The practical application of linear shaped explosive for the penetration of armor steel dates back to 1888 and the efforts of C.E. Monroe who discovered that the greatest effect is produced when the explosive is not in direct contact with the steel. Monroe found that by increasing the depth of depression or cavity in the explosive, he was able to increase the depth of penetration for a given explosive quantity. A graphic of the Monroe Effect is shown below. The mechanism of the linear lined charge involves collapse of the metal liner due to the focusing of the detonating explosive high pressure wave as it becomes incident to the side wall. If the standoff distance is optimum, collapse of the liner can be complete, before it reaches the target. A small jet of metal emerges from the base of the cone while collapse continues. The jet from the cone then breaks up into small particles followed by the slug or major portion of the cone. The jet particle and not the slug constitute the penetrating agent. The high velocity jet particles generate high pressure when they impact on the target, measured in terms of several hundred thousand atmospheres. Such pressures so greatly exceed the yield strength of the target material that it is literally pushed aside from the path of the jet by plastic flow.
The penetrating action of the shaped charge is affected by a number of factors. The explosive used is of great importance; and, while the depth of penetration is indicated to be more closely related to the detonating pressure than the rate of detonation, in general, the greater effect is produced by the explosive having the greater rate of detonation. Very little effect is produced by explosives having rates of detonation of 5,000 meters per second or less. Comparative tests with liners of different metals give results that indicate, in general, the depth of penetration is greater with metals of greater density. However, ductility also plays a major role in penetration. The standoff distance or distance between the target and the base of the chevron cavity required for maximum penetration effect varies with the metals used as a liner. With a given liner, there is an optimum standoff distance above and below which less penetration effect is obtained. As stated previously, the jet is the penetrating agent; and, as standoff distance is increased, there is more time in which the jet can become extended. However, after a certain standoff distance, the jet has a tendency to break up both axially and radially.