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
THE COMBUSTION OF EXPLOSIVES
S. F. Son
Los Alamos National Laboratory, Los Alamos, NM 87545
Abstract. The safe use of energetic materials has been scientifically studied for over 100 years. Even
with this long history of scientific inquiry, the level of understanding of the important deflagration
phenomena in accidental initiations of high explosives remains inadequate to predict the response to
possible thermal and mechanical (impact) scenarios. The search also continues for improved explosives
and propellants that perform well, yet are insensitive. Currently, the most significant uncertainties are in
the processes immediately following ignition. Once ignition occurs in an explosive, the question then
becomes what the resulting violence will be. The classical view is that simple wave propagation
proceeds from the ignition point. Recently, several experiments have elucidated the importance of
reactive cracks involved in reaction violence in both thermally ignited experiments and impacted
explosives, in contrast to classical assumptions. This paper presents a view of reaction violence, in both
thermal and mechanical insults, that argues for the importance of reactive cracks, rather than simple wave
propagation processes. Recent work in this area will be reviewed and presented. Initial results involving
novel energetic materials will also be discussed. Novel materials may yield insight into the mechanisms
involved with rapid deflagration processes.
and nanoscale thermites (also called known as
Metastable Interstitial Composites, or MIC), will be
presented also. High-nitrogen (HN) compounds may
INTRODUCTION
Energetic materials, including propellants,
explosives, and pyrotechnics, are used in
applications such as rocket motors, guns, explosive
bolts, weapon systems, air bags, and of course
fireworks. Since the first energetic materials were
discovered, the safe use of these materials has been
of interest.
For example, in 1864 a major
explosion at the Nobel factory in Stockholm claimed
the lives of Alfred's brother Emil and four other
people. This accident, in part, drove Alfred Nobel
to the invention of dynamite that was much safer
than nitroglycerine. The safe use of energetic
materials remains a concern today. In this paper, an
overview of recent efforts at Los Alamos to obtain
an improved understanding of the mechanisms of
accidental initiation of energetic materials will be
presented. The importance of the interaction of
combustion with cracks in both thermally and
mechanically insulted explosives will be presented.
Ongoing work using new classes of explosives
and propellants, specifically high nitrogen materials
1059
Figure 1. This is an image of the long confined crack
experiment. A Plexiglas window insert was used. PBX
9501 was mounted to brass inserts and shimmed to a gap
of 80 \im. The two parts shown are bolted together.
Pressure ports were placed in the crack region and in the
breach area.
be key to meeting the advanced performance
objectives of next-generation energetic materials (13). High-nitrogen solids offer the possibility of high
performance (both as propellants and explosives),
reduced emissions, and lower plume signature (low
temperature and no HC1) than materials used in
current systems.
Nanoscale thermites offer the possibility of
tunable energy release rates, high density, high
energy, high temperatures, and low toxicity (4).
These materials have been shown to have very high
propagation rates in loose powders. In sharp contrast
to HN materials, the Al/MoO3 system considered
produces little gas, but yields high temperatures. In
this paper, we present some initial results and argue
that by studying these novel materials we gain
insight into the physics of unusual combustion, and
therefore gain understanding, and confidence in the
modeling, of accident scenarios involving more
200
150 -
BACKGROUND
Convective burning can be an important step in
the
deflagration-to-detonation
transition
in
explosives and other energetic materials (5-7).
Normal deflagration involves primarily conductive
heat transfer from the gas-phase flame region to the
surface, and to a lesser extent, radiation transport
from the gas to the solid. In contrast, convective
burning involves heat transfer via mass flow. Defects
increase the available surface area where combustion
can occur and are necessary for convective burning in
energetic materials. The effect of defects on
combustion has major implications for the safety
and reliability of energetic materials.
5000
4000
—— Fit of x-t Data
A
Pressure Rise
•
Ignition Front
3000
100
2000
so -
Figure 3. Damaged long confined slot experiment.
Voids and cracks in explosives may result from
numerous environmental and physical factors.
Impact, aging, and variations in temperature and
pressure associated with combustion can produce
defects. At sufficiently high pressures, the surface
area of a defect becomes accessible to deflagration.
