ХИМИЧЕСКАЯ ФИЗИКА, 2008, том 27, № 3, с. 21-30
ГОРЕНИЕ И ВЗРЫВ
УДК 541.126
SHOCK INITIATION OF COMPOSITION B AND C-4 EXPLOSIVES. EXPERIMENTS AND MODELING*
© 2008 P. A. Urtiew1, K. S. Vandersall1, C. M. Tarver1, F. Garcia1, J. W. Forbes2
1Energetic Materials Center, Lawrence Livermore National Laboratory, Livermore, CA 94550 2Center for Energetic Concepts Development, University of Maryland, College Park, MD 2074
Received 07.12.2006
Shock initiation experiments on the explosives Composition B and C-4 were performed to obtain in-situ pressure gauge data for the purpose of providing the Ignition and Growth reactive flow model with proper modeling parameters. A 100 mm diameter propellant driven gas gun was utilized to initiate the explosive charges containing manganin piezoresistive pressure gauge packages embedded in the explosive sample. Experimental data provided new information on the shock velocity - particle velocity relationship for each of the investigated material in their respective pressure range. The run-distance-to-detonation points on the pop-plot for these experiments showed agreement with previously published data, and Ignition and Growth modeling calculations resulted in a good fit to the experimental data. Identical ignition and growth reaction rate parameters were used for C-4 and Composition B, and the Composition B model also included a third reaction rate to simulate the completion of reaction by the TNT component. This model can be applied to shock initiation scenarios that have not or cannot be tested experimentally with a high level of confidence in its predictions.
INTRODUCTION
Shock initiation is one of the most important properties of energetic materials (EM). Of interest here is the determination whether the input shock wave will build in pressure to a full-fledged detonation or decay to a deflagration wave or to a non-reacting wave. So the study of this property is important to gain knowledge if a material will detonate as intended or not for different dynamic loadings.
Energetic materials are widely used in both industrial and military applications. Therefore, initiation of such materials is of particular interest for reasons of safety and understanding of their behavior under dynamic loading conditions.
In earlier publications we have reported initiation thresholds for build up to detonation and sensitivity to impact (pop-plots) of both sensitive and insensitive, HMX and TATB based explosives at various initial temperatures [1, 2]. In this publication we will report on initiation and sensitivity of more common RDX based explosives known as C-4 and Composition B and will include previously unpublished manganin gauge records for sustained pulse shock initiation.
EXPERIMENTAL TECHNIQUE
Most of the experiments in our laboratory are performed on the 100 mm bore, propellant driven gas gun, which allows precise control of the projectile velocity
* This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
and of the loading pressure imposed on the EM target. The experimental set-up is illustrated in Fig 1.
There were two types of target assemblies that were used in these experiments. One target assembly consisted of several discs of different thicknesses. Gauge packages containing manganin pressure gauges were embedded between individual discs. The other target assembly consisted of two 24° wedges with one multielement pressure gage package placed between them. The intent here was to eliminate the effect of inert package material on each consecutive gauge element. The gauges are armored with thin (125 ^m) Teflon insulation on both sides to prevent shorting of the gauges in a conductive medium when the material becomes reactive. Other details of the manganin pressure gauges are described in our previous publications [3, 4].
For better control of the impact pressure, a thin buffer plate of the same material as the impact plate is placed in front of the target assembly for symmetrical impact. Also included in the two target assemblies are six tilt pins placed around the periphery of the target flush with the impact surface to measure the tilt of the impact plate as it strikes the target, and two velocity pins sticking out some known distance from the target to measure the velocity of the impact plate just before it strikes the target.
During the experiment, oscilloscopes measure change of voltage as result of resistance change in the gauges which were then converted to pressure using the hysteresis corrected calibration curve published elsewhere [5]. From the data of the shock arrival times of the gauge locations, a plot of distance vs. time ("x-t plot") is constructed with the slope of the plotted lines
Table 1. Listing of shock initiation gun experiments
Shot # Target Density, g/cc Velocity, km/s Pressure, GPa Dist. to Det., mm
4565 C-4 1.590 0.6 2.2 25
4547 C-4 1.630 0.737 2.86 17
4564 C-4 1.621 0.987 4.2 9
4359 Comp. B 0.835 3.78 14
4544 Comp. B 1.692 0.929 4.35 11
4540 Comp. B 1.690 1.005 4.80 10
4545 Comp. B 1.681 1.307 6.84 5
yielding the shock velocities with two lines apparent, a line for the unreacted state as it reacts and a line representing the detonation velocity. The intersection of these two lines is taken as the "run-distance-to-detonation," which is then plotted on the "pop-plot" showing the run-distance-to-detonation as a function of the input pressure in log-log space.
