CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
DETONATION PHENOMENA OF PBX MICROSAMPLES
I. Plaksin, J. Campos, J. Ribeiro and R. Mendes
Laboratory of Energetics andDetonics, Mech. Eng. Dep., Faculty of Sciences and Technology,
University ofCoimbra, Polo II, 3030 Coimbra, PORTUGAL
Abstract. Detonation study of PBX micro-samples, based in HMX with an inert (HTPB, epoxy)
or energetic (GAP) binder was performed on the meso-scale level, using the multifiber optical
probes of 50 Jim of maximum resolution, connected directly to a fast electronic streak camera
with 0.6 ns resolution. The direct 2D observation of particle to particle successive transition of
transmitted shock wave, through the binder, allows to analyse and to discuss, not only the
cooperative formation of a multihead detonation front (DF), in the collection of particles
surrounded by binder, but also the synenergetic effect, behind the DF, by the appearing of
dissipative structures drawing spatial and temporal DF oscillations.
main different kinds of detonation wave [DW]
regimes, showing different unsteady spatialtemporal dissipative structures [DS].
These spatial-temporal DS are consequently the
OS with the transversal SW (represented the
cellular DW with the longitudinal-transversal DF
oscillations) and OS without transversal waves, but
with the additional longitudinal shocks (DF as a
oscillating giant monocell). Characteristic sizes of
DS depend on the EM physical/chemical properties
and its rheology. We have detected both of these
regimes in PBX *~3.
The phenomenology of the origination and
transformation of naturally unstable PBX
detonation regimes has been already described 6 ,
based on the experiments on the meso-scale level.
The factor of divergence of the reactive flow
behind the FF (specified by front curvature) was
found to be the influent parameter determining the
regimes of DW instabilities and DS. It is obvious
that the increase of the divergence, in the reacting
particles flow, will change the rate of relaxation, in
strongly non-equilibrium temperature/stress fields,
behind the FF. Micro-mechanisms of temperaturestress transfer are very important in the OS lateral
phase formation. The kinetic relaxation level, in the
INTRODUCTION
Phenomena of pulsing detonation of PBX was
originally mentioned *~3 in the experiments, carried
out on the meso-scale level, as a result of the
application of high resolution multi channel optical
method, based on optical fiber strip1"3. Phenomena
of pulsing detonation, in chemically reacted media,
implies that the initially smooth shock front,
induced by the external source, losses its stability
and it is followed by the origination of the
oscillating structure [OS], behind the forward front
[FF]. OS appears as a result of the inter-influence,
in the strongly non equilibrium zone within the FF
and chemical reaction zone [CRZ], of a few
relaxation phenomena: kinetic relaxation involving
release of energy (due to the exothermic reactions),
thermal dissipation (due to heat/mass transfer) and
stress relaxation (due to the existing limit velocity
of impulse/shock waves [SW] propagation).
Theoretically
it was obtained4'5 that,
independently of the physical nature of the
energetic material, but according to the
combination of different main relaxation times, in
energy release and heat/mass transfer, and under
the influence of the fluctuations), it can exist two
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process of propagation in crystals of shock reactive
wave [SRW], constitutes the governing factor
determining the initial phase of OS origination. The
existing delay of the energy release, in shocked and
reacting crystals (in PBX, after the FF), could
imply the changing of process of energy transfer
and the pattern of non equilibrium temperature and
stress fields, in CRZ.
The presented study concerns the evaluation of
the relaxation phenomena of energy release and of
the stress fields, associated with SW propagation in
explosive crystals, by the direct registration of SW
within single crystals and its clusters, under the
similar conditions to the PBX detonation. This
objective also implies the clarification of the
complex pattern of the OS depending not only by
particle sizes and its compaction but also by the
binder nature (inert or reactive).
EXPERIMENTS
The experiments were carried out on the mesoscale level with the micro and mini samples of
PBX, based on HMX crystals surrounded by
polymer binders (HTPB, epoxy, GAP) or by water.
