2“Explosives”: A Definition An explosive is a substance that when initiated,exerts a sudden and intense pressure on its surroundings,by the rapid formation of large quantities of gas.INTRODUCTION TO EXPLOSIVES: (5 slides, 7 minutes)SLIDE 1 of 5: DEFINITION OF EXPLOSIVES (1 min)An “explosive” is a substance that, when initiated, exerts a sudden and intense pressure on its surroundings, by the rapid formation of large quantities of gas.There are certain types of ‘explosion’ which do not use what we would call ‘explosives’. Examples include lightening during a storm, a volcano, or a high pressure cylinder such as in a water heating system that might ‘explode’.However, given that we are concerned with ammunition and weapons, we are going to use the more refined approach and focus on ‘explosives’.We will look at this in more detail during this lesson, but just to tease out a few points from this definition:The ‘substance’ could be a solid material, a liquid or indeed a gas. It might be a solid, hard ‘lump’ or it might be a slurry. There are many types of explosive, and there are many types of role that these materials have to achieve.‘Initiation’ is the act of providing a stimulus to the explosive, such that it will react to form a chemical reaction. This is essentially the act of transferring energy into the explosive such that the chemical reaction occurs. “Setting off an explosive” is a term used by the layman. For us, as professionals in this area, we will use the term “initiate” – this is the correct technical term.The sudden pressure is due to the rapid formation of large quantities of gas as the result of the chemical reaction.The gas is formed during the chemical reaction – it is a product of the reaction.
3Products of an Explosion NoiseHeat/FlashBlast/ShockwaveAnd, depending on if there is another material involved: Projectiles/FragmentsINTRODUCTION TO EXPLOSIVES: (5 slides, 7 minutes)SLIDE 2 of 5: PRODUCTS OF AN EXPLOSION (2 min)The chemical reaction is what we see as an ‘explosion’.The usual products of an explosion are:Noise, which may be extremely loud.Heat and Flash, so the explosion appear to us as very hot and very bright.A blast effect, which is an area of high pressure in the air formed by the gases, which move outwards from the explosive material as the chemical reaction occurs. Note that in the case of an underwater explosion, the pressure also moves through the water – in a similar manner to as through air although there are some specific phenomena that are present in underwater explosions, however we won’t look at these here.Another effect of an explosion that is often seen is that of the projection hazard, or ‘fragmentation’ hazard. A ‘lump’ of explosive, on it’s own, will not produce fragmentation. It will only produce the blast, light and heat/flash. However when the blast passes through or around physical objects, the solid material will be projected away from the explosion.Let us consider three examples, each using 250grams of plastic explosive:a. The explosive, if initiated in an entirely sterile environment so that the blast doesn’t interact with anything, would not, by itself, produce any fragmentation.b. The explosive might be encased by a steel body (like a grenade body or a shell casing). In this case, the metal casing will be damaged and parts of the body will be projected outwards. This could be as carefully designed fragments if the casing is designed to produce a pre-determined fragmentation pattern, or it might be random shards of various sizes including a large part of the shell casing such as the base.c. The explosion might produce no, or minimal, fragmentation itself if there is no casing, however if the blast comes into contact with sufficiently vulnerable structures and objects, these solid objects might be damaged and the broken material will be projected away. For example, a bare charge that is initiated close to a large glass window will produce a very significant and dangerous shower of high velocity glass shards which will kill or very seriously injure people stood in their path.
4Chemical Composition of an Explosive Explosives need two fundamental components:Fuel = something that releases energyOxidiser = something to sustain the release of energyINTRODUCTION TO EXPLOSIVES: (5 slides, 7 minutes)SLIDE 3 of 5: CHEMICAL COMPOSITION OF AN EXPLOSIVE (1 min)In order to have a chemical reaction, the explosive material needs to have two key parts:A fuel, which is something that releases energy.An oxidiser, which is in simple terms something that sustains the release of energy.
