Velocity, shockwave

Explosive substances can also be detonated if they are subjected to a high velocity shockwave this method is often used for the initiation of secondary explosives. Detonation of a primary explosive will produce a... 

Explosives can therefore be classified by the ease with which they can be ignited and subsequently exploded. Primary explosives are readily ignited or detonated by a small mechanical or electrical stimulus. Secondary explosives are not so easily initiated they require a high velocity shockwave generally produced from the detonation of a primary explosive. Propellants are generally initiated by a flame, and they do not detonate, only deflagrate. 

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. 

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. 

By applying the fundamental physical properties of conservation of mass, energy and momentum across the shockwave, together with the equation of state for the explosive composition (which describes the way its pressure, temperature, volume and composition affect one another) it can be shown that the velocity of detonation is determined by the material constituting the explosive and the material s velocity. 

Primary explosives are substances which unlike secondary explosives show a very rapid transition from combustion (or deflagration) to detonation and are considerably more sensitive towards heat, impact or friction than secondary explosives. Primary explosives generate either a large amount of heat or a shockwave which makes the transfer of the detonation to a less sensitive secondary explosive possible. They are therefore used as initiators for secondary booster charges (e.g. in detonators), main charges or propellants. Although primary explosives (e.g. Pb(N3)2) are considerably more sensitive than secondary explosives (e.g. RDX), their detonation velocities, detonation pressures and heat of explosions are as a rule, generally lower than those of secondary explosives (Tab. 2.1). 

Generally, it can be said that the damaging effect of a shockwave produced by a detonation is proportional to its impulse (impulse = mass x velocity of the gaseous explosion products) and its maximum pressure, with the impulse being the most influential factor at smaller distances and the pressure being most important at larger distances. As a rule of thumb , the distance D, which offers a chance of survival, is proportional to the cube route of the mass w of an explosive, while for typical secondary explosives at larger distances, the proportionality constant is approximately 2 ... 

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. 

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... 

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. 

In light of these observations from our MD simulations, we have proposed a simple model [41] describing the role of shockwave interactions with microscopic voids that leads to significant heating, sufficient to thermally initiate chemical reactions in solid explosives, or phase transitions in metals. The key ingredients to this dramatic overshoot in temperature are shown in Fig. 13. The dependencies on both shock strength (piston velocity Up) and onedimensional gap width /, which we observed in atomistic simulations of a two-dimensional unreactive Lennard-Jones solid, for the thermal overshoot AT was well predicted by our straightforward model ... 

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