Principles of Explosives Analysis
Explosives are used for a variety of legitimate applications from mining to military operations. However, these materials can also be used by criminals and terrorists to threaten harm or cause death and destruction.
Bombs can be either explosive or incendiary devices, or a combination of the two. An explosive device employs either a liquid, a powder, or a solid explosive material; an incendiary device is flammable and is intended to start a fire.
Explosives are classified according to the speed at which they react. High explosive materials, such as dynamite, Trinitrotoluene (TNT), C-4 and acetone peroxide, react at a rate faster than the speed of sound in that material, causing a loud detonation. These substances are tightly controlled to ensure they are not easily procured.
C-4 high explosive (Courtesy of NFSTC)
Plastic explosive (Courtesy of NFSTC)
A low explosive material, such as gunpowder, smokeless powder or fireworks, burns much slower. Low explosives are often used as propellant. A low explosive must be confined to explode. Note: In the case of a low explosive, this is referred to as a “deflagration,” not detonation, since the reaction occurs at a rate below the speed of sound, in the material.1
Bomb Blast Effects
The effects of an explosion are created through the action of several rapidly occurring events. The most damaging phase of the explosion is the initial blast wave that radiates rapidly outward from the epicenter. This wave of highly compressed air inflicts the primary damage to structures and people.
The initial blast wave, also called shock waves, violently expel fragments of the casing and shrapnel such as nails or screws that may have been added to the device. In addition to the shrapnel from the device, when an explosion occurs in or near a building, fragments from high-velocity glass shards or other building materials can be a large contributor to injuries.
The blast may also produce high heat and a fireball that could start secondary fires or explosions. In most explosions the seat of the explosion should be apparent, but structural damage and physical damage to vehicles can be used to approximate the location of the seat of the explosion.
Common Explosive/Incendiary Devices
Common pipe bomb (Courtesy of NFSTC)
One of the most common explosives encountered by a bomb squad is a pipe bomb. A pipe bomb is a section of pipe that usually contains a low explosive powder mixture. The ends are capped and a fuse is inserted into the explosive powder. Upon explosion, the bomb sends shards of the pipe outward at a rapid speed. If a bomb has a hard casing, such as a steel pipe, large fragments may be found around the blast site. If a bomb has a soft casing, it will generally leave smaller fragments behind. (Image courtesy of FBI)
Improvised explosive devices (IED) - The term “IED” has become commonplace since U.S. involvement in the Iraq and Afghanistan wars. These homemade devices can be delivered in a myriad of different designs—from letter bombs to roadside bombs. IEDs employ at least four key components: a power supply, initiator, explosive material and a switch. A typical IED marries an explosive main charge with electrical fusing system that can contain components from a device such as a mobile phone or garage door opener.
Common IEDs (left to right): remote controlled IED, victim-operated IEDs (2) (Courtesy of NFSTC)
Remote controlled IEDs often use radios or cell phones as the triggering device. Victim-operated IED (VOIED) fusing systems require someone or something to complete an electronic circuit such as stepping on two pieces of metal wire, held apart by an insulator. A VOIED using two metal strips wired, but separated by matchboxes achieve the same effect as the saw blades above and show the diversity of materials that IED triggers can be made from.
A car that has been struck by a relatively small IED offers much information about the device and possibly the network that designed, built and delivered the device. (Courtesy of NFSTC)
Incendiary devices - A common incendiary device used by rioters to start fires is the Molotov cocktail. A Molotov cocktail is a glass bottle containing a mixture of flammable liquids; a rag or fuse is attached and lit just before the bottle is thrown. This ignites the liquid upon impact.
1 Practical Bomb Scene Investigation, James T. Thurman, 2011, Taylor and Francis Group, LLC