Topic 8 - Explosive Effects and Improvised Explosive Devices (IEDs)…
Topic 8 - Explosive Effects and Improvised Explosive Devices (IEDs)
blast velocity lower that 400 m/s.
e.g. nitrocellulose or gun powder.
used in firearm cartridges, rocket motors and blasting devices for mining.
most homemade explosives are LE, e.g ANFO or cordite
Subsonic combustion propagated through thermal conductivity.
Primary High Explosive
v. sensitive e.g lead styphnate or azide or nitroglycerine (primers)
Blast velocity 400-1900 m/s
Secondary High Explosive
relatively insensitive e.g. TNT, RDX. Typically used as main or booster charges for demolition.
Combustion that is a supersonic exothermic front accelerating through a medium that eventually drives a shock front directly in front of it.
Based on pressure rather than thermal conductivity.
Energy release is prompted by the spread of a shock wave in the material.
(result of detonation)
Massive release of chemical energy super heats air and causes flash burn from the radiant energy. Shadowing on surviving structures can aid in determining the centre of detonation (COD)
Detonation causes a compression wave with speed up to 19000 m/s. Overpressure of 12-15 atmospheres at blast centre. Most destructive phase. IED components scattered. Can cause damage far away from COD as the blast waves can reflect, refract and interfere.
Air filling the vacuum left by the blast wave sucks material towards the blast site. Feeds flash fires from fireball with oxygen leading to increased fire damage.
Any debris accelerated outwards by the blast wave.
Created entry and exit wounds. Travel in ballistic arc. Main cause of death, then over pressure injuries.
Depends on IED type (HE, LE), surface type (hard/soft) and placement of the IED (contacting, suspended, in a vehicle). Dimensions of crater useful in predicting type of explosive used.
Is the ability of an explosion to produce a shattering effect. Defines the rate of increase in blast front with respect to elapsed time.
Overpressure wave barotrauma (pressure related injury).
Blast lung, bowel perforation, rupture of tympanic membrane.
Blast energised bomb fragments and other displaced objects.
Bruising, puncture lacerations, penetrating wounds.
Crush injuries, blunt trauma fracture, amputations, open or closed brain injuries.
Collapsed buildings, fires.
Burns, asphyxia (suffocation), exposure to toxic inhalants.
m = (3.25 x D / k)^2
k = assessed value on how hard/soft surface is 1 = hard (concrete), 6 = soft (loose soil or sand)
m = mass of explosive (kg) 8kg is normal
D = diameter (m)
Note: detonation speed is
to be 6.5-8 km/s (standard military HE) and crater is not from a shaped charge.
Crater calculations not accepted as evidence in court. Due to subjective assessment of surface hardness. The formula is a tool for examiner to set cordons at appropriate distance. Can be used in court to justify cordon distance.
Scene, victims, burn marks, area close to detonation. HE residues - only present within flash burn area. LE residues found over wider area.
IED fuse and detonation assembly:
Search area, DNA and fingermarks can be found on components of the device. Component ID numbers. Container will be scattered like the fuse.
Undetonated explosive may be found.
Witness statements e.g. smoke colour, white indicates military grade.
Common IED Types
Pipe bomb (LE or HE), vehicle bomb (HE), letter/parcel bomb (small HE), mortar (LE or LE+HE boosters), mine (HE), adapted ordnance (military HE)
Booster Charges: Detonates main charge. Primary HE detonator detonates single or multiple charges which run through the main charge. Or used where the main charge is LE or homemade.
Focuses the effect of the explosions energy.
Parabolic explosive charge. Concave surface of the surface is covered in copper foil. On detonation, the blast front squeezes the foil towards the charge axis, compresses it then accelerates the copper rod. 17 km/s. Need to be adjacent to target due to small range.
Explosive Formed Penetrator/Projectile (EFP):
EFP charge cover remains solid and forms a heavier (thus slower) projectile. 2-3 km/s. Greater range than shaped charges (100-150 m vs a few metres)
Shaped charges leave minimal forensic data, but can still get: explosive residue, chemical composition and fusing/casing fragments. EFPs more helpful: DNA and fingermarks recovered from projectile, casing disintegrates in same way as a shaped charge.
Measured quantity of explosive material, can be a single or mixture of ingredients.