Explosives are tested to determ ine their chemical stability, sensitivity, and level of performance.
The chemical stability of explosives is checked in the same manner as is the chemical stability of propellants; the methods involved are described in Section 1.3.5.
Checks of sensitivity are made with reference to friction, impact and heat.
In tests of sensitivity to friction, the explosive to be tested is placed between two rough porcelain surfaces, which move across one another. The surfaces are pressed against one another with increasing load until the explosive begins to react. The value of the load applied at the time a reaction begins, serves as the measurement of sensitivity to friction.
To determine sensitivity to impact, the drop hammer method is used. Here, a hammer of definite w eight is dropped on the explosive to be tested from increasingly greater heights. The height from which the hammer is dropped when the reaction begins, is m ultiplied by the weight of the hammer, to give the value of sensitivity to impact.
Sensitivity to heat is determined by means of the "steel sleeve test". A certain amount of the explosive to be tested is placed in a steel sleeve. The sleeve is closed with a cover that has a hole in it. When the steel sleeve is heated with a gas burner, the material explodes, and the exhaust gases escape through the hole in the cover. The hole is made progressively sm aller until the pressure released by the reaction of the explosive becomes so great that it destroys the walls of the sleeve. The diameter of the hole is taken as the sensitivity to heat.
For a detailed description of these sensitivity tests— which were developed by The Federal Office For Materials Testing, Berlin the reader is referred to references [18] and [19] in the bibliography. In terms of performance data, it is the detonation velocity which is of prim ary interest. In homogeneous explosives the constant detonation velocity is attained after a starting distance. The measurement of velocity involves determining how long it takes for the "fro n t” of the detonation to travel a specified distance along a column of the explosive. To avoid having too short a time interval to measure the velocity with precision, it is necessary to ensure that the measurement distance is not too short (i.e., it should be between 100 and 300 mm). The elapsed time is measured with electronic timers (counters). The counter is started and stopped by open double w ires placed in the explosive at the start and finish of the measurement distance. The w ires are closed when the explosion wave passes across them.
The classic procedure is that developed by DAUTRICHE. He used a detonating cord of precisely determined detonation velocity to measure the time. The DAUTRICHE method is depicted schemati cally in Figure 107.
Blasting caps 1 and 2 are placed in the test sample at measuring points 1 and 2 which are separated by the distance /. These caps relay the explosion to the two ends of a detonating cord, when the explosion front of the sample of the explosive passes across
Figure 107. Arrangem ent for measuring the detonation velocity in the Dautriche method.
them. There is a lead plate under the center of the detonating cord, where a notch arises at the point at which the two detonation waves, advancing towards each other, come together. The distance of this meeting point from the center of the cord is equal to “ a” ; this means that the detonation velocity D of the sample (with Dz equal to the detonation velocity of the cord) is equal to
In addition to the method described above, the detonation velocity can be measured by using the streak camera. In this procedure, the light trace of the detonation front is recorded as it moves through the explosive and the results are then analysed. This method also provides inform ation about the variations of the detonation velocity caused by inhomogeneity of the explosive. These variations can also be measured electronically.
Other performance data, concerning the “ strength" or the brisance of an explosive, can be obtained by using the TRAUZL lead-block test, the ballistic m ortar, the sand test, and the upsetting test of KAST or HESS.
In TRAUZL's lead-block method, the sample explosive and an electric detonating cap are placed in the hole of a lead cylinder, covered with sand, and exploded. The increase in the volume of the hole caused by the explosion is then measured.
The ballistic m ortar consists of a pendulum mounted cylinder with a hole at one end. A sample of the explosive to be tested (10 g) is placed in the hole. The opening is sealed with a projectile of
appropriate size. When the explosive is detonated, the impulse is transferred to the projectile and to the mortar, which causes the mortar to swing like a pendulum. The pendulum deflection provides a measure of the performance of the explosive. Blasting gelantine ( = 100%) is used for calibration.
In the sand test (USA), the sample explosive is encased in sand of a specific quantity and with grains of a certain size. After detonation, a sieve is used to determine the amount of sand which has been broken into sm aller grains by the explosion. The brisance (shattering power) of an explosive is determined experim entally by placing an explosive charge against a copper cylinder (KAST) or a lead cylinder (HESS) and measuring the amount of compression.
KAST also devised a mathematical formula for determining the brisance of a substance. He defines brisance as the product of charge density, specific energy ( f —nR T) and the detonation velocity.