Introduccion a explosivos


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Introducción a explosivos

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  • There are three classes of energetic materials of interest to bomb technicians and investigators: Pyrotechnics, which are typically used to produce an effect such as sound, light, heat, smoke or a time delay in other explosive devices; Propellants, which generate gas pressure at a relatively slow rate (compared to explosives) and are used to propel projectiles from a firing system. When provided with the proper confinement, they can produce an “explosion” through the failure of the containment vessel as a result of the generation of large volumes of gas. In this context, they are referred to as “Low Explosives”. Explosives, which detonate (a specific chemical decomposition reaction characterized by the presence of a shock wave in the material) which are then further broken down by use type. Primary explosives are highly sensitive explosives used in small quantities which are very easy to initiate using an impulse such as heat, shock or friction. Secondary explosives are less sensitive to these impulses and are typically initiated by primary explosives. Within the classification of secondary explosives, a further distinction is made between products which can be initiated by a detonator and those which cannot. These are called cap sensitive explosives and booster sensitive explosives. Booster sensitive explosives are also called blasting agents.
  • Explosions can be broken down into three types, two of which are of interest to us. 1. Mechanical explosions are those which are caused by the failure of a confining vessel due to a build up of pressure within the vessel. An example of this would be either a steam boiler being overpressurized and failing, or a common pipe bomb, in which the propellant or low explosive burns, generating a large volume of gas, which ruptures the container. The rupture typically produces a loud bang, the result of the pressure being released suddenly and compressing the surrounding air. 2. Chemical explosions are those which take place in explosive materials. When a sufficient impulse is provided, the explosive begins to decompose, building a shock wave within the material that transmits at a speed higher than the speed of sound in the material. This decomposition produces a large volume of gas, which varies from product to product, compressing the air around the explosive and producing a loud noise, a flash of light (from the reaction), and a large volume of gas. It is the gas generated that determines how useful the explosive may be for performing certain tasks. 3. Nuclear explosions are the result of atoms splitting or fusing and the large amounts of energy produced as a result of these reactions. This is outside the realm of this class.
  • Often times, the term high order or low order is used to refer to explosions and explosives. These terms are frequently confused with high and low explosives, which are different concepts. When discussing high order and low order explosions, we are referring to whether the detonation process functioned fully or only partially. When the process is fully completed, it is considered a high order detonation and all materials have been consumed. In other words, the explosive has functioned as designed. Sometimes, however, explosive materials don’t function as designed and have an incomplete detonation, which is referred to as a low order. These can be caused by a number of factors, including a deterioration of the explosive material, gaps in the explosive which do not allow the detonation wave to continue or propagate, and the failure to use sufficient force in the initiation system. The key point to remember is that explosives are high explosives or low explosives, and explosions are high order or low order.
  • The two different processes which are present in low explosives or propellants and high explosives are detonation and deflagration. Deflagration is the decomposition of energetic materials at a rate much lower than the speed of sound of the material without requiring any additional oxygen. This type of reaction is propagated by the liberated heat of the reaction and the direction of flow of the reaction products is the opposite to the direction the burning is proceeding. Thus, if we imagine a cylinder of propellant laid out horizontally which is initiated from the left side of the cylinder, as the burning of the propellant travels from left to right, the gases and other byproducts are pushed to the left. This tends to generate directional force. Both high explosives and propellants can deflagrate, however, for propellants, it’s the normal reaction, for explosives, it is generally the result of insufficient initiating energy and is an abnormal reaction. When an explosive is properly initiated, a front of high temperature and pressure is generated by the decomposition reaction. This front moves through the material at extremely high speeds, ranging from 5,000 to 30,000 feet per second. As the reaction progresses, the reaction generates more gases, which provide the high temperature and pressure regions necessary to propagate the reaction. The denser the material, that is, the more explosive in a given volume, the faster the reaction will generally be.
