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C. Davis Although explosives he science of high explosives is conditions, they are of little use under basically a coupling of chemistry the extremes of temperature and pres- have been known for T and fluid mechanics. While each sure generated in explosives. Similarly, over a thousand of these fields is in itself quite well- standard techniques to study the me- developed and sophisticated, high- chanics of flow in metals are of little use years, the science of explosives science is surprisingly prim- when the relevant stress and strain rates explosives is still very itive. The fluid-mechanical phenomenon produce a violent reaction in the studied of detonation is reasonably well under- material. As a result, few academic in- young. We are only stood, but the detailed chemical reac- stitutions have deemed it fruitful to es- tions and thermomechanics that cause a tablish a research program in explosives. beginning to understand detonation are still largely a mystery, The second reason is that applications the nonlinear interaction For many explosives, even the final of explosives technology in the past have chemical composition after detonation is not placed a high premium on under- between chemistry and not known accurately and the reaction standing the details. of the detonation fluid mechanics that mechanisms are only guessed at. Similar- phenomenon. Most explosive applica- ly, while it is clear that some of the most tions are in the fields of excavation, produces the rapid energetic explosives would not detonate mining, or conventional munitions. All energy release at all were it not for their nonuniform of these have well-established, albeit mechanical response to shock waves, the crude, “rules-of-thumb” as regards known as detonation. micromechanics of explosive materials is quantities and configurations of ex- not nearly so well understood as that of plosive required to accomplish the task. metals. Hence there has been little pressure to There are two basic reasons why the establish an extensive industrial research science of explosives is relatively un- base. developed, and an understanding of Recently, however, there has been a them indicates why the next decade is significant increase in both the capability likely to produce a dramatic increase in and motivation for expanded research in our understanding. First, as is obvious, explosives science. Modern instruments, measurements in the interior of a deto- particularly those employing lasers as nating explosive are extremely difficult. probes, have made it possible to selec- Whereas experimental methods have ex- tively investigate chemical phenomena isted for many decades that can charac- on time scales of less than 100 terize chemical reactions under normal picosecond. Such measurements are 48 LOS ALAMOS SCIENCE stimulating a rapid growth in experimen- nonlinear interaction between chemistry tal and theoretical techniques that should and fluid dynamics that is at the heart of soon be applied to explosives. The need the detonation process. for understanding the details of the energy release in explosives is also in- Detonation Physics creasing rapidly. Explosive systems are being demanded that function with in- Our understanding of explosives be- creased precision and efficiency and at gins with empirical observations. Figure the same time maximize safety. For 1 shows a block of explosive as it example, in situ retorting of oil shale or detonates. The detonation wave spreads chemical mining of scarce minerals will out from the point of initiation almost require blasting techniques that can pro- like a Huygens construction. The wave duce a preselected distribution of small velocity is supersonic and almost con- cracks or fragments rather than just a stant for a particular explosive, but it displacement of the ore to facilitate me- varies from one explosive to another, chanical mining. As another example, depending primarily on the composition modern munitions are increasingly re- and density of the explosive. For most liant on the ability to focus the energy explosives, the detonation velocity is from explosives to defeat well-protected affected little by the time it has run, the targets such as tanks and armored vehi- size and shape of the block, or the Fig. 1. Detonation of a block of ex- cles, Designs of such systems are so curvature of the detonation wave front. plosive. The detonation wave spreads sophisticated that a giant computer is Because the detonation wave velocity is from the point of initiation as a nearly needed to optimize them, and the behav- faster than the velocity of sound in the spherical wave, almost like a Huygens ior of the explosive must be quite ac- explosive, the material in front of the construction. The wave is supersonic curately predicted. This combination of wave is absolutely unaffected until the (faster than sound in the unreacted ex- demand for a refined explosive technolo- detonation wave passes through it. In plosive), so there is no signal of any kind gy and the availability