October 24, 2004

Explosives - and Shock Waves 

No, this is not the nth article on the Net on explosive chemistry and DIY bombs. There's even too much of that stuff around.

Explosives, also called energetic materials, are materials which contain a considerable amount of chemical energy, and in response to an appropriate stimulation they will release that energy on a timescale of millisecond. For comparison, fuels contain even more chemical energy, but will release it on a timescale of seconds.

The distinctive feature of high explosives is detonation, that is something different from a very very fast combustion. Detonation proceeds through particular supersonic shockwawes, called detonation waves. A shockwave is a strong pressure wave, and it can propagate in any material, eventually causing physical or chemical changes due to its high energy. The speed of sound is an important limit in this case: the speed of sound is basically the "natural" speed of a compression wave in a medium. If a disturbance is faster than sound, the molecules of the medium cannot adjust in time, and the abrupt change of conditions will subect the material to an immense strain, causing overhating and other phenomena. The shockwave also pushes particles onwards at high speed.

A detonation wave propagates only through high explosives, destroying the propagation material. Before the wavefront there is unaffected explosive; in the wavefront there are extreme conditions of pressure and temperature (20 - 30 GPa, or 200 000 - 300 000 bar, and up to 10 000 K): the explosive molecules will rapidly decompose with exothermic reactions, whose energy is expended sustaining the process. The decomposition tipically takes only a few microseconds, in a reaction zone about 1 mm thick. As often happens, all these values change substantially from one explosive to the other, and the density, structure, size and geometry of the charge and the eventual presence of inert particles all affect the final outcome. The detonation wave propagates at speeds of 4 - 10 km/s, that is abundantly supersonic.

Propellants and low explosives (and also high explosives in certain cases) do not detonate.
They deflagrate (a very fast combustion), and the deflagration front advances at subsonic speeds. Propellants are much tamer than high explosives, and are thus used, generically, to push bodies around - usually, to push projectiles through barrels and rockets/missiles through the air, or in the space.

Behind the wavefront and reaction zone, there are hot gaseous decomposition products, that will tend to expand (nice primer document here) outwards at supersonic speed, sending a shockwave through the adjacent medium. If the originating explosion is a detonation, this shochwave will initially be supersonic, and will gradually slow down while some of its energy is adsorbed by the propagation medium. If the charge is placed in a shell, the shell will be broken and shrapnel thrown at high speed. In air, the resulting shockwave can directly kill people and destroy or damage structures. The same in water, and notice that the pressure of the shockwave in air or water is much lower than in the explosive. If the charge is placed in a solid, like rock, the material will be crushed and broken.

It is assumed that an overpressure of 0.3 bar will be enough to severely damage or destroy normal buildings: 0.3 bar (30 kPa) correspond (grossly) to a load of 3 ton/m2 of surface exposed to the shock, that is a hell of a load. For comparison, a 200 km/h wind will produce a dynamic pressure, Q, of 1.5 kPa, around 155 kg/m2. Nuclear explosions will easily produce even greater overpressures.

If an explosive charge has a cavity (shaped charge) - usually conical - and this cavity is lined with copper or another metal, the shockwave will compress and heat the metal, and then squeeze it onwards as a high-speed jet (some 5 km/s) of molten metal that will easily cut through a considerable thickness of steel and other materials. This is the principle of HE armor-piercing warheads. All the phenomena above are useful for demolition and warfare.

When an explosive charge detonates near the ground (or on it, or not deeply buried), the interaction of expanding and reflected shockwawes will dig a crater. The width and depth of the crater obviously depends from the size of the charge, but also from wether the charge is shaped, from the explosive-ground distance (and the filler material), from the soil composition and structure, humidity etc.

But a clever use of shockwaves can be used for more costructive purposes.
The main physical property for the propagations of shocks is density, and thus the behaviour of a shockwave at an interface is governed by the density difference.
In particular, when a shockwawe passes from a dense material to a less dense one, part of the shock will be reflected as relaxation wave - as the name suggests, this waves causes a pressure reduction. In gases, this will increase the expansion velocity. In solids, the interaction of shock and relaxations will induce a temendous tensile stress, that can break the material apart (spalling).

Combining fast and slow explosives in the appropriate geometry, it is possible to control the shape of the detonation wavefront, which otherwise tends to be a spherical segment.

If we put a piece of some material in contact with an high explosive and then detonate it, when the shochwave reaches the explosive-material interface, some things will happen: part of the wave will be reflected back (if the material is denser than the explosive), and part will propagate in the second material, that will receive a sort of super hammer blow on its surface. This compression effect can be used to harden the surface of metallic objects, for example: parts with a high surface hardness, but overall non-brittle are higly appreciated for numerous important applications.

And also to weld together scarcely compatible metals in big structures. With the energy levels involved, metals will behave like fluids (at least for short times), and the interface between the two metals sometimes looks like frozen waves curled into each other. And the two pieces are perfectly welded together. Shaped charges can be used to cut thick metal pieces, that is a non-trivial industrial problem. A big advantage of explosive methods is that they are very fast: it takes some time to do the calculations and place the charges, but once one fires them, the job is done in fractions of a second.

Explosive compression and heating can cause unusual structural modifications in solids, and that is useful to prepare special materials, or to simulate the conditions in the interior of the Earth, or the phenomena associated with meteor impacts. An impressive application is in the implosion cores of nuclear weapons: an accurately designed and assembled explosive device is used to deliver a powerful and almost exactly spherical coverging shockwave to a fissile material nucleus, that will be compressed beyond it critical density and thus undergo explosive fission. With the proper design, the pressure generated reaches 500 GPa, and the density of the nucleus will increase of a factor 2 to 3 in a matter of 1 microsecond.


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