CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
2002 American Institute of Physics 0-7354-0068-7
DOUBLE SHOCK INITIATION OF THE HMX BASED EXPLOSIVE
EDC-37f
R. L. Gustavsen, S. A. Sheffield, R. R. Alcon1 R. E. Winter, P. Taylor, and D. A. Salisbury2
1
Los Alamos National Laboratory, Los Alamos, NM 87545
Atomic Weapons Establishment, Aldermaston, Reading, Berks, RG7-4PR, U.K.
2
We have completed a series of double shock initiation experiments on EDC-37. EDC-37 is a unique
HMX based explosive because it has an energetic liquid binder, is composed primarily of fine (< 40
(im) particles, and has a very low void content (< 0.3%) versus 1.5 - 2.0% for other HMX based explosives. It is also considerably less sensitive to initiation by shocks than other HMX based explosives
such as PBX 9501. Double shocks were created by impacting the EDC-37 with gas gun launched sapphire impactors faced with a 1 to 1.5 mm thick layer of Kel-F. Varying the impact velocity controlled
the magnitude of the shocks, and varying the thickness of the Kel-F layer controlled the duration of the
first shock. Wave profiles were measured using embedded electromagnetic particle velocity gauges.
Results show buildup to detonation commencing after the first and second waves coalesce into a single
shock, provided there is not significant reaction in the first wave. That is, in the doubly shocked region,
the explosive is completely desensitized by the first shock. If there is significant reaction in the first
wave, the explosive is only partially desensitized.
INTRODUCTION
At least as early as the work of Gittings1, people
have been interested in multiple shock initiation, or
equivalently, shock desensitization, of pressed
granular explosives.1"8 This work has involved experiments, theory and modeling. Initiation of granular explosives is widely understood to involve "hot
spots", small regions of explosive which have been
heated more than the rest by the passage of the
shock wave. Chemical reaction in these hot spots
determines the reaction rate of the explosive. Successful numerical models of multiple shock phenomenon have understood that hot spots are generated only in the first shock.2'4'6 The second shock or
rarefaction does not generate more hot spots, nor
does it destroy them, so it has little effect on the
rate.
In the present study we will be discussing experiments in which a weak wave is followed by a
stronger second wave. Because the second wave is
traveling in pre-compressed material it will travel
faster than the first wave, and will eventually over-
take and coalesce with the first wave, forming a
single shock.
Combining this wave pattern with ideas about
hot spots in multiple shocks predicts that, if the first
shock is weak enough that minimal hot-spot reaction is generated, buildup to detonation does not
start until the two waves coalesce forming a strong
shock with high reaction rates. This is the basis for
the often quoted rule of thumb that buildup to detonation does not begin until the waves coalesce.5 In
this paper we show that if the first shock generates
significant hot-spot reaction, this simple rule no
longer applies.
EXPERIMENTAL DETAILS
We have recently done a great deal of work to
characterize the Hugoniot and initiation characteristics of EDC-37. 8"10 This work merely extends that
work to include multiple shock initiation.
The overall configuration for the experiments is
shown in Figure 1. This is the same configuration
described in Refs. 2, 5, and 11. A gas gun projectile
is faced with an impactor disk made of a thin disk of
Kel-F12, over a thick disk of sapphire13.
t Work performed under the auspices of the U.S. Dept. of Energy. Work funded by the U.K. Atomic Weapons Establishment.
999
The Kel-F launches a weak wave into the explosive, and the sapphire launches a following
stronger wave into the explosive. The strength of
the waves is controlled by the impact velocity, and
the width of the pulse is tailored by using different
thicknesses of Kel-F. Impact velocities, wave
strengths and Kel-F thicknesses are given in Table 1
below.
Electromagnetic particle velocity gauges are
embedded in the sample at 10 - 12 different depths.
Two experiments are done for each stress
level/wave width combination. In the first experiment the explosive sample is made without a front
disk (a disk is shown in Fig. 1) and the gauges cover
depths of 0.0 and 2.5 through 7.0 mm. In the second experiment, a 6 mm thick disk of explosive is
used on the front of the sample. The gauges are at
depths of 0.0, 6.0, and 6.5 through 11.0 mm.
