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BLASTWRAP® EXPLOSION TERMINOLGY
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CHEMICAL EXPLOSIVES: TYPES, CHARACTERISTICS, ETC.
Chemical Explosive - 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. For example, a mixture of nitrogen and oxygen can be made to react with great rapidity and yield the gaseous product nitric oxide ; yet the mixture is not an explosive since it does not evolve heat, but rather absorbs heat. For a chemical to be an explosive, it must exhibit all of the following:
- Formation of Gases - Gases may be evolved from substances in a variety of ways. When wood or coal is burned in the atmosphere, the carbon and hydrogen in the fuel combine with the oxygen in the atmosphere to form carbon dioxide and steam, together with flame and smoke. When the wood or coal is pulverized, so that the total surface in contact with the oxygen is increased, and burned in a furnace or forge where more air can be supplied, the burning can be made more rapid and the combustion more complete. When the wood or coal is immersed in liquid oxygen or suspended in air in the form of dust, the burning takes place with explosive violence. In each case, the same action occurs: a burning combustible forms a gas.
- Evolution of Heat - The generation of heat in large quantities accompanies every explosive chemical reaction. It is this rapid liberation of heat that causes the gaseous products of reaction to expand and generate high pressures. This rapid generation of high pressures of the released gas constitutes the explosion. It should be noted that 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.
- Rapidity of Reaction - Rapidity of reaction distinguishes the explosive reaction from an ordinary combustion reaction by the great speed with which it takes place. 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.
- Initiation of Reaction - A reaction must be capable of being initiated by the application of shock or heat 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 desired.
TYPES OF EXPLOSIONS: Explosives are distinguished between high explosives, which detonate, and low explosives, which deflagrate.
Pressure Burst - If a liquid is sealed in a container and heated, the liquid will vaporize and pressurize the container. If this process is continued, the pressure will rise until it exceeds the strength of the container and the container will burst. The pressurized gas will then escape. The higher pressures traveling faster than the lower pressures will result in a blast wave with a calculable TNT equivalence.
Low Order Explosion - Low explosives change into gases by burning or combustion. These are characterized by deflagration (burning rapidly without generating a high pressure wave) and a lower reaction rate than high explosives. The overall effect ranges from rapid combustion to a low order detonation (generally less than 2,000 meters per second). Since they burn through deflagration rather than a detonation wave, they are usually a mixture, and are initiated by heat and require confinement to create an explosion. Gun powder (black powder) is the only common example.
Deflagration - The chemical decomposition (burning) of a material in which the reaction front advances into the reacted material at less than sonic velocity. Deflagration can be a very rapid combustion which under confinement can result in an explosion, although generally it implies the burning of a substance with self-contained oxygen. The reaction zone advances into the unreacted material at less than the velocity of sound in the material. In this case, heat is transferred from the reacted to the unreacted material by conduction and convection. The burning rate for a deflagration is usually less than 2,000 meters/second.
Fuel/Air Explosion - High explosive materials contain the oxygen that they require for detonation within their chemical structure. A fuel/air explosion occurs when a chemical, which on its own will not detonate, is mixed with ambient air and is initiated by an event of the appropriate energy. The air provides the oxygen which is required to maintain the detonation oxygen balance. Fuel/air explosions are characterized by their power which can be orders of magnitude higher than TNT. An example of this kind of explosion is the propylene oxide/air reaction.
Detonation - Also called an initiation sequence or a firing train, this is the sequence of events which cascade from relatively low levels of energy to cause a chain reaction to initiate the final explosive material or main charge. They can be either low or high explosive trains. It is a chemical reaction that moves through an explosive material at a velocity greater than the speed of sound in the material. A detonation is a chemical reaction given by an explosive substance in which a shock wave is formed. High temperature and pressure gradients are generated in the wave front, so that the chemical reaction is initiated instantaneously. Detonation velocities lie in the approximate range of 1,400 to 9,000 m/s or 5,000 to 30,000 ft/s.
