Shock, shockwave

Temperature, shock, shockwaves, friction and light may be the physical agency of instability. Unsaturated organic substances can sometimes undergo violent chemical transformations under the influence of some of these but do not come within the above definition. In these specific cases, dangerous chemical reactions, which often involve catalytic impurities, are the cause and are treated in chapter 4 as dangerous reactions . 

A detonation shock wave is an abrupt gas dynamic discontinuity across which properties such as gas pressure, density, temperature, and local flow velocities change discontinnonsly. Shockwaves are always characterized by the observation that the wave travels with a velocity that is faster than the local speed of sound in the undisturbed mixtnre ahead of the wave front. The ratio of the wave velocity to the speed of sound is called the Mach number. 

Mattick, A.T., Russell, D.A., Hertzberg, A., and Knowlen, C. (1995) Shock controlled chemical processing, in Shockwaves at Marseille, Proceedings... 

Akerman, L. A. et al., Combustion, Explos., Shockwaves, 1987, 23, 178 Calcium chloride and oxide catalyse the second, violent, stage of ammonium perchlorate decomposition and increase the shock sensitivity of mixtures with sugar. 

L.P. Parshev, ZhPriklMekhan i TekhnFiz 1965(5), 130-31 CA 64, 3274(1966)(Calculation of the energy of a shock wave in water) 72a) W.A. Walker H.M. Sternberg, "The Chapman-Jouguet Isentrope and the Underwater Shockwave Performance of pentolite , 4thONRSympDeton (1965), pp 27-38(26 refs) 73) R. Cheret, "Theoretical Considerations on the Propagation of Shock and Detonation Waves , Ibid, pp 78-83 74) A.B. Amster et al, "Detona-... 

Explosive substances which on initiation decompose via the passage of a shockwave rather than a thermal mechanism are called detonating explosives. The velocity of the shockwave in solid or liquid explosives is between 1500 and 9000 m s-1, an order of magnitude higher than that for the deflagration process. The rate at which the material decomposes is governed by the speed at which the material will transmit the shock-wave, not by the rate of heat transfer. Detonation can be achieved either by burning to detonation or by an initial shock. 

PA Tech Div Lecture, 29pp) 14) G. Taylor, PrRoySoc 201A, 159-74(1950) 15) H.G. Snay J.H. Rosenbaum, Shockwave Parameters in Fresh Water for Pressures up to 95 Kilobars, NAVORD Rpt 2383(1952) 16) Armament Engrg (1954), 185 97 17) Coll, Symposium on Blast and Shock Waves, HA, England (1955) 18) F. Berry et al, PrRoySoc 227A, 258-70(1955) ... 

Trade name of a new non electric device for the firing of explosive charges. The basic unit consists, of detonating cords of a plastic hose (3 mm 0), the inner wall of which is coated with a thin layer of explosive instead of electrical wires. A shock wave initiated by a special initiator passes through the tube with a speed of approx. 2000 m/s (6700 ft/s). The spectator observes this shockwave process as a flash in the hose. The plastic tube is not destroyed by the shock. 

The material falling from the parent molecular cloud directly on the forming star is rapidly dissociated and ionized and solids are vaporized, once the matter passes the accretion shock on the stellar surface. The material falling on the inner parts of the accretion disk suffers the same fate, since the matter has to pass through a standing shockwave on the surface of the disk. At this shock the infall of matter is stopped and the flow characteristics change from infall of envelope material to nearly Keplerian rotation of disk material. The location of this shock is shown in Fig. 2.10 as the heavy line marked by crosses. 

Secondary explosives (also known as high explosives = HEs) unlike primary explosives can not be initiated simply through heat or shock. In order to initiate primary explosives have to be used, whereby the shockwave of the primary explosive initiates the secondary. However, the performance of the secondary explosive is usually higher than that of the primary (see Tab. 2.1). Typical currently used secondary explosives are TNT, RDX, HMX, NQ and TATB (see also Tab. 1.2) and, for civil applications, HNS and NG e.g. in the form of dynamite for commercial use. 

As we can see in Figure 2.7, after a detonation, the shock wave naturally requires a certain amount of time t (G < t2< t3< f4), to reach a certain point. However, the maximum pressure p of the shockwave also decreases as the distance from the center of the detonation increases. 

Immediately ahead of the detonation firont the explosive rests quietly in its metastable state, while to the rear the shocked and reacted material flows at several kilometers per second with a pressure of several hundred thousand atmospheres and temperature of several thousand Kelvins. The rapid compression and heating of matter to these extreme conditions and the associated high velocity flow are properties of detonations that can be shared by strong shockwaves. However, with detonations the heated and compressed flow is selfsustaining. Typically, detonations are maintained by the exothermic chemistry they induce. Detonations driven by first order phase transitions have been envisioned, but have not yet been observed. 

Our earliest attempt to model shock-induced chemistry with molecular dynamics was based on LEPS-like potentials, which were used to describe a nitric oxide chain with the NO molecules arranged head to tail and tail to head along the chain (NO, ON, NO,. ..). This approach was successful in demonstrating that chemically sustained shockwaves driven by exothermic reactions (in this case the half reactions N-t-NO—>-N2-t-0) could be modeled with molecular dynamics. However, the potential form assumed required a fixed number of nearest neighbors and hence was not readily generalized to higher dimensions. 

Simulations using Model I were initiated by impacting a flyer plate composed of several layers of a nonenergetic AA molecular solid with the edge of the 2-D semiinfinite energetic AB solid. The latter is initially at rest at near zero temperature and pressure. Figure 4 shows a typical snapshot of a chemically sustained shockwave that can result. A distinct shockfront is visible with reactant molecules at the right and product molecules at the far left. After initiation, the shock front rapidly approaches a constant... 

In these latter studies, strong shockwaves were produced by driving the free edge of the molecular solid with a steadily moving piston as depicted in the lower part of Fig. 3. Two-dimensional simulations were initially carried out to determine the piston driven shock-to-detonation threshold in the perfect crystal. Once this threshold was determined, a crack such as that depicted at the top of Fig. 19 was introduced. Additional simulations were then performed for a series of piston velocities near, but below, the critical piston velocity, Vp, that is necessary to cause detonation in defect-free... 

For strong shockwaves the presence of a single crack such as that depicted in Fig. 19 (top) was found to significantly reduce the shock-to-detonation threshold. This is demonstrated in Fig. 19 (bottom). This figure depicts the shockfront position versus time for a series of simulations... 

The results demonstrate that nanometer wide cracks can have severe effects on the shock-to-detonation threshold. It might be tempting to conclude that the chemical reactions caused by these defects result from a velocity doubling as atoms are spalled into the crack. Indeed, we found that the velocities of the leading particles that are spalled into the crack by the shockwave had approximately twice the particle flow velocity in the shockwave, as predicted by the continuum theory. However, we also observed that when these high velocity molecules struck the opposite side of the crack, reactions were not induced immediately. Rather, the complex motions of the many atoms within the crack appears to seed the chemical reactions that ultimately cause detonation. These studies lay the foundation for additional studies with more complex models. 

Actually, there is not one but two transitions visible in the snapshot of the Model III simulations. This second transition occurs at the interface between the dissociative zone and the rarefaction region as the material transforms from the dissociative phase to A2 and B2 molecular products. It is the second transition that produces the required behavior in the shock Hugoniot for a rarefaction shockwave. This product rarefaction shockfront appears to act as a steadily moving piston producing the observed flat-topped shockwave structure. 

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