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
EFFECT OF GMB ON FAILURE AND REACTION
REGIME OF NM/PMMA-GMB MIXTURES
Jose Gois, Jose Campos and Igor Plaksin
Laboratory of Energetics and Detonics, Av. Universidade de Coimbra, 3150 Condeixa, Portugal
Mechanical Department, University of Coimbra, Pinhal de Marrocos, 3030 Coimbra, Portugal
Abstract. The effect of the addition of small amounts of glass microballoons (GMB) on
heterogeneous explosives has been investigated with the aim of understanding mechanisms that
lead to the strong reduction of its critical diameter. However, there is no clear identification of
the changes on detonation wave propagation and its structural features. To obtain a better
understanding of the contribution of GMB as a particular heterogeneity, the detonation failure
and the re-initiation of NM/PMMA-GMB mixtures is studied. Corner turning configuration was
performed in order to determine the influence of the GMB concentration and size on failure
phenomena by observing the trajectories of the divergent shock waves around the corner. The
shape of the printed traces on a copper witness plate, coupled with detonation velocity and front
curvature measurements, was used to evaluate the evolution of the detonation reaction regime
and its cellular structure. The obtained results of printed flow lines show significant changes of
the original pattern of the propagation of the detonation wave. The structural colliding and
divergent waves were successful identified and explained.
diameter allowing the propagation of detonation
front (4).
In previous work Gois et al. (4) observed for 1
percent mass fraction of GMB in NM/PMMA-GMB
mixtures, significant changes in the original pattern
of NM/PMMA matrix. Close to the failure
thickness, the critical pressure pulse of the initiation
of the shock front to induce a CJ plane wave is just
critical for an initiation detonation. Dremin (5) has
explained the conical failure wave generated when
the detonation front (DF) reaches the section,
corresponding to an abrupt increasing of cylinder
charge diameter.
The purpose of the present work is to determine
the effect of hot-spots induced by GMB collapse on
failure phenomena and re-initiation of light
heterogeneous explosives, in order to better
understand the complex mechanism of propagation
and detonation as described in literature. A copper
witness plate and different strips of 64 optical fibres
are used to measure respectively the grid of
INTRODUCTION
The influence of the addition of a small amount
of inert particles to NM has long been known to
drastically change failure thickness (1). Glass
microballoons (GMB) were used with success to
sensitize homogeneous NM/PMMA (96/4) matrix.
Increased mass fraction and decreased particle size
were found to increase the shock sensitivity of
NM/PMMA-GMB mixtures (2). The shock pressure
to initiate detonation is lowered as manifested by the
decrease of critical diameter and failure thickness
(2,3).
The increase of the energy release rate due of hot
spots induced by GMB collapse compensates the
effect of the rarefaction, avoiding the breakdown of
the reaction zone in the detonation front. As proved
in previous work, a decrease of particle size and rise
of concentration increases the density of the reinitiation sites and strongly reduces the critical
898
perturbations and the curvature and velocity of
shock waves around the corner.
Corner turning tests and Results
Corner turning experiments of detonation front
propagation of NM/PMMA-GMB mixtures were
performed for different concentrations of GMB. The
setup consists of an aluminium channel of 80 mm
length with a square section of 12x12 mm. Copper
plate of 5 mm thickness is used as witness plate. A
strip consisting of 64 optical fibers is used in
different positions to record the light induced by the
detonation front around the corner. A fast electronic
streak camera (THOMSON TSN 506N) is used to
record the signals. Figure 1 shows the setup and
configuration used to measure the curvature of
shock wave propagation from the end of the
channel. Two more configurations not shown were
adopted to measure the radial propagation of shock
front, the conical failure and the extension of the
dark zone around the corner.
EXPERIMENTS AND RESULTS
Explosives mixtures and preparation
The heterogeneous explosives mixtures used in this
study are based in a homogeneous explosive
mixture of NM/PMMA (96/4 wt). The amount of
PMMA increases viscosity of the mixtures and
avoids buoyancy GMB, without changing the
original detonation characteristics of NM (2). GMB
(QCel 520 FPS and QCel 300), supplied by Asko
Inc., were selected to perform NM/PMMA-GMB
mixtures. QCel 300 GMB was sieved to obtain large
particle diameters. Both classes of GMB have a wall
thickness of about lum. Table 1 shows the range
and mean particle diameter of GMB's used.
TABLE 1. Range and mean diameter of GMB used in
NM/PMMA-GMB mixtures.
[mm]
[mm]
QCel 520FPS
16-79
45±1
Effective
density
[kg.rn'3]
220
QCel 300*
21 - 146
92±2
151
GMB
dio - d9o
dso
* Selected size distribution after sieving.
The diameter distribution of GMB's was
obtained by laser diffraction spectrometry. The
effective density was measured using helium
picnometry. The experiments were performed for a
range temperature of 15°C to 20 °C. Tab. 2 shows
the mass fraction of GMB's and the density of final
mixture.
FIGURE 1. Corner turning test experimental setup.
Figure 2 shows the change on the detonation
front curvature of NM/PMMA-GMB mixtures
caused by the abrupt transition of detonation front
between the channel and the corner. For 1% of
GMB, the conical failure of shock front induced by
the rarefaction waves at the corner is identified.
