Shock Initiation of Homogeneous Explosives

The experimental observations are that approximately the same amount of decomposition was observed for 0.1 cm of air, Krypton, methane, or vacuum. Our most favorable calculations indicate that heat conduction alone is insufficient to cause appreciable reaction in air or vacuum and that the methane filled gap should give TNT temperatures at least 300°K lower than the Krypton-filled gap. Therefore, it would appear that some phenomenon other than plane surface heat conduction dominates the initiation process of explosives when gaps are present. Some mechanism is required for heat in the gas to be concentrated in local areas of the explosive surface, or some other source of initiation energy is required such as shock interactions or internal void compression. The concentration mechanism appears to be relatively independent of the gas temperature and essentially independent of the gas. 

It is disturbing that the mechanism of initiation that is probably important in the accidental premature initiation of explosives in shells is unknown. Our calculations confirm what the experimental evidence indicates, that some phenomenon other than plane surface heat conduction and adiabatic gas compression is dominating the initiation process. 

TABLE 3.3 PETN Equation of State and Rate Parameters 

Explosion time data for nitromethane are available for pressures of 67 to 104 kbar. While some of the data are not one-dimensional, it is interesting to determine whether the data are consistent with an adiabatic explosion model with the same kinetics and temperature calculations as used in the numerical hydrodynamic calculations. To perform the adiabatic explosion time calculations the same nitromethane constants and a heat of decomposition of 1000 cal/g were used. 

A summary of the available data and calculated temperatures follows. 

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Mader, "The Hydrodynamic Hot Spot and Shock Initiation of Homogeneous Explosives", LASL Rept LA-2703 (1962) PhysFluids 6,... 

J. Zinn R.N. Rogers, JPhysChem 66, 2646-53(1962) CA 58, 3262(1963) (Thermal initiation of expls) 40b) C.L. Mader, "The Hydrodynamic Hot Spot and Shock Initiation of Homogeneous Explosives ,... 

Campbell et al (Ref 7) have successfully used Eq 5 to interpret their observations on the shock initiation of homogeneous explosives such as NM, liq TNT single crystal PETN... 

Experimental and theoretical studies suggest that shock initiation of homogeneous explosives results from simple thermal explosion caused by shock heating the bulk of the material. 

The theory of detonation has also been extended to study the process of initiation of reaction by the commonest means used in practice, namely, by the shock wave arising from another high explosive. Campbell, Davis and Travis have studied the initiation by plane shock waves of homogeneous explosives, particularly nitromethane. Initiation occurs at the boundary of the explosive after an induction period which is of the order of a microsecond and which depends markedly on initial temperature. During the induction period the shock wave has proceeded through the explosive and compressed it. The detonation initially in compressed explosive has a velocity some 10% above normal, but the detonation soon overtakes the... 

The classical view of the mechanism of shock initiation of homogenous expls, ably presented in Ref 3, has been discussed in some detail in Vol 7 under Liquid Explosives, L31-L to L32-R. Briefly stated this mechanism involves a thermal expln at or near the shock entry face... 

F lire 20. Distance-time plots for shock initiation of homogeneous (a) and heterogeneous (b) explosives. 

The initiation of homogeneous explosives, for example, liquid explosives, may occur as a result of homogeneous heating in the ( namic compression zone. However, for successful initiation, the shock wave pressure should exceed 10 GPa, causing the hot spot temperature to rise up to 1000 K. 

Charles L. Mader, Shock and Hot Spot Initiation of Homogeneous Explosives , Physics of Fluids 6, 375 (1963). 

Mode d) is operative only at strong shock inputs and may be the main mode of initiation and propagation in homogeneous explosive liquids or defect-free explosive single crystals... 

Figure 2.39 shows the experimental data and the calculated velocities as a function of scaled Dural thicknesses for yields of 0 and 5.5 kbar. In contrast to the underdriven 9404 data, the nitromethane data scale and do not exhibit build-up. Because initiation of a homogeneous explosive begins in the previously shocked but undetonated explosive and proceeds through the compressed explosive at a velocity and pressure greater than the infinite-medium velocity and effective C-J pressure, initiation of a homogeneous explosive results in an overdriven detonation that then decays toward the infinite-medium effective C-J pressure. 

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