Defects can trap the hot reaction products, creating
the necessary pressure gradient for convective
burning. The pressure from this burning may induce
further cracking. A few studies exist on the effects
of voids and cracks on the combustion of some
common propellants (7-8), but relatively few studies
exist of the effects of voids and cracks on the
combustion of high explosives (HE) (7). One
example is the study by Ramaswamy and Field who
studied hot spot and crack propagation in single
crystals of RDX (9). Formulated explosives, using
HMX,
(octahydro-1,3,5,7-tetranitro-1,3,5,7tetrazocine), typically include a binder that makes it
possible to desensitize and shape the explosive
formulation. Binder affects the number, shape, and
size of voids, as well as influences somewhat the
combustion.
Recent experiments highlight the importance of
1000
- - - Propagation Speed
-50
0
50
100
150
Time (its)
Figure 2. Flame front location in long confined slot
experiment.
typical materials.
In accident scenarios, relatively low-speed
deflagration processes play a critical role in
determining the ultimate reaction violence. Reaction
propagation may begin as conductively driven
(normal) combustion, transit to convective
combustion (advective energy transport), induce
compaction-initiated
combustion
(mechanical
dissipation), and possibly lead to detonation
(propagation involving shock waves).
Normal
deflagration and detonation are relatively well
studied.
Convective and compaction driven
combustion are much less well understood. The
objective of this paper is to present an overview of
recent and current work proceeding at Los Alamos in
the area of convective burning.
1060
sample. They detect reaction, indicated by luminous
emission, throughout cracks that are caused by
pressurization due to production of reactive gases
(13). The fast reactive waves, indicated by the
luminosity, propagate through the cracks at
velocities on the order of 500 m/s. An interesting
question is whether reaction is spread into these
cracks via convective processes, or whether crack tip
dissipation ignites the material.
Crack tip
dissipation is expected to provide only a small
amount of energy. However, in the MCCO
experiments the material is already at an elevated
temperature so very little energy is needed for
reaction to occur. Recent initial experiments have
been performed on unheated pristine materials that
show similar reactive cracks. These results are
shown below.
cracks and voids in the ignition, combustion, and
reaction violence of PBX 9501. The Steven Test
determines the critical impact velocity of a lightly
confined energetic material to the low-speed impact
of a blunt steel projectile. Radial cracks emanating
from the impact point are apparent for a test where
no sustained reaction occurred (10). Idar et al. (10)
find that damaged PBX 9501 has a significantly
lower impact threshold for violent reaction than
pristine material.
lil
Figure 5. Images from radial burn experiment. Part (a) shows the
initial ignition in the center and part (b), 0.5 ms later, shows
illuminated cracks that extend from the center ignition site to the
confinement ring.
In recent carefully heated cook-off (thermal
explosion) experiments involving larger-scale
explosive charges in an annular configuration (14),
nearly symmetric compression of the inner wall was
observed, although thermocouple records indicate
ignition occurred asymmetrically. Consequently, it
is surmised that some physical mechanism must
spread reaction around the annular explosive charge,
followed by violent reaction yielding a nearly
symmetric compression of the inner walls.
Spreading of ignition sites must occur at a rate on
the order of 1000 m/s. Connected porosity is
expected in this material because it is heated for
several
hours
at
elevated
temperatures.
Consequently, convective burning is one possible
mechanism to provide this spreading of ignition.
Convective burning in this material has not been
observed before at speeds near 1000 m/s in PBX
9501. Recent experiments have shown that these
rates are achievable in PBX 9501. Initial results are
presented below.
Figure 4. Schematic of radial burn experiment.
Sample is ignited in center using a coiled nichrome
wire. Pressure is measured in the center of the
sample.
Henson et al. have conducted shear impact
experiments using thin samples of PBX 9501 (11).
A rectangular steel plunger is driven into the lightly
confined sample at about 100 m/s.
Plunger
intrusion causes both shear and non-shear fracturing
with reaction initiated along fracture zones.
Skidmore et al. (12) have used microscopy to study
damaged samples recovered from the shear impact
experiments and find that the HMX along the
fracture zones shows clear signs of heating and
quenched reaction (12).
Evidence of the importance of crack-sustained
combustion also appears in elevated-temperature
experiments, such as the Mechanically Coupled
Cookoff (MCCO). Dickson et al. slowly heat a
confined sample of PBX 9501 to a well-defined
temperature field, then ignite the center of the
1061
can reach ignition spread rates over 1000 m/s, which
was a primary aim of this experiment.
