C. Effects of Explosions

An explosion is a gas dynamic phenomenon there are a variety of effects of the spherically expanding heat and pressure wave. These effects assist the investigator in understanding the dynamics of the explosion and the sequence of events that took place during the explosion.


1. Blast Pressure Front Effect

The initiation of chemical explosives and fuel gases result in the generation of quantities of gases. These gases expand at a high speed and move outward equally in all directions until an obstruction is encountered.

a. Positive Pressure Phase


The positive pressure phase is when the blast pressure front is moving away from the point of origin. The positive pressure phase is stronger than the negative and is responsible for most of the damage.

(1.) Spherical shape of the Pressure Wave

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Under ideal conditions the shape of the blast front from an explosion is spherical in nature, moving outward from the origin equally in all directions. However, obstructions or vents will change the configuration of the blast front.

(2.) Rate of Pressure Rise vs. Maximum Pressure

The damage caused as a result of the blast pressure front of an explosion is a result of two items. The maximum pressure and the rate of pressure rise. If the rate of pressure rise is rapid, then the containing vessel will not have sufficient time to resist the forces.

For information concerning pressures, see NFPA-921 (1998) p921-92 Table 13-13.3.1(b), NFPA 68, Guide for Venting of Deflagrations, and Explosion Investigation and Analysis, Investigations Institute, Chicago, IL.


Pressure Damage Chart

Restricting Element

Typical Failure Pressure (PSI)

Light partition walls dislodged

(Dry wall on Wooden Studs)

 

0.3 - 0.7

Glass windows  

(Broken or Dislodged)

 

 0.3 1.0

Vehicles Overturned

6.0 8.0

        Chart from Explosion Investigation and Analysis pg.45

b. Negative Pressure Phase

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As a result of the movement outward from the origin of the positive pressure phase, a low pressure area is created. This low pressure area allows the movement back towards the origin of the surrounding air. The negative pressure phase causes additional or secondary damage, and it is common to find the debris moving towards the point of origin or in opposite directions from the positive pressure phase.

2. Fragmentation (Shrapnel) Effect

Containers, structures, or vessels that contain or restrict the movement of the positive pressure fronts may rupture and produce pieces of debris that may travel long distances and cause considerable damage or injury. The distance that they may travel depends on a variety of factors, including the types of fuels involved, the strength of the container, type of initiation, and venting that may be present.

a. Fragmentation

Debris from the container, for example the structure.

b. Shrapnel

Debris that was placed in the area of a device, in order to inflict harm to those in the area.

3. Thermal (Incendiary) Effect

Combustion explosions release quantities of energy that may heat light solid combustibles or gases present to their ignition temperature. Often diffuse fuels (vapors or gases) or high surface to mass solids (example flexible urethane foam cushions or vehicle interiors) are more likely to be ignited as a result of the thermal effects of an explosion. Dense solid fuels are not as likely to be ignited due to the duration of the heating that takes place.

a. Heat/Thermal

High Temperature Gas

Often sufficient to ignite diffuse fuels or lightweight fuels.

4. Secondary Blast Pressure Or Ancillary Effects

a. Reflection

Reflection is caused when the blast pressure front contacts an object and as a result the front moves in a different direction. This "reflection" often causes damage in other directions or locations.

Refraction is often caused by different temperature layers. Blast focusing can be caused as a result of reflection and refraction. The key is that the damage can be amplified as a result of the movement of the blast pressure front in one direction. Often in cities this damage can be increased as a result of the configuration of the street and surrounding structures.

b. Seismic

As the blast pressure wave expands, and damaged portions of buildings collapse, significant localized seismic waves may be transmitted throughout the ground. Velocity of the wave and the affect of the wave will vary depending on soil conditions.

c. Water

As water is basically a non-compressible product. A shock wave will transmit through water or other with limited reduction in strength for long distances.

d. Ceiling

A low ceiling can have a dramatic effect on reflection of the blast front and resultant damage to the surrounding area. Given similar types of materials and the quantity present, with only a change in the ceiling there will be a noticeable change in the blast front.

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D. Factors Controlling Explosion Effects

There are a number of factors that effect explosion effects. The nature of these factors and their various combinations in any one explosion incident can produce a wide variety of physical effects that the investigator may encounter. Factors including the following:

1. Nature of the Fuel

The type of fuel, mixture with air by volume, location of the fuel in the containing vessel, and method of initiation may have an effect on the condition of the scene and evidence found. In addition, depending on the fuel present, subsequent explosions may occur and continue to alter the scene.

a. Dust

b. Gas

c. Liquid Vapor

d. Explosive

2. Quantity of the fuel present

While it is simple to think about the damage that might be caused by 10 sticks of dynamite vs. 1 stick in a building, consider the damage as a result of the formation of gasoline vapors in a structure from 1 gallon vs. the effects of 10 gallons.

3. Configuration of the Fuel

Homogeneous mixture of vapors or gases in a compartment or structure, may produce greater overall damage to the structure. Dust particle size will also have a bearing on the rate of reaction as well as others.

4. Containment Vessel

Considerable information can be provided concerning the effects of the containment vessel and its effect on the explosion and resultant damage. Basic containment vessels can include the wrapping of the explosive material; paper, wax impregnated Kraft paper, thin plastic coverings used on "Tovex", metal coverings of Detonators, rooms, buildings, configuration of dust collection systems and explosion venting of facilities. The amount of damage and resultant evidence of the explosion is dependent on the following items:

a. Size and volume

b. Shape

c. Construction

(1.) Materials

(2.) Design

5. Venting

Venting is a term used to describe the release of pressure as a result of structural failure of the confining vessel or the operation of specifically designed "vents". Venting of the containing vessel can have a dramatic effect on the resultant explosion scene. When evaluating explosion scenes it should be remembered that the rate of pressure rise is more important than the maximum pressure attained. Consider slow and fast rates of pressure rise and damage. The maximum pressure attained by any explosion is limited by the venting capabilities.

a. Size of the Vents

b. Location of the Vents

Rate of Pressure rise more important than the maximum pressure