Gas Bubbles in Explosives

J hc role of gas bubbles in explosives has already been discussed (pp. 298,547). Goffart [96) described the importar.ee of gas bubbles which increase the cup sensitivity of slurry explosives containing akiniiniuin dust. However, the presence of gas bubbles of course reduces the density and subsequently the rale of detonation, for example a density of 1.33 falls to 0,8.3 and the rate of detonation frosn 3600 m/s to 2710 m/s (fv>r a diameter of 36 nun). 

Originally Bowden s school suggested that heat from the adiabatic compression of such gas bubbles initiated explosion in the surrounding liquid. Johansson coworkers (Ref 4), however, pointed out that heat flow from a compressed gas bubble to the surrounding liquid is much too slow to account for the observed phenomena, particularly at low impact energies. They have shown that to achieve explosion fine droplets... 

Some of the quantitative consequences of hot spot theory were presented in Ref 19. A qualitative discussion of heat flow in a compressed gas bubble hot spot was also presented in Ref 19. The necessity of having enough, but not too much, liquid or solid spray or foam within a compressed gas bubble (in order to have sufficient heat flow from the bubble to the surrounding condensed explosive) provides another hard-to-control variable in impact testing and thus increases the variability of test results... 

Bowden and Yoffe 79. 80] pointed out the role of small trapped gas bubbles in the sensitivity of both liquid and solid explosives. The adiabatic compression of small trapped gas bubbles creates hot spots manifested by high temperature. Also friction occurs at the surface of the containing walls, on grit particles or between the crystals of explosives and forms an additional factor governing the sensitivity of explosives to impact. 

A.P. Sukhotln, Disruption of a Solid Medium by an Explosion and B.D. Khristoforov, Parameters of a Shock Wave and Gas Bubble in an Underwater Explosion of Charges of PETN and Lead Azide of Different Density , ZhPrikl-Mekhan i TekhnFiziki 1961, No 4, pp 99—101 118—27 [Joint Intelligence Bureau, Division of Scientific Intelligence Transln No 791, British Ministry of Defense (1962)] 75) M.E. Sereb-... 

As Mader showed [4,5], solid particles or gas bubbles in an explosive perturb shocks, causing shock focusing. To examine whether this was an initiation mechanism in the present situation, experiments of the type illustrated in Figure 7 were performed in which solid particles and gas bubbles were placed on the... 

Other. Because a foam consists of many small, trapped gas bubbles, it can be very effective as a thermal insulator. Usually soHd foams are used for insulation purposes, but there are some instances where Hquid foams also find uses for insulation (see Eoamed plastics Insulation, thermal). Eor example, it is possible to apply and remove the insulation simply by forming or coUapsing the foam, providing additional control of the insulation process. Another novel use that is being explored is the potential of absorbing much of the pressure produced by an explosion. The energy in the shock wave is first partially absorbed by breaking the bubbles into very small droplets, and then further absorbed as the droplets are evaporated (53). 

By adiabatic compression of gas. This is particularly noticeable in liquid explosives such as nitroglycerine, where eveq the moderate compression of small gas bubbles can readily lead to initiation of the explosive. 

The maximum potential power of an explosive can be calculated, or it can be measured by techniques such as those developed by Cook. A typical method consists of firing the explosive under water and measuring the energy liberated in the various forms, such as shock wave in the water, the work of expansion of the gas bubble, etc. These figures have limited practical value as the methods of application of explosives are of low and variable efficiency. A more practical measurement of strength can be obtained by the measurement of cratering efficiency. This, again, demands considerable expense and also requires the availability of uniform rock. 

The momentum of the water as it moves outward causes the gas bubble to over-expand so that its pressure drops below the hydrostatic pressure, and then to collapse as liquid flows back in. The return flow also overshoots the gas pressure rises again, but to nowhere near the detonation pressure. However, the bubble expands again in rebound, behaving in effect as a second explosion. Several successive rebounds may occur, each becoming weaker. To evaluate the total damage, both the shock wave and the subsequent bubble pulses must be considered... 

A modification of the above underwater method studied by Cook (p 37) is the measurement of the spall-dome velocity at the surface , caused by explosion at a fixed distance beneath the surface. The method (which is not described in Cook s book) is best applied by use of calibration curves employing as suitable standard a selected explosive. It has been claimed that the method is reproducible within 5 to 10% and gives data generally in fair accord with expectations from theoretical calculations, provided the depth and extent of the pond are sufficient to avoid shock reflections. In many cases, however, there was a necessity of taking into consideration the rate of evaporation of water at the gas bubble-water interface (Ref 17, p 37)... 

The spots may be formed in two main ways 1) By friction on the confining surfaces of expls, on grit particles, or on crystals of the explosive itself and 2) By the adiabatic compression of small occluded gas bubbles the presence even of a small bubble can render most expls very sensitive to impact (Ref 19)... 

It is known that initiation of solid TNT to explosion cannot be caused by local heating to high.temperature, unless the material contains some bubbles of gas, such as air, At, etc. This means that the presence of gas bubbles is of importance, although they do not seem to take part in the chem reaction. The luminescence of these gases on shock heating plays the decisive role. By 2-color pyrometry with electronic amplification the temp... 

Gas (or Yapor) Pockets (Bubbles) in Liquid and Solid Explosives. 

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