In addition to the 10 - 12 particle velocity
gauges, these experiments also used the "shock
tracker" gauges.11 Outputs from these gauges can
be used to construct x - t plots of the shock front
position with time, similar to those obtained in optical or pinned wedge tests. If a transition to detona-
Kel-F(1-1.5mm)
FIGURE 1. Experimental setup. Gauges are located along the
inclined line. A disk of explosive is placed on front of the sample
to put the disk farther into the explosive on some experiments.
tion occurs with depth spanned by the shock tracker,
the time and distance of the shock-to-detonation
transition can be determined.11
RESULTS AND DISCUSSION
Wave profiles of particle velocity vs. time and
x-t plots of the shock trajectories were obtained for
two pairs of experiments with wave stresses of 2.9
and 6.2 GPa, and 3.9 and 8.6 GPa. Figure 2 pre-
Table 1. Summary information for double - shock experiments in EDC-37.
Shot#
Impact Vel.
Kel-F Thickness
1st shock Pressure
2nd shock pressure
(mm)
(km/s)
(GPa)
(GPa)
0.921
1175
2.87
6.20
1.033 ±0.005
1176
0.925
2.87
6.20
1.033 ±0.005
1194
3.92
1.170
8.56
1.428 ±0.005
3.92
1195
1.165
8.56
1.428 ±0.005
0.0
0.5
1.0
1.5
2.0
2.5
Run (after coalescence) (mm)
12.3 (6.5)
9.43 (2.40)
8.4 (2.0)
3.0
Time ((is)
FIGURE 2. Results of Shots 1175 and 1176 with shock strengths of 2.9 and 6.2 GPa in the first and second waves. Gauges are located at 0.0
and 2.5 through 11 mm on Vi mm intervals.
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16 -rt
sents data from the pair of lower pressure experiments. Note that there are duplicate gauges at the
impact surface as well as at 6.0, 6.5, and 7.0 mm.
The first wave appears to have no reaction whatsoever in it. The second wave appears to be reacting a
little; however, it is not until the waves coalesce at
5.8 mm and 1.6 ^is, that the reaction really gets under-way.
Figure 3 shows as-measured wave arrival times
for experiments 1175 and 1176. This is also
equivalent to a Lagrangian position - time or x-t
plot. From these plots, we can determine x-t points
for wave coalescence (5.8 mm, 1.6 (is), and more
importantly, the transition to detonation (12.3 mm,
2.9 n,s). This is about 1.3 mm beyond the last
gauge, which is located at 11.0 mm, and 6.5 mm
beyond the position where the first and second
shocks coalesce. From Figure 2, we can also determine the Lagrange velocities of the first and second
waves to be 3.6 and 5.6 km/s respectively. For this
pair of experiments, the run distance to detonation
after coalescence falls on the single shock Pop plot. This will be further discussed later in the paper.
In summary, for the first set of experiments we
saw no reaction in the first wave. While there was
considerable reaction in the following wave, it did
not contribute to the final buildup, and the run distance after coalescence fell on the single shock Pop
- plot.
In the next set of experiments we wanted to get
reaction in the first wave and even more reaction in
0.0
0.5
Detonation 12.3 mm, 2.9 jis
6.5 mm after coalescence
Coalescence 5.8 mm, 1.6 jo,s
Wave Arrival Time (jis)
FIGURE 3. x-t plot obtained from shock arrival at shock tracker
elements and particle velocity gauge elements for the first pair of
shots. The x-t points for coalescence and detonation are shown..
the second wave. We also wanted to increase the
pulse length by using a thicker layer of Kel-F,
thereby further allowing the reaction to run longer.
Figure 4 shows the results of these experiments.
The stresses of the first and second waves were 3.9
and 8.6 GPa, corresponding to single shock run
distances of 13 and 4 mm respectively. Clearly there
is a lot of reaction in the first and second waves.
This is revealed by the positive slopes in particle
1.0
1.5
2.0
2.5
Time (|ns)
FIGURE 4. Particle velocity wave profiles from Shots 1194 and 1195. The strengths of the two waves are 3.9 and 8.6 GPa. Gauges are
spaced at 0.0 and 2.4 through 10.9 mm on l/2 mm intervals.
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velocity following the initial shock as well as the
shock amplitude increasing with depth. Coalescence of the waves occurs at 7.0 mm. From the two
experiments we got x-t plots which didn't overlay
quite as well as those of Figure 2. We also got
slightly different run distances to detonation of 8.4
mm and 9.4 mm.
Figure 5 shows the Pop - plots for single9 and
double shocked EDC-37 (this paper). For the double shock experiments, the pressure of the second or
the coalesced wave is shown. The low pressure
point is from Shots 1175 and 1176. The run distance after coalescence is plotted and it lies on the
single shock Pop - plot.9 This pair of shots follows
the rule of thumb for double shock experiments:
"Buildup to detonation does not begin until after the
two waves coalesce."5
The higher pressure experiments, 1194 and
1195, are also shown in Figure 4. Run distances
from these experiments lie significantly off the Pop
- plot. The run distance after coalescence is significantly shorter than that predicted by the Pop - plot.