5Sensitivity v Sensitiveness Sensitivity: How responsive an explosive material is to an INTENTIONAL stimulusSensitiveness: How responsive an explosive material is to an UNINTENTIONAL stimulusA firing pin hitting a percussion cap on a cartridge case for rifle ammunitionA fork lift truck fork piercing an artillery shell cartridge caseINTRODUCTION TO EXPLOSIVES: (5 slides, 7 minutes)SLIDE 4 of 5: SENSITIVITY AND SENSITIVENESS (1 min)These are two often used terms. As the words are similar, there can be confusion about which one is which.Sensitivity describes how responsive an explosive material is to an intentional stimulus. For example, the ‘primary explosives’ used in detonators are very sensitive to the usual methods of initiating a detonator, for example by using electricity or flame or percussion. Conversely, military demolition charges which use high explosives such as PE4 are quite sensitive to the designed stimulus used in an explosive train, such as from a detonating cord booster.However, while the primary explosive will be highly responsive to unplanned stimulus such as someone standing on a detonator, the bulk demolition explosive is relatively unlikely to detonate just because someone stands on it. Again, don’t try this at home though!So in terms of this unintentional – we could say accidental – stimulus, the primary explosive is said to have a high level of sensitiveness, whereas the bulk charge has a low level of sensitiveness.You might hear this concept referred to as a ‘figure of insensitivity’.
6Effects of Containing Explosives Containing an explosive creates a more significant explosion (greater power = pressure)Within the container: pressure increases, so temperature increases, which results in an increase in pressure = a reciprocal effectThis lasts milliseconds but is still significant‘Contained’ explosion will usually be significantly more powerful than an uncontained oneINTRODUCTION TO EXPLOSIVES: (5 slides, 7 minutes)SLIDE 5 of 5: EFFECTS OF CONTAINING EXPLOSIVES (2 min)If we contain an explosive material and then initiate it, the effects of the explosion will be more significant –we might say greater.The pressure is contained within the container, meaning the temperature increases, which results in an increase in pressure = a reciprocal effect. This magnifies the effect. This only lasts milliseconds but is significant – we are after all talking about extreme engineering here.This will be the case even if the explosive is only enclosed in a cardboard tube. Some military forces have and do use sound simulators (the UK army has traditionally called these “Thunder Flashes” for example). These contain a low explosive in a cardboard container – the effect of the flash and the noise is significantly more pronounced that the same quantity of, say, gunpowder in an unconfined pile.Containment is relevant to us if we consider:Shell casingPackaging boxesLogistic over-packsThe building the explosive is stored in
7The difference is simply how that chemical reaction occurs Types of ExplosiveLow ExplosiveHigh ExplosiveAn explosion is a chemical reaction between appropriate materials which results in the sudden release of heat, noise, and pressureThe difference is simply how that chemical reaction occursTYPES OF EXPLOSIVES: (5 slides, 8 minutes)SLIDE 1 of 5: TYPES OF EXPLOSIVE (1 min)There are really two types of explosive. You will have heard of these:Low Explosives, and High Explosives.We now know that an explosion is essentially a chemical reaction.The difference between high and low explosives is HOW this chemical reaction occurs.
8Low Explosives Deflagrate (they burn, although very rapidly) Types of ExplosiveLow Explosives Deflagrate (they burn, although very rapidly)High Explosives Detonate (this is a change to the molecular structure and happens instantly, rather than ‘burning’)TYPES OF EXPLOSIVES: (5 slides, 8 minutes)SLIDE 2 of 5: TYPES OF EXPLOSIVE (1 min)The chemical reaction in low explosives is essentially a burning action. It is a very, very rapid, burning, which is almost instantaneous.Conversely:The chemical reaction in high explosives is an instantaneous chemical change to the molecular structure. This is not burning. This is called “detonation”.