  • When an explosive detonates, various reactions occur which produce effects. As the explosive material decomposes, a large volume of gas is generated which compresses the area immediately around the explosive and raises the pressure. This pressure wave spreads out, decreasing exponentially as a factor of distance. The release of this pressure wave also causes the generation of a seismic pulse in whatever ground is nearby. As the rate of the explosive reaction increases, the volume of gas generated in a specific period of time increases, thus increasing the intensity of the pressure pulse and reducing its duration. At the same time, the reaction produces heat and a fireball as a result of the decomposition. The size and temperature and duration of the fireball depends on the composition of the explosive material, but this can ignite secondary fires, causing more serious damage. If an explosive charge is in the vicinity of a material, it can shatter the material and propel the resulting fragments with high velocity causing personnel injury and further property damage. In an uncased explosive charge suspended in the air or water, fragmentation is not typically a concern, however, for propellants/low explosives, the container providing confinement can produce considerable amounts of fragmentation.
  • When calculating or predicting the effect of a particular explosive, we have to know certain characteristics of the explosive. Some of the more important characteristics are: Density - this is simply how much mass is in a given volume, and is usually expressed in terms of grams per cubic centimeter. As mentioned earlier, as density increases, more explosive material is contained within the same volume, allowing the reaction to proceed faster, for higher pressures to be generated (because more gas is being generated) and higher overall explosive effects to be achieved (more pressure as a result of higher density + higher VoD) Brisance is a term used to describe the shattering power of explosives. It is typically higher in military explosives than commercial ones, and is important when dealing with hard objects like steel. Brisance is affected by the density, the amount of gas produced by unit volume of explosive, and the heat of explosion. Higher brisance means higher shattering power. Commercial explosives tend to be used for their heaving effect rather than a shattering effect. Velocity of Detonation - See next slide Sensitivity - the ease with which an explosive can be initiated through one of several impulses. Impact sensitivity is the force of impact required to initiate a particular explosive, friction sensitivity is the frictional force required to cause an explosive to initiate, and blasting cap sensitivity is whether the explosive can be initiated with a standard blasting cap.
  • Detonators can also be initiated through the use of an electric current. This can provide a means for the blaster to check the completeness of a circuit (the continuity) and ensure it is properly configured prior to blasting. There are three types of electrical detonators based on method of initiation. They will be described separately on the following pages. Exploding Bridge wire detonators are a highly specialized type of detonator used for high accuracy and precise timing applications. Unlike match or bridge wire detonators, they require high currents to cause the thin wire to actually explode into a plasma, initiating the explosive charge of the detonator directly, rather than through a primary explosive chain. These require specialized initiation systems and are not typically seen in criminal or terrorist bombing situations. Pyrotechnic delays operate by the burning of a pyrotechnic composition which burns at a specified rate. In some cases, a large diameter lead tube is filled with pyrotechnic composition, drawn to the required diameter and test burned. The delay tubes are then cut to the required length for the specified delay. In other methods, a delay composition with specific time is loaded in increments into a delay tube which is then loaded into the detonator shell. Electronic Delays utilize a computerized circuit which allow for various delay periods, controlled from a special blasting machine. The delay is typically more accurate than pyrotechnic delays. Detonators cannot be initiated through the application of current from a battery or AC source, unlike conventional electric detonators, which provides a higher degree of safety for the blaster. It is a new technology and currently quite expensive. While it may be widely adopted someday, the cost is currently prohibitive.
  • Detonators can further be classified by type of function, instantaneous or delay, as well as type of delay. Instantaneous - these are typically designed with either no delay or extremely small delays (<5 ms). Detonators used for Seismic applications have extremely high precision designed to provide <1 ms delay before detonating and represent a higher degree of precision than standard instantaneous detonators. Short Period Delay - Most commonly used delay detonators, these rely upon a short interval (or delay period) between detonators in a series to stagger the initiation of a series of explosive charges. Short Period delays typically start at 25 ms (with inaccuracies inherent in pyrotechnic delay detonators, this is the minimum required to allow for sufficient rock movement and vibration reduction) up to as much as 100 ms for some detonator series. Long Period Delay - Less frequently used are long period delay detonators in which the delays range from 250 ms to several seconds.
