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
MECHANICAL BEHAVIOR OF EXPLOSIVES AT HIGH PRESSURES
J. M. Kelley, V. S. Joshi, and R. H. Guirguis
Research and Technology Department
Naval Surface Warfare Center
Indian Head, MD 20640
Abstract. The mechanical and ignition behaviors of heterogeneous explosives are highly dependent
on the pressure. Experimental data of the compaction of composite plastic-bonded inert samples under
hydrostatic loading conditions are presented. The data reduction techniques and the procedures used to
correct errors introduced by the compliance of the apparatus are addressed.
INTRODUCTION
than the test samples, the latter can be easily submerged, thus aiding the elimination of trapped air.
Glycerin was also selected for its low toxicity (a
pharmaceutical and food ingredient), minimal
chemical interaction with most test samples, and
because of its high freezing pressure at room temperature.
Figure 1 is a schematic of the apparatus. It was
adapted from a high-pressure intensifier in which
two pistons of different cross-sectional areas are
rigidly mounted in line. Fluid is supplied at moderate pressure to the larger piston. In principle, this
pressure is amplified by the ratio of the two areas.
However, in practice, the friction introduced by the
seals reduces the pressure gain below this theoretical value.
In quasi-static loading, because the piston moves
slowly, a dial-indicator is adequate for recording the
piston displacement in real time, from which the
volume of the high-pressure cavity can be derived.
Accurately measuring the initial volume of the cavity was bypassed by conducting comparative experiments and applying subtraction techniques during data reduction as described below. Three sets of
experiments were performed, starting at a different
piston location in each set (i. e., different initial
cavity volumes). Two tests were performed in each
set, one with and the other without the sample,
starting each test at the same piston location.
To test explosive samples up to l/2\ an intensifier
with a bore measuring %" in diameter and 8" in
The quasi-static compression behavior of Vfc"
composite plastic-bonded inert samples was measured under hydrostatic loading conditions at pressures up to 8 kbar. A new apparatus was specifically
built for this purpose. Unlike previous designs using
a cylinder-piston arrangement, the current approach
subjects the samples to quasi-static compression in a
working fluid, such as to ensure hydrostatic loading
conditions. The cavity pressure is recorded using a
piezoelectric transducer, while its volume is determined by measuring the displacement of the piston.
APPARATUS
Various apparatus and seals to apply and maintain quasi-static high-pressure loads over an extended duration were proposed by Bridgman [1], In
these, hydrostatic loading conditions are achieved
by using a working fluid as an interface between the
walls of the device applying the stress and the sample. Because fluids mostly resist compression but
little shear, if the vessel wall is not impulsively
started, then hydrostatic loading conditions can be
maintained (even under dynamic loading conditions) by selecting a working fluid with a high
sound speed. In the experiments described below,
glycerin was selected as the working fluid for its
moderate density (1.25 g/cm3) and high sound speed
(1.9 mm/fas). By selecting the working fluid lighter
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length, capable of containing 200 kpsi was selected.
To the original intensifier, an off-the-shelf product
of Harwood Engineering Company, a smalldiameter extension rod was attached to the large
piston in order to measure the piston displacement.
The diameter of the low-pressure piston is 21/2".
With the W extension rod attached, the area ratio
was reduced from 15.5:1 to 14.88:1. The highpressure closure plug was fitted with a T-section to
accommodate: (1) a piezoelectric pressure transducer; (2) a manually-operated needle-valve to relieve any residual pressure in the high pressure section (cavity and conduits); and (3) a burst-tube designed to fail at 220 kpsi and release the pressure.
An Enerpac® 10 kpsi hydraulic hand-pump
equipped with a pressure regulator and Bourdontube pressure gauge was selected to remotely power
the low-pressure hydraulic cylinder. Test-section
pressure was acquired via a Kistler Model 6213B
quartz piezoelectric transducer nominally rated to 10
kbar. A Kistler Type 501 OB dual-mode amplifier,
recommended for measuring quasi-static pressures
because of its "long time constant" charge mode,
and a Nicolet Integra Model 40 12-bit digital oscilloscope were used to acquire the pressure data. Piston displacement was measured at the extension rod
using a Fowler-Sylvac electronic dial indicator capable of 0.00005" (1.3 jam) resolution and a total
travel of 1" and the data recorded every Is using a
PC. For safety purposes, the tests were remotely
conducted and monitored via a closed-circuit television camera.
TEST SAMPLES
All the data presented in this paper are for inert
samples, similar to plastic-bonded explosives, but in
which the energetic crystals were replaced by a
simulant. Sucrose was selected from several possible candidate simulants because of its availability in
several granulations, its low toxicity, and relatively
low cost. The particle-diameter specifications for
several grades were provided by Tate-Lyle Company, the manufacturer of Domino® sugars.
A solid filler with a bimodal distribution was
used such that the coarse-to-fine ratio was 3: 1 and
the ratio of the mean diameters was 10:1. Domino®
"Industrial 10X Confectioner's" grade sucrose was
used as the fine fraction, and a blend of larger particles sieved from various grades was used for the
coarse component. Procedures representative of
standard processing techniques were used to prepare
a small batch of the mock-composition. The cured
material was machined into ¥2 ' x ¥2 ' right circular
cylinders. The volume and density of each sample
were measured with a helium pycnometer.
