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
For special copyright notice, see page 844.
REACTION OF SHOCKED BUT UNDETONATED
HMX-BASED EXPLOSIVE
P. Taylor, D.A. Salisbury, L.S. Markland, R.E. Winter and M.I. Andrew
Hydrodynamics Department, AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK
Cylindrical samples of the pressed plastic bonded HMX based explosive EDC37, backed by metal
discs, were shocked through a stainless steel attenuator by an explosive donor. Reaction of the EDC37
sample was diagnosed with embedded PVDF pressure gauges and a distance to detonation for the
geometry was determined. Sample length was then reduced to less than the observed detonation
distance and laser interferometry was used to record the free surface velocity of the metal backing disc.
The results provide data on the metal driving energy liberated by explosive which is shocked and
reacting but not detonated. The results are compared with 2-D Eulerian calculations incorporating a 3term ignition and growth reactive burn model with desensitisation. It is found that a parameter set for
the reaction model which replicates the PVDF pressure profiles before reflection also gives good
agreement to the metal disc velocity history at early times. The results show that an appreciable fraction
of the metal driving potential of an explosive can be released without detonation being established.
experiments in a similar geometry have been
performed with the aim of simultaneously recording
the reaction profiles in the shocked explosive and
the resultant velocity history of adjacent metal
reflector discs. Measurement of both the reaction
history and metal driving ability of the shocked
explosive allows comparison with the energy
release prediction of a reactive burn algorithm such
as the Lee and Tarver ignition and growth model3 to
be made.
INTRODUCTION
Prediction of the response of explosive
containing systems to external shock stimuli is an
increasingly important area of study for application
to system safety assessments. A key question for
any predictive tool is whether the explosive is
detonated by the insult, as this is likely to lead to
the highest energy responses. However, the work
reported by Craig and Marshall1 on planar shocks in
explosive samples shorter than their natural run to
detonation distance shows that absence of
detonation conditions does not preclude significant
energy release from the shocked and reacting
explosive material.
Previous work by the authors of this paper2 on
the reaction of the HMX based explosive EDC37
suggested that normal reflection of strongly reacting
shocks from a high impedance barrier did not lead
to detonation, but did produce significant energy
release as observed from indentation depths in the
stainless steel reflector blocks. In order to better
quantify the amount of energy liberated from the
shocked but undetonated explosive, further
EXPERIMENTAL
The basic SI2D experimental geometry used in
this study is depicted in Figure 1. An EDC29 donor
charge initiated by a detonator drives a divergent
shock wave through a 45 mm thick stainless steel
barrier into an acceptor charge of EDC37 explosive.
This sample usually consisted of a set of machined
discs with PVDF pressure gauges mounted between
them. The sample was terminated with a reflector
disc or discs. For the reactive drive experiments, the
free surface velocity of this disc was
841
EDC37 Sample 1Discs
F-P Velocity Probes
V.
Perspex >^^
Rings
\^
Stainless
Steel ^^^
Barrier ^^~-
|
1
H
X, U /"
K^-————————————\ \ _ _—————————
N
^
(50
|-c
PVDF
^ Gauges
s^
-
80
EDC29 Donor
Charge
~~~~
consisting of a pulsed dye laser output via fibre
optic cable, with Fabry Perot analysis of the
returned light. The system has a velocity sensitivity
of 0.9 km/sec per fringe order (etalon spacing of 50
mm wavelength 602 nm). The dynamic fringe
patterns were recorded on Thomson 506 N electrooptical streak cameras. These records were
calibrated by the use of a pulsed laser diode time
marker system.
eflector
isc(s)
r—-~ ^
,
Perspex ^--~-T——————— Ji
/
Detonator
en
-o
RESULTS AND DISCUSSION
150
Dimensions mm
The first shock or detonation wave arrival time at
each on axis gauge location is recorded in Table 2,
where T NN.N is the arrival time (JLIS) of a wave at
gauge location NN.N (mm), relative to the shock
entering the sample. The time from detonator
current zero (lo) to shock arrival in the sample was
recorded as 15.6 ± 0.05 jus over the series.
SI2D/10 and SI2D/14 were control experiments
to establish the experimental distance to detonation
for the 45 mm barrier geometry with and without
embedded gauges. From the total charge transit
time alone a simple 1 -D equivalent run distance can
be calculated with the unreacted EDC37 Hugoniot
and the Pop plot. This procedure leads to a run
distance discrepancy of 5 mm between the two
experiments. This suggests the effect of the
embedded gauges is to reduce the effective 1-D
equivalent input pressure from about 24 kb to 21
kb. The 2-D divergent nature of the geometry leads
us to expect that the 1-D Pop plot will actually be
FIGURE 1. SI2D geometry with 3mm reflector disc shown.
diagnosed
with
Fabry
-Perot
velocity
interferometry at locations on and off the axis.
