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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
EXPERIMENTAL INVESTIGATION OF HETEROGENEOUS HE
DECOMPOSITION MECHANISM IN DETONATION WAVE FRONT
A.V.Fedorov
Russian Federal Nuclear Center, VNIIEF, 607190, Sarov, Russia
Abstract. The mechanism of decomposition of heterogeneous HE in hot spots is considered.
Experimental investigations have shown that the main reason of formation of locally heated areas is the
existance high-velocity microjets, which penetrate HE layers laying ahead. Chemical reaction, thus, can
begin in hot spots already before detonation wave arrival. Different structures of detonation wave and
different profiles of particle velocity in the chemical reaction zone can be explained by jets penetration.
There is also discussion on the relation of von Neumann spike and Chapman-Jouguet state for highdensity condensed HE.
Shock wave passes it in «0.15 ns. Laser spot size at
the surface is «100 jum.
INTRODUCTION
In recent years the methods with high time
resolution are used to study the detonation wave
structure and chemical reactions zone (CRZ) of
condensed HE [1-8]. We have performed studies of
detonation front of liquid homogeneous and solid
heterogeneous HE [6-8]. The model of hot spots,
which explains the mechanism of decomposition of
heterogeneous HE in detonation wave, is well
known. Voids collapse and occurrence of hot area,
as well as friction between HE particles, shifts, and
dislocations are now considered as the basic reasons
of hot spots formation. However, already in the 40's
A. Apin wrote [9] that detonation of heterogeneous
HE propagating by there is penetration of reaction
products into HE layers laying ahead, and velocity
of these products is higher than detonation velocity.
FIGURE 1. 1.-detonator; 2.-liquid HE in cylindrical cell or
sample of solid HE; 3,-Al covering (0.2-1.5 ^M)) or Al foil (5-10
JIM); 4.- LiF crystal; 5.-laser beam (X=694.3 nm); 6.-focusing
lens.
HE is initiated by a divergent shock wave.
Pressure of this wave in aluminum cap of electric
detonator is 22 GPa.
Von Neumann spike of high-sensitive HE is
formed in this case on thicknesses close to value of
critical diameter [6, 7].
EXPERIMENTAL
The experimental set-up is depicted in Fig. 1.
Fabry-Perot laser interferometer records particle
velocity profile at the HE-LiF window interface. To
obtain good time resolution, aluminum covering
(w 1 jam) is placed between HE and window.
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The majority of experimental facts testify that
Al covering and LiF layer adjacent to it are
destroyed by microjets and the recording is broken.
The following experimental facts are confirming
that the microjets are moving with a velocity higher
than the detonation velocity, penetrate into the
unreacted HE and are the main reason for formation
of locally heated areas (hot spots):
1. In a series of experiments (see Fig.l), laser
interferometer records have shown that even before
the detonation wave the arrival or before
Al covering is broken, the individual HE - window
interface starts to be smoothly accelerated. Time of
such record is «5-7 ns, the maximum velocity
before break is «100 m/s. Approaching particles of
the microjets are, most probably, the reason of such
an acceleration.
2. Figure 3 shows schematically the results of
one of the model experiments, with conical cavities
(simulating voids in HE) 0.3 - 4 mm deep and
vertex angle of 30 - 60 degrees in plastisized
composition of PETN. HE was in contact with to an
aluminum plate. After the explosion, parameters of
the cavity formed in the Al plate were measured.
Sizes of the cavity formed in the Al plate are
approximately equal to sizes of the conical hollow
in the HE. In the case, when the cavity is filled with
glue (which simulated the binder), size of the
hollow in the Al plate approximately 3-4 times
smaller cavity. If there was a small conical cavity in
the Al plate, the size of this conical cavity was 3-4
times larger because of explosion products.
EXPERIMENTAL RESULTS
Earlier in [6, 7] two types of profiles of
detonation wave were recorded at the HE-LiF
interface: smooth concave profiles falling from
von Neumann spike and protuberant profiles with
decelerating oscillations of velocity. The profiles
with velocity oscillations are depicted in Fig.2.
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200
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FIGURE 2 Profiles of particle velocity with decelerating
oscillations
Before the explosion
HE
Vertex angle
ct=40°
;avity filled
with glue
5mm
^2 mm
In this paper we present an experimental proof
that the formation of such profiles is caused by
microjets which occurred, when the detonation front
interacts with voids existing in a heterogeneous HE.
When such microjets outstrip detonation front
and decelerate in unreacted HE in immediate
vicinity of surface to be recorded (HE-LiF
interface), the laser interferometer records
perturbations caused by jet in profile of detonation
wave.