EXPERIMENTAL PROCEDURE
Because glycerin was used as a transfer medium, special precautions were taken during the
tests in order to ensure the accuracy of the measured
mechanical properties:
1. Pressure-loading of the samples was done in a
step-wise fashion, with sufficient dwell-time at
each step to allow the materials in the cavity to
equilibrate before the pressure was increased in
the next step.
2. To increase the signature of the test samples
versus that of the glycerin used as a working
fluid, the number of samples used in each test
was selected such that their total volume occupied most of the intensifier cavity.
3. Prior to placement in the apparatus, each sample was lightly coated with Fluorolube® GR290 (Hooker Chemical Co.) to minimize intrusion of the glycerin.
4. To eliminate entrapped air, each leg of the apparatus was manually filled with glycerin.
piston
high-pressure
cavity
FIGURE 1. Schematic of high-pressure apparatus.
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5.
6.
The glycerin in the intensifier bore, which was
replaced for each test, was evacuated in situ
until no bubbles appeared.
After compaction, each sample was blotted free
of glycerin and weighed. Weight changes were
negligible in all but one instance, when a sample apparently lost weight. Removal of some of
the Fluorolube® coating was determined to be
the cause.
Next, the compaction data of mock samples and
glycerin illustrated in Fig. 4 were reduced using
(2)
tample
where A is the area of the piston, and Vs is the volume of the sample(s) at atmospheric pressure. In Eq.
2, xs(p) is the piston location at pressure p, whereas
xg(p) is the corresponding piston location in the test
conducted in the same series but with glycerin
alone.
When calculating V/V0 for the mock samples,
the average fit for V/V0 of glycerin described above
was used. The results are illustrated in Fig. 5. It is
important to notice that the scatter in the values of
(VW0)sampie, portraying the variation from one test to
another, is smaller than the corresponding scatter in
(V/V0)glycerin observed in Fig. 3. That is because unlike glycerin which is sensitive to its environment,
the samples made of sucrose and binder are much
more stable.
Figure 6 compares the compaction properties of
the test samples to the compression behavior of
glycerin. In the lower pressure regimes the test samples exhibit a somewhat higher compressibility than
does the glycerin, but as the pressure increases the
sample becomes more rigid. This is attributed to
lock-up of the filler particles.
DATA REDUCTION
As explained above, in order to eliminate the
errors arising from flexing of the apparatus at high
pressure, three sets of experiments were performed,
starting each set at a different piston location. Two
tests were performed in each set, one with and the
other without the sample, starting each test at the
same piston location.
First, the measured pressure vs. piston displacement data describing the behavior of the neat glycerin and illustrated in Fig. 2 were reduced using
X2-X,
(1)
where V denotes the specific volume (per unit mass)
at pressure p and V0 is the corresponding volume at
atmospheric pressure. In Eq. 1, \i(p) - x2(p) is the
difference in piston locations between two glycerin
tests begun at Xj and X2, respectively, both resulting
in the same pressure p.
The calculated values of V/V0 for glycerin are
illustrated in Fig. 3. The individual data sets are
represented by different symbols. The solid curve is
a fit of the points resulting from averaging all three
sets.
In theory, V/V0 is an intrinsic material property
and should not depend on the piston starting location. However, in practice, because glycerin is hygroscopic, moisture sorption varies with duration of
exposure to atmospheric air, resulting in a slightly
different material for each individual test, with different mechanical properties. In addition to the other
possible errors committed during data acquisition
and reduction, the scatter in the data illustrated in
Fig. 3 is attributed to the differences in the properties of glycerin used in each test.
Glycerin Only
Small Vo
2
4
6
8
10
Pressure, kbar
FIGURE 2. Piston displacement vs. pressure in glycerin compression tests.
866
1
Squares:
Piston Positions 1-2
0.98
0.98
0.96
0.96
0.94
0.94
Circles:
Piston Positions 1-3
0.92
0.92
0.9
0.9
0.88
0.88
4
4
Pressure, kbar
Pressure, kbar
FIGURES. V/V0 for glycerin.
FIGURE 6. Comparison of (V/Vo),^ to (V/V0)giycerin.
CONCLUSIONS
Glycerin-i-Samples (1,2,3)
Urge Vo
A new apparatus capable of quasi-static compaction of explosives under hydrostatic loading
conditions reaching 8 kbar was built. The errors
arising from flexing of the apparatus at high pressure were eliminated by conducting comparative
experiments and applying subtraction techniques
during data reduction. The compaction properties of
test samples were expressed in terms of the compression behavior of glycerin, selected as a working
fluid for its high sound speed and moderate density.
Preliminary results describing the compression of
glycerin and the compaction of an inert analog of a
typical plastic-bonded explosive composition are
presented.
Glycerin + Samples (4,5)
Intermediate Vo
-2
0
2
4
6
8
10
Pressure, kbar
FIGURE 4. Piston displacement vs. pressure for samples
compacted in glycerin.
REFERENCES
1
Bridgman, P. W., The Physics of High Pressure,
Dover Publications, Inc., New York, NY, 1970.
is—————————————•———————————'•—— Circles:
0.98
Intermediate Vo, _
Samples 4,5
0.96
0.94
0.92
0.9
0.88
2
4
10
Pressure, kbar
FIGURE 5. V/V0 for test samples.
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