Control experiments without embedded gauges,
with an inert impedance match sample and with a
long sample length to determine the run to
detonation distance were included in the
experimental series which is detailed in Table 1.
The PVDF gauges were commercially available
Dynasen gauges insulated with Kapton and having a
thickness of less than 90 jam. Outside the gauge
area the void between the explosive discs was filled
with Sylgardl84 potting compound after assembly.
One experiment incorporated strain gauges in place
of PVDF pressure gauges, and the strain data from
this trial was used to provide a strain compensation
correction for the pressure data.
The Fabry-Perot measurement was performed
with a BMI Doppler Laser interferometer system,
TABLE 1.45 mm barrier SOD Experiments
Experiment
Number
SI2D/10
SI2D/1 1
SI2D/12
Sample
Material
EDC37
EDC37
EDC37
Derived 1-D
Reflector
Reflector
Gauge Planes
Run Distance*1
Material(s)
mm into sample
Thickness
31.5mm
40mm
St.Steel
0,10,17.5,22.5,42.5,62.5
40mm
St.Steel
None
Ob,10,17.5,22.5b
3mm
Oh,10,17.5,22.5h
None
Al.Alloy
3mma
St.Steel
KEL-F
22.5mm
Al.Alloy
3mm
Oh,10,17.5,22.5h
None
SI2D/13
a
3mm
St.Steel
EDC37
40mm
SI2D/14
60mm
26.5mm
0,60
StSteel
SI2D/15
EDC37
22.5mm
Copper
3mma
0,10,17.5,22.5°
None
a
The free surface velocity of these plates was monitored with F-P interferometry, on axis and 20 mm off axis.
h
These gauge positions contained two gauges, one on axis the other 20 mm off axis.
c
SI2D/15 contained strain gauges in identical positions to the PVDF gauges in SI2D/11-13.
d
Derived from transit time through the complete sample length the unreacted Hugoniot and the Pop plot.
Sample
Thickness
62.5mm
22.5mm
22.5mm
842
TABLE 2. PVDF Pressure gauge timing data.
T10
Expt.
T17.5 T22.5 T42.5 T60
US
MS
MS
MS
4.84 6.24 10.86 4.88 6.30
6.48 8.67
MS
SI2D/10 2.73
SI2D/1 1 2.77
SI2D/12
SI2D/13 3.62
SI2D/14
-
-
-
returning to the input face of the sample, at a typical
shock velocity, well below the normal detonation
propagation speed in the explosive.
T62.5
MS
13.17
.
11.75 -
Gauge 1 S-Steel/EDC37
Gauge 3 10mm into EDC37
Gauge 4 17.5mm into EDC37
EDC37/S-
displaced to higher run distances for a given input
pressure. This is confirmed by the embedded gauge
shock and detonation wave arrival times for
SI2D/10 shown in Fig. 2. A simple run distance of
37 mm is suggested which corresponds to a 1-D
Pop plot pressure of 19 kb. However the observed
average shock velocity of 3.615 mm/us equates to a
shock pressure of 34 kb.
2
PVDF Gauge Arrival limes
f
t+
Shock
fj
— Detonation
8
10
Figure 4 shows the pressure gauge data from
SI2D/12 where the reflector consisted of 3 mm
thick Aluminium Alloy and stainless steel discs.
The impedance mismatch between the two discs
leads to a double shock reflection, and there appears
to be increased reaction behind both of the reflected
shocks. The short duration release wave between
the two reflected shocks seen on the 22.5 mm gauge
indicates a small assembly gap between the two
discs. The double shock nature of the reflected
pulse is still evident when the shocks return to the
input face of the sample.
^
•
6
FIGURE 3. Comparison of PVDF gauge data from SI2D/11 22.5 mm sample (black) and SI2D/10 - 62.5 mm sample (grey).
^* **
S
4
Time from main shock into samplers
-
Distance/mm into sample
FIGURE 2. SI2D/10 x-t plot from gauge times.
The results from the two run distance
experiments suggest that most of the offset from the
1-D Pop plot run distance observed in SI2D/10 is
due to the 2-D divergent shock flow in the
experiment, with a small proportion of the increased
run distance due to the presence of the embedded
gauges. As the main study involves reflection of the
reactive shock well before the experimental run
distance of 37 mm, the small degradation of the
shock pressure and reactive growth profiles due to
the embedded gauges was judged to be acceptable.
