A Layman’s Guide to Understanding the Science of Explosives
The science and mathematics of explosives is astonishingly complex. Research is underway everyday to better understand the destructive power of explosions and substances that create them. This overview will help you understand the processes that make ALLGard’s Blast Mitigation technology so effective.
CHEMICAL EXPLOSIVES AND THEIR CHARACTERISTICS
TYPES OF EXPLOSIONS
SENSITIVITY
EXPLOSION CHARACTERISTICS
EXPLOSION PHENOMENA
Explosive defense solutions
Blast Mitigation – Reducing the Destructive Effects of an Explosion
CHEMICAL EXPLOSIVES AND THEIR CHARACTERISTICS
A chemical explosive is defined as a compound or mixture that reacts, decomposes, or rearranges with extreme quickness upon the application of heat or shock, yielding high levels of gas and heat. There are many substances that can do one or more of these things but are not considered to be explosives. For a chemical to be an ‘explosive,’ it must exhibit all of the following:
1. Formation of Gases - Gases may be produced in a variety of ways. When wood or coal is burned, the carbon and hydrogen in the fuel react with the oxygen in the atmosphere, releasing heat in the form of flame, particulates as smoke, and gases, carbon dioxide and steam. When the fuel is pulverized, so that the surface in contact with the air is increased, the burning becomes faster and the combustion more complete. When the fuel is reduced to the form of dust or is immersed in liquid oxygen, the burning occurs with explosive violence. In each case, the same thing happens: a burning combustible forms a gas.
2. Evolution of Heat - The generation of large quantities of heat accompanies every explosive chemical reaction. This rapid liberation of heat causes the gases produced in the chemical reaction to expand very quickly. This generates tremendous pressures, and constitutes the explosion. We must note that liberating heat too slowly will not cause an explosion. For example, a pound of coal yields five times as much heat as a pound of nitroglycerin, but the coal cannot be used as an explosive because the rate at which it yields this heat is too slow.
3. Rapidity of Reaction – The speed of the reaction distinguishes an explosive reaction from an ordinary combustion reaction. Unless the reaction occurs extremely quickly, the heated expanding gases will be dissipated, and there will be no explosion. Reconsider a wood or coal fire. As the fire burns, heat is produced and gases are created, but neither is produced quickly enough to cause an explosion.
4. Initiation of Reaction - A material that satisfies the first three factors has one more requirement to be considered an explosive. The reaction must be made to occur when desired, through the application of shock or heat to a small portion of the mass of the material.
TYPES OF EXPLOSIONS
Deflagration - The burning of a material is in fact a kind of chemical decomposition. When the reaction front advances into the un-reacted material at lower than sonic velocity, this is called deflagration. Deflagration can be a rapid combustion, which can result in an explosion when under confinement, although generally it implies the burning of a substance with self-contained oxygen. In this case, heat is transferred from the reacted to the un-reacted material by conduction and convection. The burning velocity for a deflagration is usually less than 2,000 m/s.
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 much larger chain reaction to set off the main explosive material. They can be either low- or high- explosive trains. Detonation is a chemical reaction that propagates through an explosive material faster than the speed of sound in the material, forming a shock wave. High temperature and pressure gradients are produced in the shock wave front. Detonation velocities are in the range of 1,400 to 9,000 m/s or 5,000 to 30,000 ft/s.
Fuel/Air Explosion – High-explosive materials contain oxygen within their chemical structure. This oxygen is used during detonation. By contrast, a fuel/air explosion occurs when a chemical, which will not detonate by itself, is mixed with ambient air and is set off by an event with enough energy. The air provides the oxygen required to maintain the detonation-oxygen balance. The power of fuel/air explosions can be orders of magnitude higher than TNT.
High-Order Explosion – High-explosives (HE) are capable of detonating, and are therefore used in military ordnance, blasting, mining, etc. They react extremely rapidly, develop a high pressure, and produce a super-sonic detonation wave (Mach 1 or higher, or 331.46 m/s at sea level). "High Order Explosion" often also means that because the HE carries all of the necessary oxygen for complete combustion and oxidation of the explosive material in a charge. High-order explosives are chemical compounds that are set off by heat or shock, and have a high brisance (the shattering effect of an explosion). Nitroglycerin, dynamite, TNT, and C4 all produce high-order explosions.