9Low Explosives “Deflagration” – very rapid burning The burning takes place along a surfaceThe rate of deflagration is determined by :Surface areaMaterial densityThe surrounding pressureTYPES OF EXPLOSIVES: (5 slides, 8 minutes)SLIDE 3 of 5: LOW EXPLOSIVES (2 min)The burning, or deflagration, occurs along a front at the surface, and this burning front travels at subsonic speed. Imagine a line of gunpowder in the old cowboy films burning along towards the barrels of gunpowder in the old abandoned mine, if you like. The technical term for this kind of very rapid burning is “deflagration”.The rate of deflagration is in part determined by the surface area, the density of the low explosive material, and the surrounding pressure. For example, a solid ‘lump’ of compressed gunpowder would be expected to have a different burn rate than a loosely powdered substance.The higher the pressure, the faster the rate of burn as well. This is most easily seen if one takes a length of burning fuze, which has a fairly predictable burn rate. However if the fuze is stood on, the pressure inside increases and the burn rate actually speeds up. The picture illustrates that burning fuze can be held in the hand quite safely as the gunpowder contained within the waterproof sheath burns along its length.Gunpowder is a low explosive materialA type of ammunition containing a low explosive is burning fuze
10High ExplosivesA supersonic shockwave which travels through the materialThe “Velocity of Detonation” (VOD)Detonation rate is not determined by material density, surface area or outside pressureTYPES OF EXPLOSIVES: (5 slides, 8 minutes)SLIDE 4 of 5: HIGH EXPLOSIVES (3 min)The chemical reaction in a high explosive also progresses through the material in a wave – called a detonating wave - but this occurs, effectively, throughout the entire material. The speed at which the detonating wave travels significantly supersonic speed and this is called the “Velocity of Detonation” or “VoD”. To give you an idea, the VoD for a high explosive, often used in detonating cord, is between 7000 and 8000 meters per second! So if I initiated a length of detonating cord from 10 miles away, and the other end was under your chairs, you would in theory have 2 seconds to get away. In practice, it is unlikely that this amount of detonating cord would actually propagate the detonating wave that distance due to its physical characteristics.The effect is that high explosive generally produces a much more powerful or significant effect than the same quantity of low explosive.The photo on the slide shows a British military ammunition box which is made of metal and is quite robust. A detonator had been taped to the inside wall of the box and the lid sealed. The detonator was then initiated. The pea-sized amount of high explosive in the detonator punched a hole through the side of the box and ruptured and deformed the sealed container. During the actual event, the box and lid were thrown about 10 meters into the air.Detonators contain a pea-sized amount of high explosive.This metal ammo box had a single detonator inside it.
11High Explosive Brisance High Explosives produce an effect upon adjacent metal“Brisance” - shatters the metalIn grenades, artillery shells and mortar bombs this is used to produce fragmentationThis does not happen with low explosivesTYPES OF EXPLOSIVES: (5 slides, 8 minutes)SLIDE 5 of 5: HIGH EXPLOSIVE BRISANCE (1 min)Another property of a high explosive, which low explosives do not possess, is that of “brisance”. This shatters metal that is in contact with the explosive. This is used as a design feature in grenade bodies, mortar bombs and artillery shells to produce highly effective fragmentation in terms of achieving a lethality effect on targets.
12The Explosive TrainA method of initiating an explosive “main charge” using a sequential assembly of explosive componentsTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 1 of 7: THE EXPLOSIVE TRAIN – CONCEPT (1 min)The “explosive train” may be seen as a method of initiating an explosive ‘main charge’ using a sequential assembly of explosive components.Of note here, the main charge is taken to mean the final component in an explosive device, for example. This might be a large quantity of bulk demolition explosive if one is trying to blow up a bridge or a blind shell. It might also be the high explosive payload in missile which functions to cause the desired target effect at whatever it is fired at.
13The Explosive Train – An Analogy Think of how you start a barbeque: You don’t hold a match to the charcoal …you use a match to a firelighter… …and the firelighter to the charcoalThis is a more efficient transfer of energy from a small volatile and short lived device, to something that holds and ‘boosts’ the energy of the match, into something that is difficult to light but, once lit, delivers the desired resultTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 2 of 7: THE EXPLOSIVE TRAIN – AN ANALOGY (2 min)Another way to think of why we might need an explosive train is think of a comparison, in this case lighting a barbeque.We just wouldn’t hold a match or a cigarette lighter straight to a piece of charcoal. We know that the energy in the match flame is, by itself, insufficient to ignite the charcoal.We need some way to take the energy of the match and boost it, so that the energy is magnified in a way that is sufficient to ignite the charcoal. We need some sort of intermediary in the process that takes the energy form the match and magnifies it sufficiently and for long enough to ignite the charcoal. In practice, this is a fire lighter.The firelighter, once lit by the match burns more intensely and for longer that the match. The flame produced provides sufficient energy to the charcoal to get that to bur too.
14The Explosive TrainThe explosive train is the same: It takes a small, highly sensitive explosive (so sensitive you wouldn’t want a large quantity or it as this would be potentially highly risky)… …to transmit its energy into a booster that magnifies this power… …which in turn is sufficient to initiate a main chargeTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 3 of 7: THE EXPLOSIVE TRAIN (1 min)The explosive train is basically the same as this analogy.This applies to both low and high explosive systems.Read over the slide and think about the similarity between the analogy of lighting a barbeque and an explosive system such as an item of ammunition.
