EXPLOSIVE FORCES
OF IMPROVISED EXPLOSIVE DEVICES
Jeff Slotnick
In today’s world we are constantly bombarded with news of Improvised Explosive Devices (IED’s). Such devices are regularly used against U.S. Forces and security personnel in hostile environments like Iraq, Afghanistan and Colombia. Bombings are clearly the method of choice by most terrorists. For example, improvised explosive devices were used for the 1993 and 2001 bombings of the World Trade Center, the 1995 attack on the Murrah Federal Building in Oklahoma, the 2000 attack on the USS Cole, the 1998 attacks on the East African Embassies, the 2003 Earth Liberation Front bombings in California, and about 13,510 other incidents from 1993 to 1997. These bombings have exacted both a physical and mental toll on their victims, as well as, the general population. Additionally, we are now confronted with homicide bombers, and dirty bombs.
Before anyone can start to study these events and digest the design, construct, and methods chosen by terrorists, it is most important to understand the basics of explosives, explosive forces, and how they work.
Explosives are substances that, through chemical reaction, rapidly and violently change to gas, accompanied by high temperatures, extreme shock and a loud noise. An explosion is the process of the substance transforming into the gaseous state.
Explosives are classified as low or high according to the detonating velocity or speed at which this change takes place and other pertinent characteristics such as their shattering effect. An arbitrary figure of 3300 fps is used to distinguish between burning/ deflagration (low explosive) and detonation (high explosive).
There are three types of explosions atomic, mechanical (characterized by a gradual build-up of pressure in a container until it overcomes the structural resistance of the container and an explosion occurs such as a pipe bomb), and chemical the rapid conversion of a solid or liquid explosive compound into gasses having much greater volume than the substances from which they are generated. The entire conversion takes place in only a fraction of a second and is accompanied by shock, heat, light and a loud noise.
In all chemical explosions, the changes that occur are the result of combustion or burning. Combustion (of any type) produces several well-known effects: heat, light, and release of gas. The burning of a log in a fireplace and the detonation of a stick of dynamite are similar because each changes its form and, in doing so, produces the same effects through combustion. The difference between a burning log and the detonating dynamite stick is the rate of the combustion process.
There are three rates of combustion; ordinary combustion, explosion (Rapid Combustion), and detonation. Detonation can be defined as instantaneous combustion, although there is actually a time interval where combustion passes from one particle of explosive compound to the next. When an explosive is detonated, the block or stick of chemical explosive material is instantaneously converted from a solid into a rapidly expanding mass of gasses.
The velocity of instantaneous combustion has been measured for most explosives and is referred to as the detonation velocity of the explosive. Detonation velocities of high explosives range from approximately 3,300 feet per second (fps) to over 29,900 fps. To bring this speed down to our terms – If we took a five-mile length of garden hose and filled it in with a high explosive and then detonated one end of the hose, it would only take one second for the chemical reaction to reach the other end.
In a detonation, the chemical reaction moves through the explosive material at a velocity greater than that of sound through the same material. The characteristic of this chemical reaction is that it is initiated by and, in turn, supports a supersonic shock wave proceeding through the explosive.”
In a deflagration, the chemical reaction moves rapidly through the explosive material and releases heat or flames vigorously. The reaction moves too slowly to produce shock waves.”
There are two types of Explosives Low Explosives and High Explosives. Low explosives are said to burn or deflagrate rather than to detonate or explode. The burning gives off a gas which, when properly confined, will cause an explosion. Most low explosives are mechanical mixtures or a mechanical blending of the individual ingredients making up the low explosives.
High Explosives do not require confinement to shatter and destroy. It must generally be initiated by a shock wave of considerable force. This is usually provided by a detonator or blasting cap.
The varying velocities of explosives and configuration have a direct relationship to the type of work they can perform. The difference in velocities determines the type of power exerted by high or low explosives. Low explosives have pushing or heaving power and high explosives have shattering power(Brisance).
A high order detonation is a complete detonation of the explosive at its highest possible velocity. A low order detonation is either an incomplete detonation or a complete detonation at lower than maximum velocity.
Explosives have several effects, blast pressure effect (most powerful of all explosive effects). When the explosion occurs, very hot (between 3,000 and 7000 Fahrenheit) expanding gases are formed in a period of approximately 1/10,000 of a second. These gases exert pressures of about 700 tons per square inch on the atmosphere surrounding the point of detonation at velocities of up to 13,000 miles per hour or 29,900 fps. The expanding gas rolls out from the point of detonation like a ripple in the water and is known as the blast pressure wave.
This wave has two distinct phases positive and negative. Positive; the blast pressure wave moves outward from the point of detonation and delivers violent force to everything in its path. It lasts a relatively short period of time and delivers the highest pressures and velocity. Negative; more descriptively known as the suction phase. It is three times longer in duration but of less intensity than the positive phase. It is formed as the out rushing air is compressed and forms a vacuum at the point of detonation. The vacuum causes the displaced air to reverse its movements and return to the point of detonation. This accounts for much of the debris that is found at the seat of the explosion and nearby.
Fragmentation Effect; missiles are produced by the explosive container, objects around the detonation point and the intended target. Fragmentation adds to the destructive force of the explosive device. Fragments can travel at velocities up to 2,700 fps.
Incendiary Thermal Effect can vary greatly from one explosive to another. In general, low explosives will produce longer incendiary thermal effects than will high explosives. A high explosive will produce higher temperatures but for a shorter time. The effect is seen usually as a bright flash or fireball at the moment of detonation. The low explosive fireball is more likely to cause a secondary fire than a high explosive detonation.
Ancillary explosive effects are secondary blast pressure effects (reflected); created by blast waves that are shattered, reflected or shielded by reflective surfaces. The reflective blast wave off of surfaces surrounding it may actually reinforce the original wave by overlapping it in some places (i.e. corners of a room). Certain unusual effects may be noted at a crime scene that can be attributed to the secondary blast pressure effects.
Ground and Water shock; occur when an explosive is initiated while buried in the earth or submerged under water. Both earth and water are less compressible than air and tend to propagate a shock wave further and with more force than air. Therefore, structural damage may be substantially greater under those circumstances where earth and water are involved. Water cannot be compressed at all and, therefore, will transmit energy much faster and farther than any other medium (tamping).
Overpressure; types of overpressure include Incident Overpressure: a result of the explosive pressure wave itself and Reflective Overpressure; a result of the explosive pressure wave hitting a surface and rebounding, increasing the overpressure value. The effect of overpressure on the human body varies depending on; distance from explosion, nature of surroundings, and the age and physical condition of the individual.
I hope this article in some measure has given you the ability to understand explosive forces and their effects. This information is critical to developing appropriate training, polices, and procedures for reacting to bomb threats, especially when it comes to protecting the lives of our families, principals, and employee’s.
About the Author
Jeffrey A. Slotnick, PSP is Regional Director for NWTC Inc. and Lead Instructor for Hazardous Devices; Recognition and Avoidance for the Corporate Security Community. NWTC Inc. is a national non-profit organization committed to low cost professional training of Law Enforcement, Military, and Select Corporate personnel. Jeff’s experience and background stem from his 28 years as an explosives specialist for the United States Army and the Washington State Military Department where he currently holds the rank of Command Sergeant Major. Jeff will be presenting on this material at the ASIS International Emerging Trends Conference and can be reached by email at nwtcjeff@earthlink.net or by telephone at 253-538-9848.