Comparison of the pressure gauge data from
SI2D/10 and SI2D/11 shows the effect of reflecting
the reacting shock wave from a thick high
impedance barrier. Figure 3 shows that the reaction
profiles
before
reflection
are
reasonably
reproducible, and the reflection of the shock seems
to produce an increase in reaction rate behind the
reflected shock. The reflected shock is observed
Gauge 3 - 10mm
Gauge 4 - 17.5mm
Gauge 5 - 22.5mm
2
4
6
8
10
Time from main shock into sample/us
FIGURE 4. SI2D/12 pressure data against calculation in grey.
Also plotted in the figure, in grey, are pressure
profiles calculated using an Eulerian hydrocode
with a 0.5 mm mesh. The reactive burn model used
is a three-term ignition and growth model with a
shock desensitisation model. The desensitisation
model is required in the calculation to prevent the
843
good to around 5 jus, but at late times the hydrocode
overestimates the drive and terminal velocity.
explosive detonating when the shock pressure
increases on reflection. The parameters for the
model have been adjusted to give a reasonable fit to
the experimental reaction profiles close to the
reflection plane.
Figure 5 compares the experimental free surface
velocity measurements from SI2D/12 with
calculated histories from the same calculation
shown in Fig.4. Agreement is good for the first 3-5
jas, but the ignition and growth calculation appears
to be over-predicting the late time drive, giving
significantly higher terminal velocities than the
experimental observations.
—SI2D/15 data - on axis
—Calculation - on axis
—SI2D/15 data - 20mm off axis
—Calculation - 20mm off axis
FIGURE 6. Free surface velocity measurements from 3 mm
Copper reflector experiment SI2D/15 compared to calculation.
CONCLUSIONS
SI2D/12-onaxis
Calculation - on axis
Detonation calculation - on axis
SI2D/13 data - 20mm off axis
Calculation - 20mm off axis
SI2D/13 data (Kel-F samole) - on axis
The data presented clearly shows significant
energy release from the shocked but undetonated
explosive. A 2-D Eulerian hydrocode with an
ignition and growth reactive burn model and a
desensitisation model provides a reasonable fit to
the recorded reaction profiles recorded with
embedded pressure gauges. This calculational
modelling provides a good fit to the early time
reflector plate velocity but tends to provide too
much late time drive. A possible explanation of this
discrepancy is that the calculated reaction rate
remains finite at late times whereas experimentally
the presence of 2-D divergence and release waves
from the free surface of the reflector quench the
reaction.
35
Time/Ms from lo
FIGURE 5. Free surface velocity measurement from 3 mm
Aluminium Alloy / 3mm stainless steel reflector experiments
SI2D/12 and SI2D/13 compared to calculations of SI2D/12.
A calculation was also performed on the same
geometry where the EDC37 was modelled with
programmed burn and a detonation point, rather
than ignition and growth reactive burn treatment,
and the resultant velocity profile is also plotted. It
shows a terminal velocity of 1.75 mm/jas compared
to 1.2 mm/us for the experiment and 1.6 mm/us for
the reactive burn calculation. The free surface
velocity trace on axis for the inert impedance match
Kel-F sample experiment is also included in the
figure, showing the drive given to the reflector discs
from the donor charge alone. A simple velocity
squared analysis suggests almost 50 % of the total
detonation drive is released in the SI2D/12
experiment without detonation being established.
SI2D/15 incorporated a single 3 mm Copper
reflector disc. The free surface velocity
measurements for this experiment are shown in Fig.
6 for comparison with a hydrocode calculation
using the same reactive burn model used to
calculate SI2D/12. The early time agreement is
REFERENCES
1. Craig, B. G. and Marshall, E. F., "Decomposition of a
Shocked Solid Explosive", in 5th Symposium (Int.) on
Detonation, 1970, pp. 321-329.
2. Winter, R. E., Taylor, P. and Salisbury, D. A.,
"Reaction of HMX Based Explosive Caused by
Regular Reflection of Shocks", in 11th International
Detonation Symposium, 1998, pp. 649-656.
3. Lee, E. L. and Tarver, C. M, "Phenomenological
model of shock initiation in heterogeneous
explosives", Phys. Fluids 23(12), pp.2362-2372
(1980).
© British Crown Copyright 2001/MOD
Published with the permission of the Controller of Her Britannic
Majesty's Stationery Office.
844