Low-Order Explosion – Low-explosives change into gases combustion, causing deflagration (burning rapidly without generating a high-pressure wave), not detonation, due to reaction rate that is lower than HE. The effect ranges from rapid combustion to a low-order detonation (generally less than 2,000 m/s). Since low explosives burn rather than detonate, they are usually a mixture, rather than a compound, and are set off by heat. They also require confinement to create an explosion. The most common example is gun powder.
Nuclear Explosion - In nuclear fission processes, a nucleus absorbs a neutron, becomes unstable, and splits into two nuclei. In an atomic weapon, there are more neutrons available to continue the process, creating the ‘chain reaction.’ These extra neutrons come from using radioactive isotopes of certain heavy metals, such as uranium and plutonium. Breaking the bonds that hold these nuclei together creates enormous amounts of energy, using only a relatively small portion of fissionable material.
Pressure Burst - If a liquid in a sealed container is heated, the liquid will vaporize and produce higher pressure inside the container. If this process is continued, the pressure will increase until it bursts the container, releasing the gas. If there is sufficient pressure, the higher pressures will travel faster than the lower pressures, resulting in a blast wave with a calculable equivalence to TNT.
SENSITIVITY
Explosives are also classified by the amount of energy required to set off the reaction. This is called sensitivity and can be anything, from impact, heat, shock, friction, electrical discharge, or the detonation of another explosive. There are two basic classes of sensitivity.
Primary Explosives - Extremely sensitive to shock, friction, and heat, they require only a small quantity of energy to go off. They are commonly used in detonators to set off secondary explosives.
Secondary Explosives - Relatively insensitive to shock, friction and heat, they require a large amount of energy to set off the reaction. However, they have much more power than primary explosives, and as such are used in demolition. They require a detonator.
EXPLOSION CHARACTERISTICS
Air Blast - The airborne shock wave or acoustic transient generated by an explosion. An air blast has the characteristics of overpressure, duration, and impulse.
Impulse – Measured in Newton-seconds (Ns), impulse is the product of average net force and change in time. It causes the exchange of momentum between the explosive charge and the target. Structures are generally more sensitive to the effects of impulse than to peak pressure, due to the quarter wavelength of the natural frequency typical structures being longer than the duration of the blast wave.
Overpressure (Peak Pressure) - The pressure measured above the ambient pressure at the time of measurement.
Quasi-Static Pressure - Quasi-static pressure can be measured in situations where the duration of a high-pressure event, such as from the liberation of gas and/or heat from an explosion, is significantly longer than the response time of the structure surrounding. The loading on the structure can be treated like a static or quasi-static event, commonly occurring in internal explosions in poorly vented structures.
Pressure – Pressure is used to describe the intensity of an explosive blast wave. Pressure is determined by the ratio of force to surface area, commonly expressed in units of atmospheres, bars, or dynes.
Shock - A shock front is a discontinuity in the physical properties of the medium through which it is passing. The thickness of the shock front in a gas at standard temperature and pressure is about 100 nm. This discontinuity is characterized by a near instantaneous rise in pressure. The velocity of the shock, sometimes measured by Mach number, depends on the magnitude of the pressure.
EXPLOSION PHENOMENA
There are several phenomena associates with explosions, and each of them poses its own set of issues when conceiving a means of explosive defense.
Afterburn – Post-detonation combustion as products of detonation 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 start post-blast fires.
Collateral Damage - Inadvertent casualties and destruction inflicted on property or individuals. Unintended damage to material surrounding a controlled explosion.
Flash – Visible light and infrared emissions generated by most explosives upon detonation. Near the blast source, the flash can cause severe burns. Some energetic material liberates a significant proportion of its energy as radiated heat with reduced blast.
Fragmentation - The breaking and scattering of the fragments of a projectile, bomb, or grenade, or the breaking of a solid mass into pieces by an explosion. Fragments generated by a cased explosion can have a velocity greater than 2500 m/s, and are potentially lethal at long distances from the explosion, making fragmentation one of the worst threats to personnel. Addressing this threat effectively has, in the past, required heavy armor plating, or expensive personal ballistic protection.
Secondary Fragmentation - Material near the explosion can be propelled by the blast and projected some distance from the event at lethal velocities. In any blast mitigation system, secondary fragmentation must be reduced to an absolute minimum.
Explosive defense solutions
Defense solutions come in two types, hardening and mitigation.