High Order Explosion - High explosives are capable of detonating and are used in military ordnance, blasting and mining, etc. These have a very high rate of reaction, high pressure development, and the presence of a detonation wave that moves faster than the speed of sound (Mach 1, or 331.46 meters per second, at sea level). "High Order Explosion" often also means that because the HE carries all of the oxident required for complete combustion of the explosive material in a charge, there is, in fact, a complete oxidation or a High Order Explosion of all of the explosive material. Without confinement, they are compounds, which are initiated by shock or heat and have high brisance (the shattering effect of an explosion). Examples include primary explosives such as nitroglycerin that can detonate with little stimulus and secondary explosives such as dynamite (trinitrotoluene, TNT) that require a strong shock (from a detonator such as a blasting cap).
Nuclear Explosion - In fission processes, a fissionable nucleus absorbs a neutron, becomes unstable, and splits into two nearly equal nuclei. In an atomic weapon the number of neutrons producing additional fission is greater than 1, and the reaction increase rapidly into a runaway explosion. In the process a small proportion of fissionable material is converted into huge amounts of energy: E = mc2, where "E" is energy, "m" is mass, and "c" is the speed of light.
EXPLOSION SENSITIVITY: Explosives are classified by their sensitivity, which is the amount of energy to initiate the reaction. This energy can be anything, from a shock, an impact, a friction, an electrical discharge, or the detonation of another explosive. There are two basic divisions on sensitivity:
Primary Explosives - They are extremely sensitive to shock, friction, and heat and require a small quantity of energy to be initiated. They are mainly used in detonators to initiate secondary explosives.
Secondary Explosives - They are relatively insensitive to shock, friction and heat and need a great amount of energy to initiate decomposition. They have much more power than primary explosives and are used in demolition. They require a detonator to explode.
Impact - Sensitivity is expressed in terms of the distance through which a standard weight must be dropped to cause the material to explode.
Friction - Sensitivity is expressed in terms of what occurs when a weighted pendulum scrapes across the material (snaps, crackles, ignites, and/or explodes).
Heat - Sensitivity is expressed in terms of the temperature at which flashing or explosion of the material occurs.
EXPLOSION CHARACTERISTICS:
Pressure - When a force acts perpendicular to a surface, the pressure (p) exerted is the ratio between the magnitude of the force and the area of the surface: pressure = force/area, and may well be expressed using other terms such as bars, atmospheres or dynes. Pressure is used to characterize one of the main parameters, sometimes known as the intensity of the blast wave.
Overpressure - The pressure measured above the ambient pressure at the time of measurement.
Shock - A shock front is a virtual discontinuity in the physical properties of the gas through which it is passing. The shock thickness is of the order of ten mean free paths, which for a gas at standard temperature and pressure is approximately 100nm or close to the wave length of light. This discontinuity is characterized by a near instantaneous rise in pressure. The velocity of the shock, or Mach number, is dependent on the magnitude of the pressure.
Air Blast - The airborne shock wave or acoustic transient generated by an explosion that has the characteristics of overpressure, duration and impulse.
Impulse - The product of average net force and change in time. It can be measured in Newton x Seconds (Ns) and is equal to (causes), the exchange in momentum between the explosive charge and the target. It is the integral of the positive portion of the pressure/time history (unless stated otherwise). Structures are generally more sensitive to the effects of impulse rather than peak pressure. This is due to the quarter wavelength of the natural frequency of the many structures of interest being longer than the duration of the blast wave.
Quasi-Static Pressure - A process taking place relatively slowly so that all the intensive variables can have definite values through the entire path taken by the process. Such a process is called a quasi-static process. Quasi-static pressure occurs in situations where the duration of a pressure event from the liberation of gas and/or heat from an explosive event is significantly longer than the response time of the structure. The loading can be treated like a static or quasi-static event. This is a common phenomenon for internal explosions in poorly vented structures.
EXPLOSION PHENOMENA:
Flash - Light and infrared emissions generated by an explosion are generally known as the "flash". The flash can cause severe burns close to the source of the blast. Some energetic material liberates a significant proportion of its energy as radiated heat with reduced blast, like Magnesium / Teflon / Viton. Most explosive materials generate flash unless they have been specifically developed not to do so such as "permitted" explosives used in the mining industry.
