Desensitization by Preshocking

The experimental geometries studied using PHERMEX were numerically modeled using the reactive hydrodynamic code 2DL, described in Appendix B. For explosives that had been previously shocked, Craig observed experimentally that the distance of run to detonation for several multiply shocked explosives was determined primarily by the distance after a second shock had overtaken the lower pressure shock wave (the preshock). 

To approximate this experimental observation and the results obtained from the three-dimensional numerical model study, the multiple shock Forest Fire model was used. The Forest Fire rate was determined by the first shock wave or the rates determined by any subsequent release waves that result in lower pressures and lower decomposition rates. As suggested by the three-dimensional study described in Chapter 3, the Arrhenius rate was added using the local partially burned explosive temperatures to the Forest Fire rate. 

Chick and Hatt and Chick and MacIntyre have studied the effects of desensitization by preshocking that occur when jets interact with barriers in contact with explosives. 

A steel jet with an initial velocity of 0.75 cm/psec and diameter of 0.15 cm initiated detonation in bare Composition B, but failed to initiate the Composition B if it first interacted with a 0.3 cm steel barrier on the surface of the Composition B. 

The system studied by Chick and MacIntyre was modeled using the Eulerian reactive hydrodynamic code 2DE described in Appendix C with the multiple shock Forest Fire rate. 

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Charles L. Mader and James D. Kershner, Three-Dimensional Modeling of Explosive Desensitization by Preshocking , Journal of Energetic Materials 3, 35-47 (1985). 

The mechanism of explosive desensitization by preshocking has been studied using a three-dimensional reactive hydrodynamic model of the process. With the mechanism determined, it was possible to modify the Forest Fire heterogeneous shock initiation decomposition rate to include both the desensitization and failure to desensitize effects as will be described in Chapter 4 and Chapter 6. 

The study to determine the mechanism of the explosive desensitization by preshocking used the three-dimensional reactive hydrodynamic code, 3DE, described in Appendix D. A constant velocity piston was applied to the bottom of a TATB explosive cube shocking the explosive to the desired pressure. When a higher pressure second shock was to be introduced, the piston velocity was increased and other piston state values changed appropriately for a multiple shock of the required pressure. The Arrhenius reactive rate law was used for TATB with the constants determined for solid TATB by Raymond N. Rogers shown in Table 3.1 of an activation energy of 59.9 kcal/mole and a frequency factor of 3.18 x 10 t sec T... 

To model desensitization by preshocking, the modification indicated by the three-dimensional study, described in Chapter 3, to the Forest Fire decomposition rate was to limit the rate by the initial shock pressure and to add the Arrhenius rate law to the limited Forest Fire rate. The Forest Fire rate for TATB is shown in Figure 4.21, along with the Arrhenius rate calculated using the temperatures from the HOM equation of state for the partially burned TATB associated with the pressure, as determined by Forest Fire. The multiple shock Forest Fire model (MSFF) uses a burn rate determined by Forest Fire, limited to the initial shock pressure, and the Arrhenius rate using local partially burned explosive temperature. 

Figure 4.26 The density and mass fraction contours at 1.5 nsec for a 0.15 cm diameter steel rod initially moving at 0.75 cm/ usec penetrating a 0.3 cm thick steel plate in contact with Composition B. The density contour interval is 0.2 g/cc. The mass fraction contour interval is 0.1. The calculation includes desensitization by preshocking. The Composition B is not initiated.

To model desensitization by preshocking, described in Chapter 4, or Multiple Shock Forest Fire, a minimum required pressure for desensitization (pcrush) is introduced (about 10 kbar) and if a burning cell pressure does not change for a time of... 

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 desensitization of explosives by preshocking with a shock pressure too low to cause propagating detonation in the time of interest is described in Chapter 4, section 4.3. 

The preshock desensitized the explosive by closing the voids and making it more homogeneous. The higher pressure second shock wave proceeded through the preshocked explosive until it caught up with the preshock. 

The experimental observations of Travis and Campbell are consistent with the desensitization being caused by the preshock, making the HMX explosive more homogeneous and reducing the explosive temperature upon arrival of the detonation wave by the multiple shock process. 

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