The Plate Dent Experiment

The observed agreement between the observed short and long time behavior of an underwater detonation and the detailed one-dimensional compressible hydrodynamic calculations indicates that the calculated energy partition between the detonation products and the water is accurate enough to be used in multidimensional studies of water wave generation mechanisms. 

Explosively generated water cavities modeled using NOBEL are described in Chapter 

The most useful and simplest experiment that can be performed to obtain a good estimate of the detonation C-J pressure is the plate dent test described by Smith and performed by M. Urizer for almost 50 years at Los Alamos. Of the usual experiments used to study detonation performance, this experiment is also one of the most difficult to simulate numerically. Lagrangian codes, such as TDL, cannot describe the highly distorted flow around the surface of the dent. The Eulerian code, 2DE, described in Appendix C has realistic calibrated equations of state and elastic-plastic treatments for solid materials, so it has been used to model the plate dent test. 

Similar calculations were performed with the Aluminum plate replaced by steel. The elastic-plastic properties used for steel(1018 CRS) were a yield of 7.5 kbar and a shear modulus of 0.987 Mbar taken from reference 8. 

Diameter (cm) Explosive Metal Experimental Plate Dent (cm) Calculated Plate Dent (cm) 

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The plate dent experiment would be just another integral experiment except that the depth of the dent has been observed to correlate linearly with experimentally determined C-J pressures of large charges of explosives that exhibit ideal behavior. Another interesting observation is the scaling of the dent with charge radius. ]... 

The plate dent test is described in Chapter 5 and in reference 35. The plate dent test consists of detonating a cylindrical charge of explosive in contact with a heavy steel plate and measuring the depth of the dent produced in the plate. The depth of the dent correlates with the C-J pressure as shown in Figure 2.21 for most explosives. The depth of the dent in the plate dent experiment is much greater for tungsten or lead-loaded explosives than expected for their observed detonation pressure. For example, a 60/30/10 volume percent RDX/Pb/Exon at 4.6 g/cc has a detonation pressure of 150 kbar as determined by metal plate acceleration data and 345 kbar as determined by the plate dent vs. C-J pressure correlation described in reference 35. The experimentally measured detonation velocity is 0.5012 cm//iis. The BKW-calculated C-J pressure is 270 kbar and velocity is 0.5096 cm/)us. Similar calculated results are obtained whether the lead is considered as compressible or incompressible and whether the lead is in temperature and pressure equilibrium with the detonation products or is in pressure equilibrium alone. 

The plate dent experiment has been numerically modeled for conventional explosives. Since these explosives have similar isentrope slopes in the pressure range of interest, the major difference among them is a function of the peak detonation pressure. The observed correlation of the plate dent depth with C-J pressure (Figure 2.21) is simply a consequence of similar expansion of the detonation products of most explosives down to about 10 kbar. 

The plate dent experiments are so difficult to understand or model that reference 16 did not consider Munroe jets as a mechanism for the formation of the dents or connect the results of the experiments with the PHERMEX Munroe jet data base. Such experiments are similar to attempting to do biology from road kill. 

The plate dent is also used to study the properties of a material by the measurement of the dent depth generated by well characterized explosives such as TNT or PBX-9404. This is called the Inverse Plate Dent experiment and described in reference 19. The plate dent has been used to determine the yield strength in many common materials and even unusual materials such as Beryllium. 

The BKW equation of state was shown earlier in this chapter to be adequate to describe the expansion of an explosion under water from several hundred kilobars to a tenth of a bar. The BKW equation of state was also shown to be adequate to describe the plate dent test from over 500 kbars to less than 10 kbars. While it increases the confidence in using the BKW equation of state for describing such integral experiments, there is nothing unique about the BKW equation of state. The experimental data is also described, within experimental error, by explosive equations of state that describe the pressure-volume-temperature-energy relationship for detonation products quite differently. The cylinder test is used to calibrate the JWL equation of state for PBX-9404/9501 assuming C-J pressures varying from 380 to 305 kbars in the Livermore explosive data compendium. They all are forced to fit the cylinder test data ... 

The explosive mixture 60/10/30 by volume of RDX/Exon/Pb has a density of 4.60 g/cc, a detonation velocity of 0.5 cm//its, and a detonation pressure determined from aluminum plate push experiments of 150 kbar. The experimental plate dent of 10.23 mm corresponds to a detonation pressure of 346 kbar. The BKW-calculated C-J performance is 270 kbar, 0.5096 cm/pLS, and 2242 K. This is one of the most nonideal inert metal-loaded explosives. The very small particle size of the lead powder used results in the failure of many individual detonation wavelets as they pass between the lead particles. 

The experiment starts with a certain content of talcum in the test explosive. If the complete detonation of the test explosive charge occurs, as observed on the basis of the depth of the dent in the lead witness plate, a new trial is performed with an increased amount of talcum content. The testing proceeds until determination of the talcum content at which the given detonator is no longer capable of causing the complete detonation of the test explosive charge. 

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