Shock Energy

We have learned from previous article, there are two basic forms of energy released when high explosives react, the shock and gas energy. Now in this article we will learn about the shock energy, while the gas energy you can learn in the next article release.

In high explosives, a shock energy pressure spike at the reaction front travels through the explosive before the gas energy is released. There are, therefore, two distinct separate pressures resulting from a high explosive and only one from a low explosive. The shock energy pressure is a transient pressure that travels at the explosives rate of detonation. The gas pressure follows thereafter.

Shock energy and detonation pressure

The shock energy is commonly believed to result from the detonation pressure of the explosion. The detonation pressure is a function of the explosive density times the explosion detonation velocity squared and is a form of kinetic energy. Determination of the detonation pressure is very complex. There are a number of different computer codes written to approximate this pressure. Unfortunately, the computer codes come up with widely varying answers. Until recently, no method existed to measure the detonation pressure. Now that methods exist to produce accurate measurements, one would hope that the computer codes would be corrected. Until that time occurs, one could use one of a number of approximations to achieve a number that may approximate the detonation pressure. As an example, one could use:

Detonation Pressure formula

Shock energy as kinetic energy

The detonation pressure or shock energy can be considered similar to kinetic energy. Correspondingly with kinetic energy, it is maximum in the direction of travel.  Mean that the detonation pressure would be maximum in the explosive cartridge at the end opposite that where initiation occurred. It is generally believed that the detonation pressure on the sides of the cartridge are virtually zero. That is due to the detonation wave does not extend to the edges of the cartridge. To get maximum detonation pressure effects from an explosive, it is necessary to place the explosives on the material to be broken and initiate it from the end opposite that in contact with the material.

Laying the cartridge over on its side and firing in a manner where detonation is parallel to the surface of the material to be broken reduces the effects of the detonation pressure or shock energy. Instead, the material is subjected to the pressure caused by the radial expansion of the gases after the detonation wave has passed. Detonation pressure or shock energy can be effectively used in blasting. Moreover when shooting with external charges or charges which are not in boreholes. This application can be seen in mud capping. Also we can see in plaster shooting of boulders. The same thing also can be seen in the placement of external charges on structural members during demolition.

Illustration of mud cap blasting
Mud Cap Blasting

To maximize the use of detonation pressure or shock energy one would want the maximum contact area between the explosive and the structure. The explosive should be initiated on the end opposite that in contact with the structure. An explosive should be selected which has a high detonation velocity and a high density. A combination of high density and high detonation velocity results in a high detonation pressure.

 

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