Shock Initiation of Heterogeneous Explosives

To increase the understanding of the basic processes involved in shock initiation of inhomogeneous explosives, numerical two and three-dimensional hydrodynamics have been used to study the formation of hot spots from shocks interacting with single and multiple voids, air holes, and other density discontinuities. 

The process of heterogeneous shock initiation is described by the Hydrodynamie Hot Spot Modelf which models the hot spot formation from the shock interactions that occur at density discontinuities and describes the decomposition using the Arrhenius rate law and the temperature from the HOM equation of state described in Appendix A. 

The studies of the one-dimensional hot spot generation and propagation were extended to two dimensions in the 1960 s. 

The development of the three-dimensional Eulerian code, 3DE, described in Appendix D, allowed the Hydrodynamic Hot Spot Model of heterogeneous shock initiation to be used to investigate the shock interaction with a matrix of holes and the resulting formation of hot spots, their interaction and build up toward a propagating detonation. The Hydrodynamic Hot Spot Model has been used to evaluate the relative effect of explosive shock sensitivity as a function of composition, pressure, temperature, density, and particle size. It has also been used to understand the desensitization of explosives by preshocking. 

The interaction of a shock wave with a single air hole and a matrix of air holes in PETN, HMX, TATB, and Nitroguanidine was modeled in references 23 and 24. The basic differences between shock-sensitive explosives (PETN or HMX) and shock insensitive explosives (TATB or Nitroguanidine) were described by the Hydrodynamic Hot Spot Model. 

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The basic two-dimensional processes involved in the shock initiation of heterogeneous explosives have now been numerically described. 

E.E. Walker R.J. Wasley, Critical Energy for Shock Initiation of Heterogeneous Explosives , Explosivst (1), (1969), 9—13... 

It is important to understand that this discussion is about build-up of the effective C-J pressure of detonating 9404 initiated at pressures less than infinite-medium effective C-J pressure, but greater than that pressure required for prompt initiation ( 100 kbar for most explosives). This is not to be confused with the build-up to detonation, characteristic of the process of shock initiation of heterogeneous explosives initiated by a shock wave of a few tens of kbars described in Chapter 4. 

Build-Up TO Detonation - Characteristic of the process of shock initiation of heterogeneous explosives initiated by a shock wave of a few tens of kbars. 

The Hydrodynamic Hot Spot Model has resulted in an increased understanding of the effect of explosive composition, hole size, number of holes, explosive Arrhenius decomposition rate, initial temperature and shock pressure on shock initiation of heterogeneous explosives. The Hydrodynamic Hot Spot Model reproduces the observed shock initiation behavior. This indicates that the dominant features are shock heating by hydrodynamic hot spot formation, cooling by rarefactions and the Arrhenius decomposition rate as a function of temperature. 

The effects of particle size (and the remaining void or hole size) and initial temperature on the shock initiation of heterogeneous explosive charges have been modeled. Investigations have included shocks of various pressures interacting with TNT, HMX and TATB with 0.5% void fraction and hole sizes from 0.5 to 0.000005 cm radius. At the same density, the most shock-sensitive explosive is the one with particle sizes between coarse and fine material. The shock sensitivity of HMX continues to increase with decreasing hole sizes for hole sizes where TNT or TATB fail. The shock sensitivity of TNT, TATB and HMX increases with initial temperature. TATB at 250°C is calculated to be as shock sensitive as HMX at 25°C. 

Charles L. Mader and James D. Kershner, Three-Dimensional Modeling of Shock Initiation of Heterogeneous Explosives , Nineteenth Symposium (International) on Combustion, William and Wilkins, Eds., 685-690, (1982). 

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