Usually, in our experiments, we place only thin
aluminum covering («1 jim) between HE and LiF
window. In these experiments either different
profiles of detonation wave are recorded, or the
record is broken for very short time (<1 ns).
Aluminu:
plate
After the explosion
0,5...0,8 mm
*2 mm
1,5...2 mm
FIGURE 3 Model experiments with conical cavities.
Thus, the experiments have shown that the jets
make a small cavity in the Al plate, and the
subsequent effects of explosion products increase
its sizes. In the case, when the jets penefiole into the
unreacted HE rather that into the metal, this effect
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will be stronger due to less density and strength of
HE.
3. In experiments with conical cavities 2-4 mm
high(see Fig.3) we also made a measurement of the
jet velocity using time of arrival of DW to the top of
the conical cavity and time of the jet approach to Al
foil 100 jam which was in contact with LiF. Under
the jet effect the Al-LiF boundary is accelerated up
to a velocity of about 100-400 m/s for several
nanoseconds. Then the recording is broken because
of the breakage Al foil. Velocity of the jet head was
9-12 km/s and higher. The detonation velocity was
7.8 km/s.
4. Tests with various HE's were performed
according to the scheme presented in Fig.l using Al
covering («l|im). Break of interferometric
recording of the HE-LiF interface velocity occurs as
a result of fracture of the reflecting covering by
microjets. In homogeneous liquid HE the recording
breaks occur very seldom, in high-density HE (12% of porosity) the breaks occur in »30 % of the
cases. Number of breaks grows sharply with the
increase in HE porosity.
5.Velocity oscillations (see Fig.2) were recorded
in profiles of the detonation wave Similar profiles
are recorded in [1] for HMX with porosity of 35%.
Average size of HE particles of «10 \im cause
velocity oscillations of 300-400 m/s, HE particles
sizes of 120 jim - cause velocity oscillation of 801000 m/s [1]. The recorded value of spike in
window material decreases in comparison with
high-density HE («1.5 % of porosity) from 3.6
mm/jis up to 2.2 mm/jis and lower (i.e. by 40 % and
more).
6. For HE (HMX, RDX) having maximum
density and minimum porosity (1-2 %), detonation
wave passes the voids (of 10-20 jam) in times of
about 1-2 ns [14]. Duration of chemical reactions
for these HE's is about 50 ns [3]. The jets overtake
the detonation wave front. HE in these jets is,
probably, in the decomposition stage, since jet is
formed by an HE, which is in the CRZ. On the other
hand, chemical reaction is not yet completed.
Hence, the jets carry forward ions, radicals,
intermediate and final products of reactions. Thus,
chemical reaction can start in hot spots already
before detonation wave arrival. Microjets catalyze
the process of HE decomposition. It is known that
for liquid homogeneous HE (TNM, NM, etc.), as
well as for HE monocrystals (HMX, RDX, etc.) the
value of critical diameter is 10-20 mm and more.
Including 1-2 % of voids in HE reduces critical
diameter ten times. According to the Yu. Khariton's
principle, the smaller the critical diameter, the
smaller is the duration of CRZ. Thus, microjets
increase the rate of chemical reactions, reduce
duration of the CRZ, and catalyze the process of HE
decomposition.
Relation of the von Neumann spike values and
the Chapman-Jouguet state.
Last years a series of experimental works was
published, where values of the von Neumann spike
in condensed HE are measured by methods with
high time resolution with good precision [1-8]. The
values of the Chapman-Jouguet (C-J) state for these
HE are also known. It is of interest to determine the
relation between these values.
Many authors, for example, L. Al'tshuler [10]
noted that experimental data on C-J pressures of HE
is
characterized
by
extraordinary
large
incoordination. So, for TNT, which is the most
investigated HE, at density of 1.63-1.64 g/cm3, the
detonation pressures are found to be between 17.7
and 21.3 GPa. For PBX 9404 compositions, the C-J
pressures are changed from 34.5 to 39.16. Craig
[11,12] revealed that the effective C-J pressure
grows in accordance with detonation wave
propagation in HE. Using charges of large
diameters (100-300 mm), Craig revealed that the
detonation pressure at change of charge thickness
from 12.7 mm up to 101.6 mm grows from 30.5 up
to 37.5 GPa. The dependence curve of the effective
C-J pressure on charge thickness from [12] is
depicted in Fig.4. Mader writes that for thicknesses
charge within 40-100 mm the C-J pressure is 3537.5 GPa, and actual value of C-J pressure for
unlimited medium is equal to «40 GPa. We
determined the von Neumann spike pressures for
four compositions close to PBX 9404 (HMX,
agatized HMX, HMX with 5% and 10% binder).