  • Leg wires are the connecting wires from the electrical circuit to the initiation components of the detonator. The electrical circuit might be as short as a direct connection to a blasting machine to hundreds of detonators in series, parallel, or series-parallel circuits. To protect the conductors of the leg wires from shorting out, insulation is provided. Typically a form of plastic, various materials have been used and can be used as insulators. These include: polyethylene, polypropylene, polyvinyl chloride and polyethylene/polypropylene blends. In older detonators, cloth wrapped around the leg wires has been used as the insulator. Conductors can be of any number of materials. Theoretically, any material which conducts electricity could be used as a conductor, but the most common materials used in commercial production are: Copper - lowest cost, lowest resistance, it is used for most applications. Tinned Copper - a copper core with a tinned coating around the exterior. This provides a degree of protection from the elements but adds cost. Usually used in response to a specific customer requirement. Same resistance as copper. Iron - Useful for magnetic separation of leg wires from the material being mined, iron always used in permissible applications. Due to restricted leg wire lengths, higher resistance not an issue. Copper clad iron - offers magnetic separation with improved corrosion resistance.
  • Other components which are used in the construction of electric detonators are: Delay tubes: These may be made out of various materials, but typically are either lead or steel. Lead tubes are typically loaded by the draw method in which a large diameter tube is loaded with pyrotechnic compound, drawn to the required diameter, test burned and cut to the delay required based on the delay provided. Steel delay tubes are typically loaded using the increment method in which powder of a specified formulation is prepared, loaded in increments representing specific time periods into the tubes and loaded into the shell. In either case, the accuracy of the delay is a function of the quality control process…the better the delay compositions consistency, the more “accurate” the detonator will be. Primary Explosives - These are the highly sensitive explosives utilized to initiate a detonator. Because they are extremely sensitive to heat, shock, friction, flame and electrical current, quantities used are extremely small, typically as low as 50 - 100 mg. Primary explosives typically used include Lead Azide, Lead Styphnate, and Diazodinitrophenol (DDNP). Secondary Explosives - These materials are less sensitive than the primary explosives but can be initiated, when properly loaded at high densities, by the primary explosive. PETN is most commonly used, although for high temperature and other specialized applications, RDX and HMX may be used.
  • When an electric detonator is recovered, the physical and chemical characteristics can be used to identify the manufacturer and, therefore, the source of the detonator. Crimp Style - most manufacturers use unique machinery to produce the closure crimp, thus making it possible to identify a manufacturer based on the crimp. Leg Wire Color/Composition - Leg Wire colors are typically used by manufacturers to identify a particular product or delay period with a particular product type. The combination of colors, while not necessarily unique to a particular manufacture, can, in combination with other characteristics provide a product identification. Closure Plug Material - As with leg wires, the closure plug composition and construction can be used to identify a manufacturer and/or product type. Explosive materials - some manufacturers use unique combinations of primary and secondary explosives, thus allowing identification of a particular manufacturer. Shell Materials - Again, the use of a particular alloy or material may allow identification or confirmation of a manufacturer or product. Physical Dimensions - the dimensions associated with a particular detonator shell again, can be unique and used as an identification point.