15The Explosive Train Detonator Detonating cord High explosive main charge, eg SemtexTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 4 of 7: THE EXPLOSIVE TRAIN (3 min)This illustration shows a conceptual explosive train. In this case it related to high explosives.Let us first consider primary explosives, which are used in detonators. These are powerful explosives, but they have a high level of sensitiveness and sensitivity. This would make them to susceptible to unplanned stimuli, and so they are not used in any quantity within an explosive device, such as apices of ammunition.Conversely, materials which are usually used as main charges in military ordnance are not as sensitive (for example a material called “RDX” is often used as the main charge in artillery shells).For this reason, a small quantity of primary explosive is used in a detonator. This is initiated but, like a match, ideally needs its power magnifying before the detonation is propagated sufficiently to the main charge.Detonating cord is therefore used. Usually this contains a powdered high explosive which can have detonation induced in it by a detonator, but which by itself might not make a desirable main charge. However this powdered high explosive, for example “PETN” will propagate the detonation into the main charge (for example RDX) such that sufficient energy is imparted into the RDX to make it detonate as well.Thus, a chain is created to achieve a desired effect. What I have just described is an example of an explosive train.
16Examples of Initiators For high explosives: a detonator For low explosives: a matchTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 5 of 7: EXAMPLES OF INITIATORS (1 min)Initiators include, for example, matches (for low explosives) and detonators (for high explosives)Methods of initiation include:PercussionChemicalFrictionElectricalLaser
17Examples of Boosters For high explosives: detonating cord For low explosives: burning fuzeTHE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 6 of 7: EXAMPLES OF BOOSTERS (1 min)Typical boosters, or intermediaries, include high explosive detonating cord or low explosive burning fuze.
18Examples of Main Charges For high explosives: PE4 (Plastic Explosive Number 4) For low explosives: Gunpowder (eg in noise simulators)THE EXPLOSIVE TRAIN: (7 slides, 10 minutes)SLIDE 7 of 7: EXAMPLES OF MAIN CHARGES (1 min)Examples of main charges include high explosives for plastic explosive demolition charges, like PE4, of low explosives used to produce a noise and flash for training effects.
19Blast and ShockBlast wave: an area of pressure expanding supersonically outward from an explosive coreExplosive materialBLAST AND SHOCK: (8 slides, 10 minutes)SLIDE 1 of 8: BLAST AND SHOCK (part a) (2 min)The ‘blast’ effect is really just the pressure of the expanding gases which are formed during the explosion. This pressure expands outwards from the explosion and forms an area of extremely high pressure. This area moves out and the effect is seen as a blast wave. The blast wave moves outwards from the explosion.The diagram on the slide illustrates this.We are talking here about the shock wave in air emanating from an explosive, rather than the shockwave which passes through the material during the actual process of detonation of the explosive.Furthermore, as alluded to earlier, this area of pressure will also apply to an underwater explosion, however the fluid dynamics involved in underwater explosions are different – we will not go into this further here, but do be aware of this difference.Area of high pressure expanding outward
20Blast and ShockIn our context, it is an area of highly increased air pressure which moves outwardExplosive materialBLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 2 of 8: BLAST AND SHOCK (part b) (1 min)In our context the blast is an area of highly increased air pressure which moves outwards from an explosion.Area of high pressure expanding outward
21Shock FrontThe “shock front” (or shock wave) is the leading edge of the area of expanding pressureIt is made up of rapidly expanding gasesBehind the shock front is an area of negative pressureAs blast effect emanates out from an explosive, it has a shock front of highly increased pressure followed by a wave of negative pressureBLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 3 of 8: SHOCK FRONT (1 min)The “shock front” (or shock wave) is the leading edge of the area of expanding pressure. it is made up of the rapidly expanding gasesBecause of the physics involved in explosive science, as the shock front (or shock wave) moves outwards, it is followed by an area of negative pressure behind it. In massive explosions this may be experienced as the shock from hitting a target as it moved passed it followed by a sucking wind. This phenomena is typical of massive scale explosions like a nuclear explosion.
