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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
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