The total run distance (as shown circled) does not
fall on the Pop - plot either. Apparently if there is
significant reaction in the first wave, the rule about
buildup to detonation starting after wave coalescence no longer holds.
Finally, if we use the actual pressure level of
the wave at coalescence, we get about the right distance to detonation. Furthermore, it appears that a
more sophisticated reactive burn model than those
currently in use will be needed to accurately replicate these experiments.
*+u •
2.
J. E. Vorthman, and J. Wackerle "Multiple - Wave Effects
on Explosive Decomposition Rates" in Shock Waves in
Condensed Matter - 1983, Elsevier Science Publishers
(1984), p. 613.
A.W. Campbell, and J.R. Travis, "The Shock Desensitization of PBX 9404 and Composition B-3", in Proceedings of
the Eighth Symposium (International) on Detonation, Re-
5.
_j
C3) ooc
cn K>
-^
-».
n -*
Run Distance to Detonal
i [TCtal RunjUj;
*
:s
^:13
O
——
•
A
Rabie and Harry Wedge Test Data
Fit
Gas Gun Data
Double Shock After Coalescence
i
J.1
i
2.5
3 3.5 4 4.5 5
6
7
8
Input Pressure (GPa)
6.
7.
PBX 9502", in Proceedings of the Tenth Symposium (International) on Detonation, Office of Naval Research Report ONR 33395-12, p. 459 1993.
C.M. Tarver, T.M. Cook, P.A. Urtiew, and W.C. Tao, "Multiple Shock Initiation of LX-17", in Proceedings of the
Tenth Symposium (International) on Detonation, Office of
Naval Research Report ONR 33395-12 p. 696 1993.
R. E. Winter, P. Taylor, and D. A. Salisbury, "Reaction of
HMX Based Explosives Caused by the Regular Reflection
of Shocks" in Proceedings of the Eleventh Symposium (International) on Detonation, Office of Naval Research Report ONR 3300-5 p. 649 1998.
8.
9.
R. L. Rabie and H. H. Harry, Characterization of the British Explosives FD16, EDC29, EDC35 and EDC37, Los
Alamos National Laboratory Report # LA-UR-92-1928.
R.L. Gustavsen, S.A. Sheffield, R.R. Alcon, L.G. Hill, R.E.
Winter, D.A. Salisbury, and P. Taylor, in Shock Compression of Condensed Matter - 1999, American Institute of
Physics (AIP) Conference Proceedings 505 (1999), p. 879.
10. R. E. Winter, Lee Markland, and S. D. Prior, in Shock Compression of Condensed Matter - 1999, American Institute
of Physics (AIP) Conference Proceedings 505 (1999), p.
883.
11. S.A. Sheffield, R.L. Gustavsen and R.R. Alcon, in Shock
Compression of Condensed Matter - 1999, American Institute of Physics (AIP) Conference Proceedings 505 (1999), p.
1043
12.
p 0 =2.14g/cm 3 , C/ s =1.99 + 1.76wJ,km/s S.A. Sheffield
and R.R. Alcon, in Shock Compression of Condensed Matter - 1991, S.C. Schmidt, R.D. Dick, J.W. Forbes, D.G.
Tasker (editors), Elsevier Science Publishers (1992), p. 909.
13. L. M. Barker and R. E. Hollenbach, "Shock Wave Studies of
PMMA, Fused Silica and Sapphire," J. Appl. Phys., 41,
4208, 1970.
port NSWC-MP- 86-194, p. 1057, 1986.
4.
"^
FIGURE 5. Pop - plots for EDC-37 and double shocked EDC37.
REFERENCES
E.F. Gittings, "Initiation of a Solid Explosive by a Short
Duration Shock", in Proceedings of the Fourth Symposium
(International) on Detonation, Office of Naval Research
Report ACR-126, p. 373, 1965.
^
2
ACKNOWLEDGEMENTS
Robert Medina operated the gas gun for all of
the experiments.
1.
30
25
20
16
12
D
^
E
E
.2
C.L. Mader, Numerical Modeling of Explosives and Propellants, 2nd Ed., CRC Press, 1998, p. 213 - 220.
R.N. Mulford, S.A. Sheffield, and R.R. Alcon, "Initiation of
Preshocked High Explosives PBX 9404 PBX 9501, and
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