon is a type of explosive weapon that derives its destructive force from nuclear fission or from a combination of fission and fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives available, with a single weapon capable of completely destroying an entire city.
Properties of explosions
Force
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.
Velocity The speed of the reaction is what distinguishes the explosive reaction from an ordinary combustion reaction. Unless the reaction occurs rapidly, the thermally expanded gases will be dissipated in the medium, and there will be no explosion. Again, consider a wood or coal fire. As the fire burns, there is the evolution of heat and the formation of gases, but neither is liberated rapidly enough to cause an explosion. This can be likened to the difference between the energy discharge of a battery, which is slow, and that of a flash capacitor like that in a camera flash, which releases its energy all at once.
Evolution of heat The generation of heat in large quantities accompanies most explosive chemical reactions. The exceptions are called entropic explosives and include organic peroxides such as acetone peroxide[2] It is the rapid liberation of heat that causes the gaseous products of most explosive reactions to expand and generate high pressures. This rapid generation of high pressures of the released gas constitutes the explosion. The liberation of heat with insufficient rapidity will not cause an explosion. For example, although a pound of coal yields five times as much heat as a pound of nitroglycerin, the coal cannot be used as an explosive because the rate at which it yields this heat is quite slow. In fact, a substance which burns less rapidly (i.e. slow combustion) may actually evolve more total heat than an explosive which detonates rapidly (i.e. fast combustion). In the former, slow combustion converts more of the internal energy (i.e. chemical potential) of the burning substance into heat released to the surroundings, while in the latter, fast combustion (i.e. detonation) instead converts more internal energy into work on the surroundings (i.e. less internal energy converted into heat); c.f. heat and work (thermodynamics) are equivalent forms of energy. See Heat of Combustion for a more thorough treatment of this topic.
When a chemical compound is formed from its constituents, heat may either be absorbed or released. The quantity of heat absorbed or given off during transformation is called the heat of formation. Heats of formations for solids and gases found in explosive reactions have been determined for a temperature of 15 °C and atmospheric pressure, and are normally given in units of kilocalories per gram-molecule. A negative value indicates that heat is absorbed during the formation of the compound from its elements; such a reaction is called an endothermic reaction. In explosive technology only materials that are exothermic—that have a net liberation of heat—are of interest. Reaction heat is measured under conditions either of constant pressure or constant volume. It is this heat of reaction that may be properly expressed as the "heat of explosion."
Initiation of reaction A chemical explosive is a compound or mixture which, upon the application of heat or shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat. Many substances not ordinarily classed as explosives may do one, or even two, of these things.
A reaction must be capable of being initiated by the application of shock, heat, or a catalyst (in the case of some explosive chemical reactions) to a small portion of the mass of the explosive material. A material in which the first three factors exist cannot be accepted as an explosive unless the reaction can be made to occur when needed.
Fragmentation is the accumulation and projection of particles as the result of a high explosives detonation. Fragments could be part of a structure such as a magazine. High velocity, low angle fragments can travel hundreds or thousands of feet with enough energy to initiate other surrounding high explosive items, injure or kill personnel and damage vehicles or structures.
2. DETONATION
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives,[1] as well as in reactive gases. The velocity of detonations in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail (higher resolution).
Gaseous detonations normally occur in confined systems but are occasionally observed in large vapor clouds. They are often associated with a gaseous mixture of fuel and oxidant of a composition, somewhat below conventional flammability limits. There is an extraordinary variety of fuels that may be present as gases, as droplet fogs and as dust suspensions. Other materials, such as acetylene, ozone and hydrogen peroxide are detonable in the absence of oxygen; a more complete list is given by both Stull and Bretherick. Oxidants include halogens, ozone, hydrogen peroxide and oxides of nitrogen.
In terms of external damage, it is important to distinguish between detonations and deflagrations where the exothermic wave is subsonic and maximum pressures are at most a quarter[ citation needed ] of those generated by the former. Processes involved in the transition between deflagration and detonation are covered thoroughly for gasses by Nettleton.
Contents
Etymology
French détoner, to explode; from Latin detonare, to expend thunder; from de-, ~ off + tonare, to thunder.
Theories
The simplest theory to predict the behavior of detonations in gases is known as Chapman-Jouguet (CJ) theory, developed around the turn of the 20th century. This theory, described by a relatively simple set of algebraic equations, models the detonation as a propagating shock wave accompanied by exothermic heat release. Such a theory confines the chemistry and diffusive transport processes to an infinitely thin zone.
