Detonation

A detonation is an explosion in which the propagation of the chemical reaction in the explosive is coupled with a shock wave . In contrast to a deflagration , i.e. the slow burning compared to the speed of sound of the explosive, there is a bang with a detonation even without containment .

reaction

When explosives are detonated, a very narrow shock front passes through the explosives. It is a shock wave that increases pressure and temperature significantly. The pressure can reach 500 kilobars , the temperature 6000 ° C ; the matter is ionized (becomes electrically conductive) and emits light , recognizable as a detonation flash. The release of the chemical reaction energy requires a rearrangement of atoms, which can take several nanoseconds, corresponding to a width of the reaction zone in the order of one millimeter, depending on the explosive. In it, the density drops roughly to the original value, but the temperature and pressure not so much due to the released reaction energy. On the microscopic scale, this drives the shock front, which would otherwise run out sooner or later due to dissipation , and possibly increases the explosive effect on a larger length and time scale. It helps if small molecules, i.e. gaseous end products, form during macroscopic expansion.

speed

The enormous density and temperature behind the shock front causes it to propagate at a speed, the detonation speed, which is greater than the speed of sound in front of the front, and which only depends on the type of initiation over a start-up distance, then only on the properties of the explosive and the Curvature of the detonation front.

The values for the detonation velocity given in the explosives data apply to a flat detonation front and lie between 1500 and 10000 m / s. High values give shaped charges their penetrative power. Lower values are chosen in mines and quarries, for example. The near area should not be pulverized there, but cracks should appear in a larger area.

The detonation speed depends on the specific energy and the physical density of the explosive, whereby only the reaction energy released within 0.1 µs after the arrival of the detonation front contributes to the detonation speed.

Geometry of the cargo

In the case of an explosives column with a constant circular cross-section, the detonation speed is lower, the smaller the diameter of the column. If a certain critical diameter, which depends mainly on the properties of the explosives and slightly on the strength of the inclusion, is not reached, the detonation cannot propagate reliably along the column and breaks off even after very strong initiation.

pressure

The decisive factor for the strength of an explosive is the detonation pressure, which is roughly proportional to the square of the detonation speed and the density of the explosive. That comes from the relationship

{\ displaystyle {\ frac {p_ {1}} {p_ {2}}} = \ left ({\ frac {V_ {2}} {V_ {1}}} \ right) ^ {k}}

with the correction parameter for a chemically homogeneous explosive. Halving the volume leads to an eightfold increase in pressure . For comparison: applies to the isothermal compression of the ideal gas ; the larger exponent takes into account the quadratic temperature increase that is necessary for the compression. ${\ displaystyle k \ approx 3}$ ${\ displaystyle V}$ ${\ displaystyle p}$ ${\ displaystyle k = 1}$

If a detonation front hits an adjacent body, it is subjected to a very strong acceleration due to the extremely rapid rise to very high pressures. The forces that arise are a multiple of the interatomic binding forces. There is no material that can withstand the detonation shock of a high-explosive substance. In a more or less wide zone, the mechanical and chemical structure of the target material is torn apart by a detonation shock.

Reaction environment

A detonation can except in solid and liquid explosives in explosive gas mixtures and even in nuclear fuel material (eg. As in a Super Nova type Ia) may occur. Contrary to widespread statements to the contrary, in the case of atomic bomb explosions, however, as a rule no detonation occurs in the nuclear component; with nuclear fission bombs, for example, there is no reaction front at all.

The shock front that occurs in the explosive spreads into the surrounding medium after the explosive has been used up and forms a typical detonation wave . However, deflagration can also trigger a shock wave in the surrounding medium if the speed of sound in it is considerably lower than in the deflagrating fuel.

The undesired pre-ignition known as knocking in internal combustion engines can lead to a detonation and cause considerable damage to the engine.

Ideal detonation

If the chemical conversion within the detonation front is practically complete, it is an ideal detonation , which is described with sufficient accuracy by the Chapman-Jouguet theory. Non-ideal detonations with delayed reactions and a broader, three-dimensional reaction zone are attempted to be simulated with complex computer simulations (LS-Dyna, etc.). An important example of a non-ideally detonating explosive is triaminotrinitrobenzene .

Differentiation from other forms of explosion

In common parlance, the term detonation is used for explosions in which a sharp bang or an intense pressure wave occurs, even if the process is physically not a detonation, e.g. B. in nuclear explosions or pyrotechnic bangs. Often, based on English usage, this also means the ignition of an explosive charge and not the actual explosion process.

In contrast to detonation, propellants should explode in the form of a deflagration , i.e. burn off very quickly and in a controlled manner with evolution of gas and perform mechanical work, such as driving a projectile out of a gun barrel. The deflagration is pressure and temperature dependent. A deflagration can accelerate under inclusion by inertia or damming and in some substances it can turn into a detonation. Detonation in a rifle would destroy it.

literature

DL Chapman: Phil. Mag . (Lond. Edinb. Dubl.) 47, 90 (1899)
E. Jouguet: J. Math. Pure App l. 60, 347 (1905); 61, 1 (1906)
J. Taylor: Detonation in Condensed Explosives . Clarendon Press, Oxford 1952.
J. Neumann, RD Richtmeyer: J. Appl. Phys . 21, 232 (1950)
CE Anderson, JS Wilbeck, JC Hokanson, JR Asay, DE Grady, RA Graham, ME Kipp, in: YM Gupta: Shock Waves in Condensed Matter - 1985 . Plenum Press, New York 1986.
JM Walsh, RH Christian: Phys. Rev . 97, 1544-56 (1955)

Individual evidence

^ Georgi I. Pokrowski: Explosion and demolition. Teubner, 1st edition Leipzig 1985, pp. 30-40.

[Pokrowski,_Explosion_und_Sprengung-1] Georgi I. Pokrowski: Explosion and demolition. Teubner, 1st edition Leipzig 1985, pp. 30-40.