of new research particular, a second detonation wave in ahead of the detonation. The entire tools should produce a dramatic im- the block propagates independently of chemical reaction takes place in a thin provement in the state of explosives the first wave until the two intersect. layer just behind the wave front. science. In this article we review our The wave front is the moving surface understanding of explosives as it has that separates explosive material in mo- evolved by bursts and starts from the tion from stationary material. In solid or temperatures necessary to drive the reac- turn of the century to the present. We liquid explosives, the pressures just be- tions. In other words, the inertia of the begin with empirical observations and hind the front are very high, a few explosive itself provides the confinement trace the development from simple to hundred thousand atmospheres (a few necessary to maintain the conditions for more complex fluid-dynamical models of tens of gigapascals), and the tem- the fast chemical reaction rates and the energy release and propagation. peratures are from 2000 to 4000 K. self-sustaining propagation of the deto- Because the time scales for energy The high temperatures and pressures nation wave. The distinguishing feature release are so fast, the simplest model are produced by the very rapid release of of detonation is the self-inertial confine- that ignores all details of the chemistry chemical energy in the explosives. Typi- ment of the chemical reaction. Thus, has been remarkably successful in pre- cally the chemical reaction is 90% com- there is an intimate relationship between dicting the performance of many ex- plete in 10-6 to 10-9 second. The energy chemistry and mechanics, and neither plosives presently in use. But the more goes into the motion of the explosive can be treated as an independent process complex models give us insight into the products, creating the high pressures and in a realistic detonation model. 50 LOS ALAMOS SCIENCE HIGH EXPLOSIVES THE INTERACTION OF CHEMISTRY AND MECHANICS Chemical Reaction Zone such as the metal of a hand grenade or the rock around a borehole, by the expansion of the explosive gases after the reaction is finished. Although the two parts are interrelated, until recently they have been treated as separate problems. In most practical cases, the chemical reaction zone is so thin compared to the size of the explosive charge that its length is neglected completely in ex- plosive performance calculations. We as- Fig. 2. Plot of pressure versus distance for a detonation wave. The shock wave, at the sume that the reaction takes place in- right, is the leading element of the detonation wave. The explosive behind it, heated by stantaneously at the detonation front the sudden compression, begins its chemical reaction there. Pressure falls as the and calculate the expansion of final reaction proceeds, and reaction is finished at the point markedfinal state. Behind that explosive products as they push what- point are a rarefaction and a constant-state region; they reduce the pressure and ever material may enclose them. If this particle velocity to match the motion of the external confinement, shown here as a idealized calculation is compared with piston. The reaction zone, between the shock wave and the final state, is a subsonic measurements, the effect of finite reac- flow region; energy liberated in it can flow forward to drive the shock wave. The tion zone length (or finite time of region behind the final state is a supersonic flow region; neither energy released there chemical reaction) appears as a small nor any perturbing waves can move forward to affect the reaction zone or the shock. rise in pressure or velocity at the detona- tion front. The explosive and the inert material it tion front is a shock wave, supersonic In many cases, this simple way of drives are usually solids, but the detona- relative to the material ahead of it, so no treating detonation phenomena is suffi- tion pressures are so high that material signal precedes it. Compression heats the cient for determining the equation of strength may be neglected and the prop- explosive, and rapid chemical reaction state of the explosive products and for agation of energy may be understood follows, Finally, reaction is complete, calculating the inert flow and explosive through the equations of reactive fluid and the product gases expand as an inert performance. However, modern applica- dynamics. Furthermore, energy trans- flow. The inert flow of the explosive tions of explosives have stimulated at- port by heat conduction, viscosity, and products is affected by the surrounding tempts to treat the entire problem as a radiation is negligibly small compared inert materials. In other words, the inert whole, to learn in more detail the with the transport by motion. The theo- flow must match the boundary condi- chemical reaction rates in the reaction retical basis for treating one-dimensional tions provided at the left of Fig.

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