Near the vertex of conical failure, a semihemispherical re-initiation point appears and
curvature is strongly reduced. From the erosion in
the witness copper plate (vd. Fig. 3) the angle
corresponding to conical failure, the quenching
distance at the edge of the corner, the dark zone and
the interaction of divergent shock waves and the reinitiation front from the vertex of conical failure can
be seen.
TABLE 2. Mass fraction of GMB's and density of
NM/PMMA-GMB mixtures.
dso
[mm]
45±1
92±2
GMB
mass fraction
[%1
1
3
Density
[kg.m-3]
1
3
1088±10
1090±5
104019
928±15
899
axis at the end of the channel (vd. Fig. 4). Previous
work (4) has shown no light at 90° immediately at
the corner. Results show that divergent shock wave
velocity is reduced when GMB concentration
increases. Shock front velocity decreases
continually from the axis of initiation of divergent
shock wave.
FIGURE 2. Typical streak records of detonation front
curvature for NM/PMMA-GMB mixtures (1% GMB,
d5o=45um) obtained with the test arrangement of Fig. 1.
FIGURE 4. Experimental divergent shock wave velocity
obtained in three directions (0°, 30° and 60°) from the
axis, at the end of the channel.
Three strip of fibers, each one on them divided
in three groups, and fixed at different angles from
the axis of initiation channel have shown that the
divergent shock wave is not circular. Fig. 5 shows
the photochronogram obtained from the light
transmitted from the three groups of fibers
distributed in three rings around the axis, fixed at 50°, 0° and +50° from the axis of the channel around
the corner. The result allows evaluates of the shape
of DF as function of radial distance and time.
The following
are observed in the
photochronograms: (I) the preferential DF is
developed in the range angle of 45° to 60°, from the
explosive mixture - confinement interface; (II) the
detonation front in the range angle of 60° to 90°
accelerates, reducing the delay of detonation front,
at the middle of the mushroom shape, characteristics
of divergent detonation front.
The steady state interactions of transversal
waves induced by rarefaction waves from the walls
of the channel (vd. Fig. 6) are identified from the
erosion profiles on the witness copper plate. The re-
FIGURE 3. Picture of witness copper plate showing erosion
around the corner induced by failure and re-initiation of
NM/PMMA-GMB mixtures with 1% GMB (d5o=45^m).
The comparison between the results obtained for
NM/PMMA-GMB mixtures and its of reference
NM/PMMA(96/4) matrix allows to verify the
influence of GMB on the reduction of the angle of
conical failure.
The radial velocity of divergent shock wave was
measured at three angles (0°, 30° and 60°) from the
900
reaction zone generating an inversion of two
hemispherical detonation fronts created from the
conical failure boundaries. The collisions of
transverse waves and microdetonations, induced by
hot spots, sustain a regular cellular structure
proportional spaced to the reaction zone thickness.
The grid of regular pattern changes with the
interparticle distance of GMB.
initiation of detonation starts at the periphery of
conical failure from the collision of transverse
waves. The detonation starts again at the conical
failure and expands to the lateral pre-compressed
zones giving rise to two hemispherical detonation
fronts. The shock reflection at the collisions of the
adjacent hemispherical detonation is initially
regular, but as the angle of collision increases, a
Mach stem appears. This Mach stem overtakes the
original front and accelerates the detonation front.
of
FIGURE 6. Erosion profiles on witness copper plate,
around the corner, induced by propagation of detonation
front of NM/PMMA-GMB mixture with 3%
GMB(d50=45um).
REFERENCES
1. Engelke R (1979) Effect of a physical inhomogeneity on
steady-state detonation velocity. In Phys. Fluids, vol. 22, (9),
pp. 1623-1630.
2. Gois JC (1995) Influencia das micro esferas ocas de vidro
na detona9&o da mistura nitrometano-polimetilmetacrilato,
Ph.D. Thesis, University of Coimbra, Portugal.
3. Gois JC, Campos J, Mendes, R (1995) On extinction
detonation behavior of NM-PMMA-GMB mixtures. In
Proceedings of the Conference of the American Physical
Society on Shock Compression of Condensed Matter,
Seattle, Washington, Part. 2, pp. 827-830.
4. Gois JC, Campos J, Plaksin I, Mendes R (1997) Failure and
re-initiation detonation phenomena in NM/PMMA-GMB
mixtures. In Proceedings of the Conference of the American
Physical Society on Shock Compression of Condensed
Matter, Amherst, Massachusetts, pp. 691-694.
5. Dremin A. Razanov SD, Trofimov VS (1963) On the
detonation of nitromethane. In Combustion and Flame, vol.
7, pp.153-162.
FIGURE 5. Signals of DF at the angles -50°, 0° and 50°
from the axis of the channel in three equal spaced rings
around the corner.
CONCLUSIONS
The optical records obtained using an optical
technique, based in several strips of fibers, and the
erosion profiles on a witness copper plate are
coupled to determine the influence of GMB
heterogeneities in NM/PMMA-GMB mixtures. The
increasing of GMB concentration reduces the angle
of conical failure and the extension of dark zone.
The presence of GMB induces hot spots into the
901