The multifiber optical probes [MFOP] of the matrix
type, with 250 um of spatial resolution (50 pm
maximum, in colimation mode)7, were connected
directly to a fast electronic streak camera
(THOMSON TSN 506N). The streak records, with
0.6 ns of maximum temporal resolution, allow the
2D and 3D analysis of SW and DW in PBX microsamples and HMX crystals. This procedure allows
the registration of, not only the emitted irradiation
from the front surface (in SW propagation inside
the p-sample), but also the induced stress
amplitudes and the front geometry (in thin layers of
kapton), from the input and output SW.
1.5
2
Z(mm)
2.5
0
50 100150
Intensity of light (%)
f) g)
FIGURE 1. Registration on the SW propagation in HMX
crystal, surrounded by HTPB binder, [a) experimental set-up; b);
c); and d) micro-photos of HMX crystal, matrix MFOP and the
MFOP in the background of the crystal; e) photochronogram; f)
z-t diagram and velocity of SW before, during and after the
crystal; g) histogram of the relative intensity of the light emitted
from the central zone of the propagated S W front.
The experimental results, presented in these
figures, show the significant effects of the SW
propagation inside the HMX crystal: 1 - The non
monotonous built up and following decrease of SW
velocity D, in order of the successive increasing
and decreasing of the crystal cross section,
followed by the enhance of the light emission
intensity, at the end of the SW run. Pike of D
corresponds to 2/3 of SW total run in the crystal; 2
- The delay time of the maximum stress phase, in
the down kapton barrier, corresponding to the delay
of the pike of energy release.; 3 - Anisotropical
effect of the SF propagated through the binder,
with the enhanced stress, not in the central, but in
the preferential zone of the crystal/binder interface,
proved by preferential development of SW in
kapton barrier (Figure 2 e)).
RESULTS AND DISCUSSION
Non Monotonous Shock Reaction and Energy
Release in Coarse HMX Crystals
Two experiments have been conducted for the
direct time registration of the SW propagation in
single coarse HMX crystals, surrounded HTPB
binder (Figure 1) and by water (Figure 2).
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FIGURE 2. Experiment with HMX crystal, surrounded by
water, a) experimental set-up, b) HMX crystal and side MFOP,
c) HMX crystal and the bottom MFOP, top view
d) photo-chronogram showing the pulsing SW propagation
inside the crystal, e) detail of the obtained photo-chronogram
(bottom MFOP), showing the surface edge effect of the
enhanced SW propagation in the peripheral zone of crystal.
1.5
FIGURE 3. Experiments with vertically oriented group of three
HMX crystals in epoxy binder, a) experimental set-up; b) HMX
crystals and side MFOP; c) photo-chronogram showing the
pulsing SW propagation; d) z-t diagram of SW propagation
from crystal to crystal.
The obtained results clarify the mechanism of
relaxation of the HMX crystal reaction, induced by
strong SW, proving the existing time delay in its
energy release, behind the FF. The experimental
time relaxation seems to be greater than the time
needed for the shock propagation inside the
crystal6. Future developments will be focused to the
quantitative evaluation of the ratio relaxation time
vs. ignition delay.
The results of the experiment with the vertical
group of three HMX crystals, in the epoxy binder,
are presented in Figure 3, showing the non
monotonous (pulsing) process of the SW
propagation, inside the crystals, and its transition
from crystal to crystal, through the 10-20 pm thick
inter-particle space. The mean velocity of SW
propagation, inside the crystal's chain is estimated
in 7.0 mm/ps, that exceeds in ~1.4 times the SW
velocity in the surrounding epoxy binder (but in
—1.2 times less than the maximum velocity within
the individual crystals).
From the presented results it can be concluded
that the defined relaxation effect of the unstable
kinetics of shock reaction of a single crystal,
accompanied by the surface edge effect, shows the
same essential properties that can be observed in
the group of crystals, creating the conditions for
high order of fluctuations of energy dissipation,
behind the FF, followed by formation of the DS by
the mechanism of the transversal micro shock
interactions.