  • Commercially used high explosives represent the vast majority of explosives used in the United States each year. Interestingly, commercial high explosives are a tiny fraction of the materials used in the construction of improvised explosive devices in the United States. The 5 billion pounds can be broken down approximately as follows: 3.25 billion pounds of unprocessed ammonium nitrate 1 billion pounds of ANFO Blasting Agents 800 million pounds of Water Gels, Slurries and Emulsions 90 million pounds of High Explosives other than permissibles Includes Dynamite, Boosters, Binaries, Etc. 8 million pounds of permissibles Includes Permissible Dynamites, Emulsions, and Water Gels Source: USBM and USGS “ Less than 5% of all bombing incidents...” Source: Bomb Data Center
  • Although nitroglycerine had been discovered by Asconio Sobrero in 1846-47, it did not find widespread use in commercial blasting until the development of dynamite by Alfred Nobel in 1864. Nitroglycerine, a viscous yellowish liquid is very sensitive to even slight shocks, and it was difficult to use effectively in mining practice. In 1866-67, Alfred Nobel discovered that nitroglycerine could be absorbed into kiselguhr, a diatomaceous earth, and wrapped in paper to form cartridges. Also in 1867, he discovered the non-electric blasting cap which provided a safe method of initiation for dynamite and other explosive products. Later developments came in 1867 when two Swedish inventors proposed absorbing the nitroglycerine in pulverized ammonium nitrate mixed with charcoal. Nobel developed his own “active base” dynamite with combustibles (sawdust, charcoal, rosin and starch) and oxidizers (sodium nitrate, potassium nitrate). These active base dynamites replaced an inert ingredient (kiselguhr) which absorbed energy from the explosion with active ingredients which contributed to the power of the explosion. The two Swedish inventors later sold their patent for Ammonium Nitrate base dynamite to Nobel. In 1875, Nobel developed gelatinous dynamites and blasting gelatin and, finally, in 1879, he developed ammonium nitrate gelatins or ammonia gelatin dynamites. By varying the formulation of dynamites, the performance could be altered to suit a specific need. One problem not yet fixed was the tendency of dynamite to freeze when the temperature dropped. Although several accidents claimed lives when thawing dynamites, it was not until 1907 that the first dynamite antifreezes were developed. The current anti-freeze, Ethylene Glycol Dinitrate (EGDN or nitroglycol) was not available until used as an automobile antifreeze and nitrated. Now it supplements or even replaces nitroglycerine in dynamite formulations.
  • Straight Dynamite - The Kiselguhr was inert and actually absorbed energy from the explosive reaction. While an excellent first step, it reduced the power available from the explosive. Straight dynamites are no longer produced. Gelatin dynamite or Blasting Gelatin involves mixing nitroglycerine with nitrocellulose to form a gelled product. Straight blasting gelatin is the most powerful dynamite, with 100% of the material being energetic in nature. They have excellent water resistance and high power. Ammonia Dynamite - by absorbing the nitroglycerine into oxidizers and fuels such as ammonium nitrate, sawdust, wood meal, etc., these materials were able to contribute to the explosive reaction and provide additional energy. They have poor water resistance due to the ammonium nitrate, are among the cheapest dynamites available, and vary in strength depending upon formulation. These are referred to by some manufacturers as Extra Dynamites.
  • Ammonia Gelatin dynamites - by adding nitrocellulose to the ammonia dynamite, a more cohesive, water proof product is formed, which is more expensive, more water resistant and generally more powerful than ammonia dynamites. Sometimes referred to as Extra Gelatin Dynamites. Semi-Gelatin Dynamites - less nitrocellulose is added to the dynamite mixture than with Ammonia Gelatin Dynamites, with reduces the water resistance somewhat. They are slightly cheaper than Ammonia Gelatin dynamites. Permissible Dynamites - contain flame dampeners and must be approved for use in underground blasting for safety reasons. By adding salt and other flame reducers, the intensity of the explosion and flame can be reduced, thus reducing the likelihood of accidental initiation of methane and/or coal dust.
  • The addition of water gels or emulsions to an ANFO mixture can provide the blaster with considerable benefits. The mixture has greater water resistance (depending upon the proportions), higher weight and bulk strength, higher detonation pressures and velocities of detonation. In the proper ratios, the power of a Heavy ANFO or pumped blend can be almost two times that of regular ANFO, as measured by Relative Weight Strength, Relative Bulk Strength and Detonation Pressure. Water resistance can increase from no water resistance (ANFO) to Excellent Water Resistance (45:55 blend), allowing the use of blasting agents in wet boreholes.
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