EXPERIMENTAL RECORDS
Total of 7 shots were fired of which 3 of them were with the C-4 explosives in the disc target configuration
and 4 with the Composition B in the wedge target configuration. Densities, impact velocities and pressures imposed on the targets material are listed in table 1.
Figure 2 shows pressure records of a shock loaded RDX-based high explosive C-4 (91 weight % RDX and 9 weight % of other additives) pressed to 98.5 of theoretical maximum density. The explosive samples in these experiments were shock loaded with an aluminum impactor flying at a velocity of 0.6 mm/|s, imposing a pressure on the target material 2.2 GPa. As in all heterogeneous explosives these traces exhibit the characteristic features of their initiation: some reac-
Flyer Plate
Single
Element
Gauges
Velocity & Tilt Crystal Pins
Multiple Element Gauge
Wedge Target
Fig. 1. Schematic diagram of experimental set-up for gun experiments.
Time, |is
Fig. 2. A typical pressure gauge record for the C-4 material.
Time, |is
Fig. 3. A typical pressure gauge record for the Composition B material.
tion occurs just behind the shock front causing it to grow in pressure, but most of the reaction occurs well behind the leading shock, creating a pressure wave that overtakes the initial shock wave causing the process to finally transit to detonation. In this case the transition to detonation occurred at about 25 mm into the target.
Figure 3 illustrates the records obtained with a wedge type experiments performed on another RDX based explosive Composition B (63 weight % RDX, 36 weight % TNT and lweight % of Wax) at ambient room temperature. It shows initially a steady shock wave and then, after the reaction becomes significant, a strong growth in
pressure just before the transition to detonation. The loading pressure in this experiment was 4.8 GPa.
Shock sensitivity of both C-4 and Composition B for various initial pressures is illustrated in Fig. 4 on a so-called "pop-plot", which displays the dependence of the distance to detonation on the initial impact pressure. On this plot one can easily compare the relative sensitivity of these explosives to a more sensitive explosive HMX-based PBX 9404 and a TATB-based insensitive high explosive at their ambient conditions. On a log-log plot the run distance to detonation versus shock pressure data mostly fall on a straight line. The closer the
Run distance to detonation, mm
1 10 100
Impact pressure, GPa
Fig. 4. Pop-plot showing data from C-4 and Composition B along with reference lines for other sensitive and insensitive High Explosives.
line is to the origin of the plot, the more sensitive is the material. Shown here are also previously published data of Gibbs and Popolato [6].
EQUATION OF STATE ANALYSIS
For any new material that was tested in our laboratory we also determine their experimental equation of
state in the form of a new shock velocity - particle velocity (Us - up) relationship. This analysis is done by using the well-known impedance matching technique and is illustrated in Figs. 5 and 6 for the case of C-4 and Composition B respectively.
Assuming that the EOS relations of the impactor at room temperature are very well known, one can plot the inverse adiabat of the flyer plate originating at the flyer
Pressure, GPa 25
20
Shock velocity, mm/^s 5
15
10
0 0.2 0.4 0.6 0.8 1.0 Mass velocity, mm/^s
Fig. 5. Impedance matching and Us - Up for C-4.
Pressure, GPa 25
20
15
10
Shock velocity, mm/^s 5
-4
-3
2
1
J0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Mass velocity, mm/^s
Fig. 6. Impedance matching and Us - up for Composition B.
5
5
30-
20-
10-
24
gauge 1 at 0 mm gauges 2 & 3 at 5.31 mm gauges 4 & 5 at 10.66 mm gauges 6 & 7 at 16.01 mm gauges 8 & 9 at 21.27 mm gauges 10 & 11 at 26.76 mm gauges 12 & 13 at 32.28 mm gauge 1 gauges 2 & 3
10 - gauges 4 & 5
11 - gauges 6 & 7
12 - gauges 8 & 9
13 - gauges 10 & 11
14 - gauges 12 & 13
\14
13
26
28
30
32
34
Time, |is
Fig. 7. Calculated Pressure histories for C-4 impacted by an aluminum flyer at 0.6 km/s.
velocity. Experimental measurement of the initial pressure from several experiments will result in the adiabat of the new target material. Measured shock velocities between the first two or more gauge stations from the same experiments allow one to draw a line through the experimental points in the shock velocity - mass velocity plane and determine th
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.