Reactive Waves Propagation, Interaction and
Transition in the Ensemble of HMX Crystals
These kind of experiments were performed with
a collection of crystals, arranged in the vertical
position, related to the initiating SW. Other results
of experiments with horizontal clusters of HMX
crystals, surrounded by HTPB and by GAP binders,
show3 the significant role of the binders nature in
the interaction process between the induced SW in
the inter-crystal space. Performed tests show
colliding SW's, generating the strong reaction in
GAP binder, generate a multi-head front less
fluctuated (more homogeneous) that appears with
the HTPB binder.
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These experimental results confirm the
synergetic effects in PBX detonation described in
the past3'6.
//Kapton
7 5 0 jim
100 jim
CONCLUSIONS
Matrix of MFOP
,„„„. 4x20 fibers
To THOMSON
TSN 506 N
The detonation study of PBX on the meso-scale
level has been carried out applying the highresolution multi-fiber optical technique, in original
tests, with single, two and more particles,
surrounded by inert and energetic binders.
The obtained results show the kinetic instability in
shock reaction of coarse HMX crystals, surrounded
by binder, the cooperative formation of a multihead
detonation front (DF), in the collection of particles,
and also the synergetic effect, behind the DF, by
the appearing of dissipative structures with spatial
and temporal DF oscillations.
FIGURE 4. Long Channel Test (80x5x5mm, PBX charge in
copper confinement) with PBX micro sample in its terminal
zone, a) terminal detail of experimental setup; b) photochronogram showing the existing of the cellular structure of DF
and OS.
TTT PB
1.06 mm
Kapton
Barrier
-Linear MFOP
To Thompson
REFERENCES
TSN-506N
FIGURE 5. Mini Gap Test, a) experimental setup; b) photochronogram.
1. Plaksin, L, Campos, J., Mendes, R., and Gois, J.,
"Interaction of Double Corner Turning Effect in
PBX", Shock Compression in Condensed Matter 1997, edited by S. C. Schmidt, D. P. Dandekar, and J.
W. Forbes, AIP CP 429, New York, 1998, p.p. 755758
2. Plaksin, L, Campos, J., Mendes, R., Ribeiro, J., and
Gois, J., "Pulsing Behaviour and Corner Turning
Effect of PBX", in Eleven International Symposium
on Detonation, pp. 679-685.
3. Plaksin, J. Campos, R. Mendes, J. Ribeiro and J.
Gois, "Mechanism of Detonation Wave Propagation
in PBX with Energetic Binder", Shock Compression
in Condensed Matter - 1999, edited by M. D.
Furnish, L. C. Chhabildas, and R. S. Hixon, AIP CP
505, New York, 2000, p.p. 817-820
4. Daniljenko, V. A., Kudinov, V. M., Dokladi.
Akademii Nauk Ukr. SSR, 1982, N. 12, p.p. 24-27
5. Daniljenko, V. A., Dissertation. Institute of
Hydrodinamics of Siberian Branch of Academy of
Sciences of USSR. 1984.
Experiments with micro and mini samples of
PBX
The experiments, demonstrating the selforganization phenomena in PBX detonation, were
carried out with p-samples and m-samples of PBX
based on 82 mass percent of HMX (80% of d50 =
240 pm, 20% d50 = 17 pm) and 18% of an
energetic binder (GAP). The results of the long
channel test6 (where DF has a big positive
curvature) show the existence of the DS forming
the multi-head, or cellular DF, and the process of
their origination and development after the DF had
propagated through a kapton p-barrier. The
characteristic size of the individual cell is equal to
5-6 d50 of coarse HMX particles.
The results of mini-gap test6 (Figure 5) with the
quasi-plane SW input, shows the existence of the
longitudinal oscillations of DF, representing in this
case a giant mono-cell. Its origin is immediately
after the shock run equal to 2xd50.
6. Plaksin, L, Campos, J., Ribeiro, J., and Mendes,
R., "Irregularities of Detonation Wave Structure
and Propagation in PBX", 32nd International
Annual Conference of ICT - Energetic
Materials, Ignition Combustion and Detonation,
Karlsruhe, July 3-6, 2001, pp. 31-1 to 31-14.
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