A more complex theory was advanced during World War II independently by Zel'dovich, von Neumann, and W. Doering.[5][6][7] This theory, now known as ZND theory, admits finite-rate chemical reactions and thus describes a detonation as an infinitely thin shock wave followed by a zone of exothermic chemical reaction. With a reference frame of a stationary shock, the following flow is subsonic, so that an acoustic reaction zone follows immediately behind the lead front, the Chapman-Jouguet condition.[8][9] There is also some evidence that the reaction zone is semi-metallic in some explosives.[10]
Both theories describe one-dimensional and steady wave fronts. However, in the 1960s, experiments revealed that gas-phase detonations were most often characterized by unsteady, three-dimensional structures, which can only in an averaged sense be predicted by one-dimensional steady theories. Indeed, such waves are quenched as their structure is destroyed.[11][12] The Wood-Kirkwood detonation theory can correct for some of these limitations.[13]
Experimental studies have revealed some of the conditions needed for the propagation of such fronts. In confinement, the range of composition of mixes of fuel and oxidant and self-decomposing substances with inerts are slightly below the flammability limits and for spherically expanding fronts well below them.[14] The influence of increasing the concentration of diluent on expanding individual detonation cells has been elegantly demonstrated.[15] Similarly their size grows as the initial pressure falls.[16] Since cell widths must be matched with minimum dimension of containment, any wave overdriven by the initiator will be quenched.
Mathematical modeling has steadily advanced to predicting the complex flow fields behind shocks inducing reactions.[17][18] To date none has adequately described how structure is formed and sustained behind unconfined waves.
Applications
The main cause of damage from explosive devices is due to a supersonic blast front (a powerful shock wave) in the surrounding area. Therefore, the detonation is primarily associated with explosives and the acceleration of various projectiles. However, detonation waves may also be utilized for less destructive purposes like deposition of coatings to a surface[19] or cleaning of equipment (e.g. slag removal[20]). Pulse detonation engines utilize the detonation wave for aerospace propulsion.[21] The first flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on January 31, 2008.[22]
In engines and firearms
Unintentional detonation when deflagration is desired is a problem in some devices. In internal combustion engines it is called engine knocking or pinging, and causes loss of power and excessive heating of certain components. In firearms, it may cause catastrophic and possibly lethal failure.
Explosion protection
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Explosion protection is used to protect all sorts of buildings and civil engineering infrastructure against internal and external explosions or deflagrations. It was widely believed[1] until recently that a building subject to an explosive attack had a chance to remain standing only if it possessed some extraordinary resistive capacity. This belief rested on the assumption that the specific impulse or the time integral of pressure, which is a dominant characteristic of the blast load, is fully beyond our control.
Techniques of explosion protection
Avoidance
Avoidance will make it impossible for an explosion or deflagration to occur, for instance by means of suppressing the heat and the pressure needed for an explosion using aluminum mesh structure such as eXess, by means of consistent displacement of the O2 necessary for an explosion or deflagration to take place, by means of padding gas (f. i. CO2 or N2), or, by means of keeping the concentration of flammable content of an atmosphere consistently below or above the explosive limit, or, by means of consistent elimination of ignition sources.
Constructional explosion protection
Constructional explosion protection aims at pre-defined, limited or zero damage that results from applied protective techniques in combination with reinforcement of the equipment or structures that must be expected to become subject to internal explosion pressure and flying debris or external violent impact.[2]
Explosion protection method selection
The technology of protection[3] can range in price dramatically but where the type of device is rational to use, would typically be from least to most expensive solutions: explosion doors and vents (dependent on quantities and common denominators, either may end up the wise price choice); inerting: explosion suppression; isolation – or combinations of same. To focus on the most cost effective, doors typically have lower release pressure capabilities; are not susceptible to fatigue failures or subject to changing release pressures with changes in temperature, as “rupture membrane” type are; capable of leak tight service; service temperatures of up to 2,000°F; and can be more cost effective in small quantities. Rupture membrane type vents can provide a leak tight seal more readily in most cases; have a relatively broad tolerance on their release pressure and are more readily incorporated into systems with discharge ducts.
Discharge Hood with Explosion Relief Panels and Fracture Clip Releases.
There are several fundamental considerations in the review of a system handling potentially explosive dusts, gases or a mixture of the two. Dependent upon the design basis being used, often National Fire Protection Association Guideline 68, the definition of these may vary somewhat. To facilitate providing the reader with an appreciation of the issues rather than a design primer, the following have been limited to the major ones only.