Afterburn - Post-detonation, aerobic combustion of fuel rich species as detonation products mix with the surrounding air. Some explosive materials are not oxygen balanced and produce fuel rich detonation products. The burning of these products increases flash and will produce quasi-static pressure, if confined. Afterburn is a significant issue for confined explosions and can initiate post-blast fires.
Fragmentation - The breaking and scattering in all directions of the pieces of a projectile, bomb or grenade or the breaking of a solid mass into pieces by blasting. Fragments generated by a cased explosion can have a very high velocity (>2500 ms) and are potentially lethal at long distances from the site of the explosion. This is one of the dominant threats to personnel from a cased explosion. Fragmentation can be difficult to effectively deal with and requires a high mass solution or expensive light-weight ballistic protection.
Secondary Fragmentation - Material close to the explosion can be propelled by the blast and projected some distance from the event. This material is potentially lethal. It is essential in any mitigation system that secondary fragmentation is effectively managed or reduced to an absolute minimum.
Collateral Damage - (Euphemism) Inadvertent casualties and destruction inflicted on material or civilians in the course of military operations. Unintended damage to material surrounding a controlled explosion. Collateral damage reduction is the mitigation of the damage from a controlled explosion.
Ground Shock - Shaking of the ground, by elastic waves emanating from a blast: usually measured in inches per second of particle velocity, where the charge is close to, or in contact with the ground. Low frequency ground shock can produce significant damage to structures at large distances from the site of the explosion. Ground shocks can become enhanced by reflections from varying density layers deep in the earth.
MITIGATION MECHANISMS:
Irreversible Changes - The laws of conservation of mass, momentum and energy for a shock wave imply that it is difficult to reduce explosive effects rapidly. The energy of the explosion must be dissipated through irreversible processes such as drag, turbulence, friction and viscosity. With BlastWrap®, this is achieved, in part, through crushing of porous media and entrainment into a two phase flow.
Two Phase Flow - The flow of two mixed materials of different phases (i.e., particulate in gas, liquid droplets in gas, gas in liquid, particulate in liquid, etc.). Energy dissipation occurs in a two phase flow through viscose drag and is a critical mitigation mechanism for BlastWrap®.
Momentum Exchange - Momentum in mechanics is the quantity of motion of a body. The linear momentum of a body is the product of its mass and velocity. The effective management of momentum exchange is an important mechanism in blast mitigation. On detonation, the momentum of the blast wind and detonation products are transferred to the surrounding media (BlastWrap®) which in turn is entrained into a two phase flow. This mechanism allows energy to be dissipated through viscose drag. Structural coupling is the negative aspect of this mechanism.
Shock Multipathing - The speed of sound for any material is given by: a2 = Eb / p, where "a" is acoustic velocity, "Eb" is the bulk modulus of elasticity and "p" is the density. The implications of this are that shocks travel at differing velocities in different materials. In a material containing two phases, this causes the shock front to be "smeared" and spread out over time due to the differential in the acoustic velocities of the two materials.
Flash Suppression - It is possible to reduce the flash output of an explosive device by reducing the afterburning of the detonation products in air. This can be accomplished by quenching the event or by interfering in the combustion process. Quenching is achieved by the rapid liberation of water into the fireball or the combustion processes can be disrupted by the use of advanced fire suppression materials.
Fire Extinguishing - Fire requires four criteria in order to develop: fuel, oxygen, heat and time. If any one of the four criteria is prevented from participating in the combustion, then the fire is extinguished. In an explosive event, the process of extinguishing must take place extremely rapidly (<50ms) if it is to be effective. Ideally, materials that breakdown into flame extinguishing components in less than 1ms should be intimately mixed with the accelerating flame front.
Shock Decoupling - A shock propagates with a given speed, pressure, and particle velocity relative to the shock impedance of the material through which it is propagating. At the interface with a material of different shock impedance, the laws of conservation of momentum energy and mass are obeyed and the shock is transmitted with little or no loss. If a shock attenuant is introduced between the two materials and the transmitted shock is significantly reduced, the assembly is said to be shock decoupled.
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