What happens when an explosive is detonated?
By David S. Moore
When an explosion goes off, we often think of the damage it does. We seldom think of explosions as constructive, but explosives have always been a critical component of industries such as mining, construction, transportation and even metal bonding.
Despite great advances in explosives since Alfred Nobel invented dynamite, the concept of detonation still baffles scientists. Although linked with the term “explosion,” detonation is quite a different matter. An explosion is defined as a sudden event that results in a loud noise and the fast, typically spectacular burst that breaks apart and sends pieces flying outward. One example of an explosion is a water heater whose pressure-relief vent for some reason plugs up. The pressure then builds in the water heater to the point that it finally bursts with so much force that the water heater rockets from the basement up into your house. Derived from the Latin word detonare, which means to thunder down, a detonation is what scientists call a supersonic burn-front. Picture someone lighting the fuse on a stick of dynamite. The point at which the material is burning is known as the burn-front — it is the location of the burn as it travels through the fuse material. If the velocity of this burn-front moves slower than the speed of sound (subsonic) along the material, the result is simple burning, also known as deflagration. If the velocity is faster than the speed of sound in a material, it is a supersonic burn-front. It builds a powerful shock wave, one whose incredible pressures can propel metal, break rocks and move earth. It is this type of supersonic burn-front that is known as detonation.
A detonation typically takes place in less than a millionth of a second. Moreover, the density of the materials during detonation is incredibly high and temperatures reach a few thousand degrees. Such a hostile and fast environment makes it extremely difficult to study detonation with the naked eye.
The mystery of detonation centers on the chemical reactions that take place right after the supersonic shock wave hits the material, a process that lasts only a billionth of a second. To unravel this mystery, scientists at Los Alamos National Laboratory combine computer simulations and innovative experiments that verify what the computers come up with, particularly the simulations of the short-lived chemical bonds formed during detonation.
One way Los Alamos scientists actually observe and study detonation is to significantly shrink the size of an explosion. The tiny scale enables scientists to drive shock waves into materials so thin it is possible to see through them. Using a laser, researchers send a pulse of light at exactly the moment the shock wave strikes the super-thin material. That lets them observe and record the impact and the resultant chemical reactions.
Such experiments began in 1998. Since then, recording technology advanced enough to enhance the detail of these reactions, enabling scientists to make significant gains in understanding the chemical reactions associated with detonation.
By better understanding how detonation works chemically, scientists will better understand what makes explosives so sensitive. An explosive’s sensitivity is related to those first chemical reactions associated with detonation. By unraveling which chemical reactions are the most sensitive, it may be possible to redesign molecules so that they retain the same performance, but are much less sensitive, and thus make the explosive safer and easier to handle.
The Holy Grail of this work is to predict the behavior of explosives. Although there are many other phenomena that are active during the initiation of real explosive materials, identifying the initial chemical reactions is key to predicting their properties and behavior.
With nearly 75 years of work in unraveling detonation, Los Alamos scientists are inching closer to this Holy Grail. Today, the United States alone uses 3.4 billion kilograms of explosives every year — that’s 24 pounds per person annually for military and industrial applications. Imagine the variety of new applications once it is possible to produce truly powerful, but easy to handle, controlled and safe explosives.
David S. Moore is a Los Alamos National Laboratory Fellow and a Fellow of the American Chemical Society studying the behavior of molecules under shock compression in the Los Alamos Shock and Detonation Physics group.
This story first appeared in the Albuquerque Journal.
