The properties of explosives

23/07/2004 Southwest Asia Power
[22] : is ability to do work, in practice it is defined as the explosive's ability to accomplish what is intended in the way of energy delivery (i.e., fragment projection, air blast, high-velocity jets, underwater shock and bubble energy, etc.). Explosive power or performance is evaluated by a tailored series of tests to assess the material for its intended use. Of the tests listed below, cylinder expansion and air-blast tests are common to most testing programs, and the others support specific applications.
* Cylinder expansion test: a standard amount of explosive is loaded into a long hollow cylinder, usually of copper, and detonated at one end. Data is collected concerning the rate of radial expansion of the cylinder and maximum cylinder wall velocity. This also establishes the Gurney energy or 2E.
* Cylinder fragmentation: a standard steel cylinder is loaded with explosive and detonated in a sawdust pit. The fragments are collected and the size distribution analyzed.
* Detonation pressure (Chapman-Jouguet condition): detonation pressure data derived from measurements of shock waves transmitted into water by the detonation of cylindrical explosive charges of a standard size.
* Determination of critical diameter: this test establishes the minimum physical size a charge of a specific explosive must be to sustain its own detonation wave. The procedure involves the detonation of a series of charges of different diameters until difficulty in detonation wave propagation is observed.
* Infinite-diameter detonation velocity: detonation velocity is dependent on loading density (c), charge diameter, and grain size. The hydrodynamic theory of detonation used in predicting explosive phenomena does not include diameter of the charge, and therefore a detonation velocity, for an imaginary charge of Infinite diameter. This procedure requires a series of charges of the same density and physical structure, but different diameters, to be fired and the resulting detonation velocities extrapolated to predict the detonation velocity of a charge of infinite diameter.
* Pressure versus scaled distance: a charge of specific size is detonated and its pressure effects measured at a standard distance. The values obtained are compared with that for TNT.
* Impulse versus scaled distance: a charge of specific size is detonated and its impulse (the area under the pressure-time curve) measured versus distance. The results are tabulated and expressed in TNT equivalent.
* Relative bubble energy (RBE): a 5- to 50 kg charge is detonated in water and piezoelectric gauges measure peak pressure, time constant, impulse, and energy.

05/02/2004 Iraq Density: we must distinguish:
- the absolute density, or real density, or specific weight, which is the weight of a certain volume in which the substance is contained without interspaces (also called density of crystal);
- the gravimetric density, or apparent, that is the relationship between the weight of a certain volume of explosive in normal condition and the volume that it occupies. This density is less than absolute density: the difference is small for plastic explosives, larger for those in powder form;
- the density of charge, that is the weight of explosive content in the unit of area of the blasting hole in which will occur the explosion.
All these density are expressed in kilograms/litre.
Increasing the division of grains and the pressure applied increases apparent density but it may never reach the absolute density. With the growth of the density of charge increases the detonation velocity, the explosive wave spreads through the same explosive as a sound wave and the speed of transmission is modified by the generated high pressure, increasing the density, and then declining the compressibility, increases the elasticity modulus and decreases the sensitivity, if we could reach the density of crystal there would not be more detonation.
Starting from small charge density the speed of explosion increases with increasing the density, but up to a certain limit beyond which the speed decreases until to melt away. The density where by speed is maximum is called charge density limit practice and is the one that is appropriate to adopt in mines to have the maximum performance.

Stability: is the attitude of an explosive to resist chemical decomposition, this resistance is characteristic of every explosive, for mixtures of explosives global stability is equal to that of the substance component that has lower stability.
Some compounds tend to decompose already at room temperature, with formation of products that act as a catalyst and make more rapid the phenomenon in time. Decomposition is accelerated by increase in temperature, moisture and light, we can have dangerous situations in storage and in the use of these explosives.

13/10/2004 Florida beach Sensitivity: is its attitude to explode under the action of outside pulse. You can consider sensitivity respect: shock, friction, heat, to initiation[2] .
Sensitivity to shock: normal explosives are sufficiently resistent to shock and not excessive friction , so they can be pushed and compressed, but not too violently. It may lower the sensitivity of some explosives, which would be too sensitive for the use as they are, by substances these stabilizers or phlegmatizers: the phlegmatizer penetrates in the mass of the explosive which remains divided into tiny particles and protected from shock.
Sensitivity to initiation: each explosive requires an external action that should provide an impetus to do that explode, this action is given by a primer, which must act with greater or lesser intensity that depends on sensitivity of the explosive. For very sensitive explosive is sufficient a small impact; for explosives sensitive, but not much, require a stronger impact or the contribution of a flame; explosives with relatively low sensitivity require a strong impact, it is generated by a small explosive wave generated by a powerful explosive (primer). For certain categories of explosives need only a very small charge contained in a detonator; for little sensitive explosives the detonator start a small cgarge of explosive with intermediate sensitivity and is the shock wave of this charge that is the detonator of the other cherge. Black powder does not require detonator, but explodes for simple inflammation.
Sensitivity to heat: high temperature in the environment of conservation of the explosives may facilitate the decomposition. Any explosive, if subjected to increasing temperature, at a certain time ignites or explodes, temperatures of ignition are generally lower than those which corresponding detonation.

Hygroscopicity: is the ability who have some explosives to absorb more or less water atmosphere, with the result of losing in a more or less long time the characteristics explosive.
Here certain categories of explosives with decreasing level of sensitivity to humidity: explosives with ammonium nitrate, black powder, explosives with chlorate and perchlorate, dynamite, picric acid, PETN, RDX, TNT.
Plastic explosives resist relative better than powdery explosives, for some hygroscopic powdery explosives the alternation of greater and smaller moisture in the air causes the agglomeration in a solid mass. The problem may be reduced with the addition of special anti-caking.

Brisance[4] : is a measure of the rapidity with which an explosive develops its maximum pressure.
In addition to strength, explosive materials display a second characteristic, which is their shattering effect or brisance (from the French briser, to break), which is distinguished from their total work capacity. This characteristic is of practical importance in determining the effectiveness of an explosion in fragmenting shells, bomb casings, grenades, structures, and the like. The rapidity with which an explosive reaches its peak pressure is a measure of its brisance. A brisant explosive is one in which the maximum pressure is attained so rapidly that a shock wave is formed, and the net effect is to shatter (by shock resonance) the material surrounding or in contact with the supersonic detonation wave created by this explosive. Thus brisance is a measure of the shattering ability of an explosive.
One of the most brisant conventional explosives is cyclotrimethylene trinitramine (also known as RDX).