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Theory of Explosives C3 Slide 1. “Explosives”: A Definition An explosive is a substance that when initiated, exerts a sudden and intense pressure on its.

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Presentation on theme: "Theory of Explosives C3 Slide 1. “Explosives”: A Definition An explosive is a substance that when initiated, exerts a sudden and intense pressure on its."— Presentation transcript:

1 Theory of Explosives C3 Slide 1

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. C3 Slide 2

3 Products of an Explosion  Noise  Heat/Flash  Blast/Shockwave And, depending on if there is another material involved: Projectiles/Fragments C3 Slide 3

4 Chemical Composition of an Explosive  Explosives need two fundamental components: Fuel = something that releases energy Oxidiser = something to sustain the release of energy C3 Slide 4

5 Sensitivity v Sensitiveness  Sensitivity: How responsive an explosive material is to an INTENTIONAL stimulus  Sensitiveness: How responsive an explosive material is to an UNINTENTIONAL stimulus A firing pin hitting a percussion cap on a cartridge case for rifle ammunition A fork lift truck fork piercing an artillery shell cartridge case C3 Slide 5

6 Effects 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 effect  This lasts milliseconds but is still significant  ‘Contained’ explosion will usually be significantly more powerful than an uncontained one C3 Slide 6

7 Types of Explosive Low ExplosiveHigh Explosive An explosion is a chemical reaction between appropriate materials which results in the sudden release of heat, noise, and pressure The difference is simply how that chemical reaction occurs C3 Slide 7

8 Types of Explosive Low Explosives Deflagrate (they burn, although very rapidly) High Explosives Detonate (this is a change to the molecular structure and happens instantly, rather than ‘burning’) C3 Slide 8

9 Low Explosives  “Deflagration” – very rapid burning  The burning takes place along a surface The rate of deflagration is determined by : Surface area Material density The surrounding pressure Gunpowder is a low explosive material A type of ammunition containing a low explosive is burning fuze C3 Slide 9

10 High Explosives  A supersonic shockwave which travels through the material  The “Velocity of Detonation” (VOD) Detonation rate is not determined by material density, surface area or outside pressure Detonators contain a pea-sized amount of high explosive. This metal ammo box had a single detonator inside it. C3 Slide 10

11 High Explosive Brisance  High Explosives produce an effect upon adjacent metal  “Brisance” - shatters the metal  In grenades, artillery shells and mortar bombs this is used to produce fragmentation  This does not happen with low explosives C3 Slide 11

12 The Explosive Train A method of initiating an explosive “main charge” using a sequential assembly of explosive components C3 Slide 12

13 The 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 charcoal This 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 result C3 Slide 13

14 The Explosive Train The 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 charge C3 Slide 14

15 The Explosive Train High explosive main charge, eg Semtex Detonating cord Detonator C3 Slide 15

16 Examples of Initiators For high explosives: a detonator For low explosives: a match CO Slide 16

17 Examples of Boosters For high explosives: detonating cord For low explosives: burning fuze C3 Slide 17

18 Examples of Main Charges For high explosives: PE4 (Plastic Explosive Number 4) For low explosives: Gunpowder (eg in noise simulators) C3 Slide 18

19 Blast and Shock Blast wave: an area of pressure expanding supersonically outward from an explosive core Explosive material Area of high pressure expanding outward CO3 Slide 19

20 Blast and Shock In our context, it is an area of highly increased air pressure which moves outward Area of high pressure expanding outward C3 Slide 20 Explosive material

21 Shock Front  The “shock front” (or shock wave) is the leading edge of the area of expanding pressure  It is made up of rapidly expanding gases  Behind the shock front is an area of negative pressure  As blast effect emanates out from an explosive, it has a shock front of highly increased pressure followed by a wave of negative pressure C3 Slide 21

22 Pressure (P) Time P ambient P max P min Arrival Time Positive Phase Duration Negative Phase Duration The Pressure-Time Curve C3 Slide 22

23 The Pressure-Time Curve  INSERT VIDEO C3 Slide 23

24 Impulse  The area under the curve shows the “impulse”  The impulse largely determines the extent of damage caused by the blast  A bigger/longer impulse (ie area under the graph) will cause more damage  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. C3 Slide 24

25 The Effects of Blast  High pressure ‘shock front’  Area of low ‘wind’ behind it  Blast interacts with physical material  Projected by blast at high velocity (and energy)  Projectiles/fragmentation Moves faster than the speed of sound Lasts only a few milliseconds at each point along its path Broken glass is a significant cause of injuries in bomb attacks in urban areas C3 Slide 25

26 The Effects of Blast Lagos, Nigeria, 2002 C3 Slide 26

27 Pressure Impulse Curves  “P-I Curves”  Graph showing impulse against pressure  Curve shows susceptibility of a given structure to pressure and impulse Impulse Peak Pressure Impulse – sensitive region Pressure – sensitive region Dynamically – Sensitive Region Curve of constant damage MORE DAMAGE LESS DAMAGE C3 Slide 27

28 Questions? C3 Slide 28

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