Nuclear weapons testing
Nuclear weapons tests are experiments carried out to determine the effectiveness, yield, and explosive capability of nuclear weapons. Throughout the 20th century, most nations that developed nuclear weapons tested them. Testing nuclear weapons can yield information about how the weapons work, as well as how the weapons behave under various conditions and how structures behave when subjected to nuclear explosions. Nuclear testing has often been used as an indicator of scientific and military strength, and many tests have been overtly political in their intention; most nuclear weapons states publicly declared their nuclear status by means of a nuclear test.
The first nuclear weapon was detonated as a test by the United States at the Trinity site on July 16, 1945, with a yield approximately equivalent to 20 kilotons. The first hydrogen bomb, codenamed "Mike", was tested at the Enewetak atoll in the Marshall Islands on November 1, 1952 (local date), also by the United States. The largest nuclear weapon ever tested was the "Tsar Bomba" of the Soviet Union at Novaya Zemlya on October 30, 1961, with the largest yield ever seen (as of September 2013), an estimated 50–58 megatons.
In 1963, three (UK, US, Soviet Union) of the four nuclear and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground nuclear testing. France continued atmospheric testing until 1974, and China continued until 1980. Neither has signed the treaty.[1]
Underground tests in the United States continued until 1992 (its last nuclear test), the Soviet Union until 1990, the United Kingdom until 1991, and both China and France until 1996. After signing the Comprehensive Test Ban Treaty in 1996 (which has as of 2012 not yet entered into force), these states have pledged to discontinue all nuclear testing. Non-signatories India and Pakistan last tested nuclear weapons in 1998.
The most recent nuclear test occurred in February 2013 in North Korea. In January 2013, it was announced by North Korea that it plans to conduct further tests involving rockets that can carry satellites as well as nuclear warheads.[2]
Explosive material
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material.
This potential energy stored in an explosive material may be
Explosive materials may be categorized by the speed at which they expand. Materials that detonate (explode faster than the speed of sound) are said to be "high explosives" and materials that deflagrate are said to be "low explosives". Explosives may also be categorized by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are primary explosives and materials that are relatively insensitive are secondary or tertiary explosives.
A wide variety of chemicals can explode; a smaller number are manufactured in quantity as explosives. The remainder are too dangerous, sensitive, toxic, expensive, unstable, or decompose too quickly for common usage.
History
Though early thermal weapons, such as Greek fire, have existed since ancient times, the first widely used explosive in warfare and mining was black powder, invented in 9th century China (see the history of gunpowder). This material was sensitive to water, and evolved lots of dark smoke. The first useful explosive stronger than black powder was nitroglycerin, developed in 1847. As nitroglycerin was unstable, it was replaced by nitrocellulose, smokeless powder, dynamite and gelignite (the two latter invented by Alfred Nobel). World War I saw the introduction of trinitrotoluene in naval shells. World War II saw an extensive use of new explosives (see explosives used during World War II). In turn, these have largely been replaced by modern explosives such as C-4.
The increased availability of chemicals has allowed the construction of improvised explosive devices.
Chemical
Main article: Chemical explosive
An explosion is a type of spontaneous chemical reaction that, once initiated, is driven by both a large exothermic change (great release of heat) and a large positive entropy change (great quantities of gases are released) in going from reactants to products, thereby constituting a thermodynamically favorable process in addition to one that propagates very rapidly. Thus, explosives are substances that contain a large amount of energy stored in chemical bonds. The energetic stability of the gaseous products and hence their generation comes from the formation of strongly bonded species like carbon monoxide, carbon dioxide, and (di)nitrogen, which contain strong double and triple bonds having bond strengths of nearly 1 MJ/mole. Consequently, most commercial explosives are organic compounds containing -NO2, -ONO2 and -NHNO2 groups that, when detonated, release gases like the aforementioned (e.g., nitroglycerin, TNT, HMX, PETN, nitrocellulose).[1]
An explosive is classified as a low or high explosive according to its rate 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.
Decomposition
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.
Deflagration
Main article: Deflagration
In deflagration, the decomposition of the explosive material is propagated by a flame front which moves slowly through the explosive material, in contrast to detonation. Deflagration is a characteristic of low explosive material.
Detonation
Main article: Detonation
This term is used to describe an explosive phenomenon whereby the decomposition is propagated by an explosive shock wave traversing the explosive material. The shock front is capable of passing through the high explosive material at great speeds, typically thousands of metres per second.
Exotic
In addition to chemical explosives, there are a number of more exotic explosive materials, and exotic methods of causing explosions. Examples include nuclear explosives, and abruptly heating a substance to a plasma state with a high-intensity laser or electric arc.