22The Pressure-Time Curve Pressure (P)TimePambientPmaxPminArrivalPositivePhaseDurationNegativeBLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 4 of 8: THE PRESSURE-TIME CURVE (2 min)This combination of effects is demonstrated here using a graph.At the point of initiation, there is an instantaneous increase in pressure as the chemical reaction occurs to rapidly form expanding gases. This forms an instantaneous peak. At the point of initiation, this peak then drops off back to normal atmospheric pressure. However the area of negative pressure then forms, so the pressure will drop to below that of normal atmosphere. This will then return to normal.This pressure increase emmenates out from the point of detonation. The arrival time will be dependent on the distance from the point of detonation, however it will be of the order of milliseconds.
23The Pressure-Time Curve INSERT VIDEOBLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 5 of 8: THE PRESSURE-TIME CURVE (1 min)(Instructor: If you wish, you may want to show a video at this point to show an explosion Less than 1 minute length!), where the blast effect can be clearly seen emanating outwards from the explosion. Try YouTube… at around the 4-minute mark. If you do choose to, you may wish to ‘unhide’ this slide as a prompt).
24Impulse The area under the curve shows the “impulse” The impulse largely determines the extent of damage caused by the blastA bigger/longer impulse (ie area under the graph) will cause more damageFor example, ‘Volumetric’ (or fuel/air) explosives prolong the explosion and development of blast, which increases/prolongs the impulse, to create more damage to structures.BLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 6 of 8: IMPULSE (1 min)The area under the curve shows the “impulse” – POINT THIS OUT TO STUDENTS.The impulse largely determines the extent of damage caused by the blast.A bigger and/or longer the impulse (ie area under the graph), the more damage will be caused.For example, ‘Volumetric’ (or fuel/air) explosives prolong the explosion and development of blast, which increases/prolongs the impulse, to create more damage to structures.
25The Effects of Blast High pressure ‘shock front’ Moves faster than the speed of soundLasts only a few milliseconds at each point along its pathHigh pressure ‘shock front’Area of low ‘wind’ behind itBlast interacts with physical materialProjected by blast at high velocity (and energy)Projectiles/fragmentationBLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 7 of 8: THE EFFECTS OF BLAST (PART A) (1 min)The blast will also interact with physical material it passes, which may be damaged by the blast. The damaged material will be projected by the blast as projectiles/fragmentation, at high velocity and with high energy.The blast effect dissipates very quickly in free air, especially relative to the danger area for fragmentation. For example, a hand grenade’s main damage mechanism is fragmentation. The danger area for this is significant. However if the same quantity of explosive was used as a ‘bare charge’ such that the only damage mechanism was blast overpressure, then one could, in theory, stand within, say, 10 meters of the charge and suffer little more damage than perforated ear drums. That said, DO NOT try this at home! Add fragmentation into the mix, however, and you will almost certainly die.Broken glass is a significant cause of injuries in bomb attacks in urban areas
26The Effects of Blast Lagos, Nigeria, 2002 BLAST AND SHOCK: (9 slides, 10 minutes)SLIDE 8 of 8: THE EFFECTS OF BLAST (PART B) (1 min)To illustrate this further - the photo shows what was left of part of Lagos in Nigeria after an ammunition depot explosion in 2002.Lagos, Nigeria, 2002
27Pressure Impulse Curves “P-I Curves”Graph showing impulse against pressureCurve shows susceptibility of a given structure to pressure and impulseMORE DAMAGEImpulse – sensitiveregionCurve of constant damagePeak PressureDynamically –SensitiveRegionPressure – sensitiveregionPRESSURE IMPULSE CURVES: (1 slide, 5 minutes)SLIDE 1 of 1: PRESSURE-IMPULSE CURVES – “P-I CURVES” (5 min)A Pressure-Impulse (P-I) curve may be generated to show response over a wide range of blast loading. The P-I curve generally takes on the form shown on the slide (noting this is a generic, representative curve for illustration purposes).The pressure – impulse (P-I) curves define different damage regions for buildings and other structures which could be useful design toolsfor generating vulnerability and design information for assessing the hazards of, for example, masonry walls within that structure when subjected toblast loads.LESS DAMAGEImpulse
28Questions? Are there any questions? (allow 5 minutes for questions) If there are few or no questions, take the opportunity to test their attention and understanding by asking about the difference between high and low explosives, effect of containing an explosive, the explosive train and parts thereof, the shockwave and impulse.C3 Slide 28