Hardening - Hardening, which essentially is trying to overmatch or resist a blast with the strength of the defensive materials, has been widely used throughout history. It is the most intuitive form of defense – and the most limited in its capabilities. Armor is used primarily as ballistic projectile defense. The effectiveness of armor is increased by increasing its mass (thickness and/or weight). But it can also be used to defend against explosions. Hardening solutions include steel armor plate, various synthetic fibers (Kevlar® and Spectra®), and fiber-reinforced composites. Most blast containment systems employ some form of hardening. Although some energy from an explosion is absorbed through deformation of the armor, hardening systems have the negative effect of reflecting the blast. This is a negative effect because, according to the laws of physics, this reflection actually magnifies blast effect up to eight times. How? Because the shock waves reflecting off a solid surface add to the original incident shock wave, which creates a destructive synergism of much greater gas density, temperature, pressure, and overpressure duration--which all contribute to increase the “impulse.” [INSERT LINK TO IMPULSE DEFINITION] All this reflected energy becomes an even bigger problem within confined spaces.
Mitigation – The dissipation or attenuation of blast energy, so that acoustic and shock waves, overpressure, reflected peak overpressure, impulse, and afterburn are reduced. This attenuation is accomplished through physical and chemical processes in and around the blast zone. The reduced explosive energy is transmitted more slowly, reducing its destructive effects.
Unlike hardening systems, the effectiveness of ALLGard Panels is not dependent upon thickness. Therefore, it has a much wider range of uses against blast effects than hardened armor.
Blast Mitigation – Reducing the Destructive Effects of an Explosion
Now that you understand the basics of what happens in an explosion, we will explain “blast mitigation” – what it is, and what it’s not.
There is no such thing as “blast-proof.” A big enough blast will penetrate any structure, knock down any wall, and blow through any door. For all the hardened armor plating and reinforced concrete in the world, there is always a bigger bomb.
However, the destructive effects of an explosion can be reduced, with blast mitigation technology.
What if you could reduce the pressure wave of an explosion by 75-90%?
That is what ALLGard Panels accomplish.
What if you could attenuate the duration of an explosion and the heat bloom by 75-90%?
How do you think that would affect secondary fragmentation?
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BLAST MITIGATION TECHNOLOGY
Blast mitigation can be achieved by any number or combination of these techniques.
Fire Extinguishing - Fire requires four things: fuel, oxygen, heat and time. Without all four, the fire goes out. In an explosion, fire extinguishing must take place within 50 ms to be effective. Materials that breakdown in less than 1 ms into flame extinguishing components should be mixed with the accelerating flame front.
Flash Suppression - Reducing the afterburning of the detonation products in air will reduce the flash output of an explosive device. This can be done either by quenching the event or by interfering in the combustion process. Quenching is achieved by the rapid release of water into the fireball. Disrupting the combustion processes can be done with advanced fire suppression materials.
Irreversible Changes - The energy of the explosion must be dissipated through attenuation processes such as drag, turbulence, friction, and viscosity. One way we achieve this in ALLGard Panels is through crushing of porous media and entrainment into a two-phase flow.
Momentum Exchange - Momentum in mechanics is the quantity of a body’s motion, the product of its mass and velocity. Momentum exchange is an effective blast mitigation mechanism. On detonation, the momentum of the blast wind and detonation products are transferred into the ALLGard Panels, which entrains that momentum into a two-phase flow. The energy is then dissipated through viscosity and drag.
Shock Decoupling - A shock wave propagates with a given speed, pressure, and particle velocity, proportional to the shock impedance of the material through which it is propagating. Shock impedance varies with the material. At the direct interface of two materials with different shock impedances, the shock is transmitted with little or no loss. Introducing a shock attenuant between the two materials significantly reduces the transmitted shock. This is called shock decoupling.
Shock Multipathing – Shock waves travel at different velocities in different materials. In a material containing two phases – such as solid particulates or liquid droplets within a gas – the difference in velocities between the two materials causes the shock front to become spread out over time, reducing the strength of the shock.
Two-Phase Flow – This refers to the flow of two mixed materials of different phases. For example, a particulate in a gas, gas in liquid, particulates in a liquid, etc. Energy dissipation occurs in a two-phase flow through viscose drag.
Contact us today for answers to your questions about how our products can protect your personnel and property.