Laser- and arc-heating are used in laser detonators, exploding-bridgewire detonators, and exploding foil initiators, where a shock wave and then detonation in conventional chemical explosive material is created by laser- or electric-arc heating. Laser and electric energy are not currently used in practice to generate most of the required energy, but only to initiate reactions.
Properties of explosive materials
To determine the suitability of an explosive substance for a particular use, its physical properties must first be known. The usefulness of an explosive can only be appreciated when the properties and the factors affecting them are fully understood. Some of the more important characteristics are listed below:
Availability and cost
The availability and cost of explosives are determined by the availability of the raw materials and the cost, complexity, and safety of the manufacturing operations.
Sensitivity
Main article: Sensitivity (explosives)
Sensitivity refers to the ease with which an explosive can be ignited or detonated, i.e., the amount and intensity of shock, friction, or heat that is required. When the term sensitivity is used, care must be taken to clarify what kind of sensitivity is under discussion. The relative sensitivity of a given explosive to impact may vary greatly from its sensitivity to friction or heat. Some of the test methods used to determine sensitivity relate to:
Sensitivity is an important consideration in selecting an explosive for a particular purpose. The explosive in an armor-piercing projectile must be relatively insensitive, or the shock of impact would cause it to detonate before it penetrated to the point desired. The explosive lenses around nuclear charges are also designed to be highly insensitive, to minimize the risk of accidental detonation.
Sensitivity to initiation
The index of the capacity of an explosive to be initiated into detonation in a sustained manner. It is defined by the power of the detonator which is certain to prime the explosive to a sustained and continuous detonation. Reference is made to the Sellier-Bellot scale that consists of a series of 10 detonators, from n. 1 to n. 10, each of which corresponds to an increasing charge weight. In practice, most of the explosives on the market today are sensitive to an n. 8 detonator, where the charge corresponds to 2 grams of mercury fulminate.
Velocity of detonation
The velocity with which the reaction process propagates in the mass of the explosive. Most commercial mining explosives have detonation velocities ranging from 1800 m/s to 8000 m/s. Today, velocity of detonation can be measured with accuracy. Together with density it is an important element influencing the yield of the energy transmitted for both atmospheric overpressure and ground acceleration.
Stability
Main article: Chemical stability
Stability is the ability of an explosive to be stored without deterioration.
The following factors affect the stability of an explosive:
Power, performance, and strength
Main article: Power (physics)
Main article: Strength (explosive)
The term power or performance as applied to an explosive refers to its ability to do work. In practice it is defined as the explosive's ability to accomplish what is intended in the way of energy delivery (i.e., fragment projection, air blast, high-velocity jet, underwater shock and bubble energy, etc.). Explosive power or performance is evaluated by a tailored series of tests to assess the material for its intended use. Of the tests listed below, cylinder expansion and air-blast tests are common to most testing programs, and the others support specific applications.
The RBE may be defined as Kx 3
RBE = Ks
where K = the bubble expansion period for an experimental (x) or a standard (s) charge.
Brisance
Main article: Brisance
In addition to strength, explosives display a second characteristic, which is their shattering effect or brisance (from the French meaning to "break"), which is distinguished and separate from their total work capacity. This characteristic is of practical importance in determining the effectiveness of an explosion in fragmenting shells, bomb casings, grenades, and the like. The rapidity with which an explosive reaches its peak pressure (power) is a measure of its brisance. Brisance values are primarily employed in France and Russia.
The sand crush test is commonly employed to determine the relative brisance in comparison to TNT. No test is capable of directly comparing the explosive properties of two or more compounds; it is important to examine the data from several such tests (sand crush, trauzl, and so forth) in order to gauge relative brisance. True values for comparison require field experiments.
Density
Density of loading refers to the mass of an explosive per unit volume. Several methods of loading are available, including pellet loading, cast loading, and press loading, the choice being determined by the characteristics of the explosive. Dependent upon the method employed, an average density of the loaded charge can be obtained that is within 80–99% of the theoretical maximum density of the explosive. High load density can reduce sensitivity by making the mass more resistant to internal friction. However, if density is increased to the extent that individual crystals are crushed, the explosive may become more sensitive. Increased load density also permits the use of more explosive, thereby increasing the power of the warhead. It is possible to compress an explosive beyond a point of sensitivity, known also as dead-pressing, in which the material is no longer capable of being reliably initiated, if at all.
Volatility
Volatility is the readiness with which a substance vaporizes. Excessive volatility often results in the development of pressure within rounds of ammunition and separation of mixtures into their constituents. Volatility affects the chemical composition of the explosive such that a marked reduction in stability may occur, which results in an increase in the danger of handling.
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