They are 47.8 - 49.5 GPa (average value is 49 GPa).
This value of the von Neumann spike is presented
in Fig.4. The fact that the value of the von
Neumann spike for high-sensitive HE is formed at
thickness equal to the critical diameter (dcr), and
then remains constant at thicknesses up to 200 dcr
and more, is verified by us using plastisized PETN.
Thus, for a stationary detonation wave, the
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Neumann spike pressure does not depend on charge
thickness.
ACKNOWLEDGEMENTS.
The author greatly appreciate the help of A.L.
Mikhaylov, I.P. Khabarov, V.M. Bel'sky, L.A.
Gatilov, A.V. Men'shikh, D.V. Nazarov, S.A.
Finyushin, V.A. Davydov in conduction of this
work and for heir helpful advises and discussions.
REFERENCES
80
1. Gustavsen, R.L., Sheffield, S.A., Alcon, R.R.
"Detonation wave profiles in HMX based explosives"
Proceedings of the Int. Conf. Shock Compression of
Condensed Matter, pp.739-742. Amherst, 1997.
2. Tarver, C.M., Breithaupt, R.D., Kury, J.W.,
J.Appl.Phys. 81(11), 7193 (1997).
3. Lubyatinsky, S.N., Loboiko, B.G. "Detonation
reaction zones of solid explosives," Proceedings of XI
Symposium on Detonation, Snowmass, USA, 1998.
4. Tarver, C.M., Kury, J.W., Breithaupt, R.D.
"Detonation Waves in Triaminotrinitrobenzene",
J. Appl. Phys. 82(8), 3771 (1997).
5. Green, L.G, Tarver, C.M, Erskine, D.J. Proceedings
of XI Symposium on Detonation, Snowmass,
USA. 1998.
6. Fedorov, A.V., Menshikh, A.V., et al. "Detonation
Front in Homogeneous and Heterogeneous High
Explosives", Proceedings of the Int. Conf, "Shock
Compression" of Condensed Matter-1999, Snowbird,
USA, 1999.
7. Fedorov A.V., Menshikh A.V, Yagodin N.B. //
Chemical Physics (Russian), Kail, pp.64-68, 1999.
8. Fedorov, A.V. Paper for International Conference
"Shock Waves of Condensed Matter", St. Petersburg.
2000.
9. A.Ya. Apin. On detonation and explosive combustion
of HE. Papers of USSR Acad. Of Sciences, (Russian),
v. 50, pp.285-288(1945).
10. Shock waves and extreme states of substance.
(Russian) Under edition of V.E. Fortov et al. Moscow: Nauka, 2000.
11.Mader, Ch.L, Craig, B.G. Nonsteady-state
detonations in one-dimensional plane, diverging and
converging geometries. - Los Alamos Sci.Lab.rep.
LA-5865, 1975.
12. Mader Ch.L. Numerical modeling of detonation. University of California Press, 1979.
13. K.K. Shvedov. Physics of combustion and explosion,
(Russian), JVs4, 1988.
14. Demol, G, Goutelle, J.C, Mazel, P. // Proceedings of
the Int. Conf. Shock Compression of Condensed
Matter, pp.353-356. Amherst, 1997.
120
FIGURE 4. Values of the von Neumann spike pressure (PN), C-J
state (Pj), and dependence of effective pressure (Pj cn) on
thickness of the charge for plastisized compositions of HMX of
the PBX-9404 type.
Evstigneev determined the maximum value of
C-J state for plastisized HMX using charges with
large diameter and large thicknesses (100-200 mm).
This value was 39.16 GPa. Thus, the von Neumann
spike exceeds the C-J state 1.25 times. Value of C-J
state, which is equal to 36±0.5 GPa, was determined
using charges with thicknesses of 40-100 mm. For
these charges the average exceess of the von
Neumann spike is 1.35 times. The dependence of
the detonation pressure on charge thickness
explains the fact that for the majority of condensed
HEs the excess of the value of the von Neumann
spike over C-J state is usually in the interval
between 1.25-1.4 and more. Shvedov analyzed in
detail the change of the C-J state value versus the
charge diameter. He revealed that with diameter
growth, this value increases and reaches the
maximum value at diameters of more than 100 dcr
[13].
Thus, with increase of thickness and diameter of
charge, the C-J pressure grows and reaches the
maximum value at d>100dcr. With reduction of
thickness and diameter of charge, the detonation
pressure drops, and the value of excess of von
Neumann spike over C-J state grows.
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