COMBUSTION IN MICROCHANNELS
Precisely machined slots in PBX 9501 have
allowed us to examine the propagation of fast
reactive waves in psuedo-cracks of PBX 9501,
focusing on the reactive wave velocity and on the
interplay of pressure and crack size in PBX 9501
(15). In other words we try to eliminate the
mechanics in the problem to study the combustion
in isolation. Initial experiments were performed
with 4 cm slots that were open to a pressurized
large-volume environment. Experiments at initial
pressures of 6.0 MPa reveal monotonic reactive wave
propagation velocities around 7 m/s for a 100-pim
slot. Reactive wave velocities as high as 100 m/s
are observed in experiments at initial pressures of
17.2 MPa and various slot widths.
Similar
experiments at lower pressure sometimes exhibit
oscillatory reactive wave propagation in the slot with
periodic oscillations whose frequencies vary with
combustion vessel pressure.
Although the
propagation rates achieved were impressive
considering the length of the channel and that an end
of the gap was open to a large volume, the question
remained concerning the ultimate ignition spread
rates attainable.
Long Highly Confined Gap Experiment
More highly confined experiments, with longer
channels were designed and initial experiments have
been performed. Figure 1 shows the experimental
setup used. PBX 9501 was mounted on brass and
shimmed to produce an 80 |Lim gap, 19 cm long.
The end of the gap was ignited in a small breach area
with an equal mixture (by weight) of AP and DHT.
Pressure was measured in the breach area and at four
locations down the length of the crack. A Plexiglas
window provided optical access to the experiment.
For low violence events it was expected that this
window would release.
In the experiment, the ignition front spread
rapidly through the slot, approaching speeds of 1500
m/s, followed by failure of the cell. The speeds
were obtained from both the visual and pressure data
(see Fig. 2). Figure 3 shows the resulting damage to
the cell. There were no clear indications of
detonation, although the damage was severe. An
additional series of experiments in this configuration
is planned and a more complete description of this
series of experiments will appear in a later
publication. However, this initial experiment clearly
shows that convective burning in slots in PBX 9501
Figure 6. High speed video image of a mound of BTATZ.
Part (a) and part (b) are taken 6 ms apart. In part (a)
movement is evident ahead of the main front in the mound
of material.
RADIAL BURN EXPERIMENT
Figure 4 shows a schematic of the radial burn
experiment, or Cindy test.
The radial burn
experiment is nearly the same as the MCCO
experiment, except that it is designed to fit into our
pressure vessel, pressure is measured in the center
ignition hole, and pristine material is considered (it
is not heated). We found that we could achieve
ignition in pristine materials if the pressure was
1062
initially sufficiently high or a good seal is achieved
(using thin Teflon sheets) such that the nichrome
wire could pyrolyze enough of the sample to raise
the pressure sufficiently.
Figures 5 shows some initial observations from
this experiment. The pressure rises very fast as
ignition occurs in the small center hole (1 mm
diameter). This quickly creates radial cracks that are
illuminated by reaction, which is very similar to the
MCCO experiments. The aluminum confinement
rings were quickly breached by the hot reactive gases,
in these initial experiments. Additional experiments
will use copper rings that may provide more robust
confinement. Since the material was not preheated,
the energy dissipated by the crack tip cannot explain
the ignition of the reaction in the cracks.
Consequently, the most probable explanation is that
the cracks are opened by the pressurization from the
burning, and convective burning spreads the reaction
through the crack.
material is clearly reacting. This appears to be a
self-confined convective spread of ignition within the
mound.
Nanoscale Thermite
Similar unconfined experiments were performed
using nanoscale thermites (MIC). Since reaction of
these materials yield solid phase products, it might
be assumed that rapid convective burning would not
occur, yet propagation rates exceed 100 m/s. This
far exceeds BTATZ in a similar configuration. With
the small size of the reactants the barrier to reaction
is small in the MIC material. Furthermore, initial
air is likely heated and behaves as a working fluid,
and gaseous intermediates are probable for this
system because of the high temperatures. Figure 7
shows an image from a high-speed video record. A
large plume is evident, with the leading edge in the
mound of material. The resulting propagation rate is
nearly constant except initial and final transients.
NOVEL ENERGETIC MATERIALS
Recently at Los Alamos we have studied the
combustion of several novel energetic materials. The
two materials considered here exhibit rapid,
presumably convective, burning in a small mound of
material. The first material is BTATZ (3,6-bis(l#l,2,3,4-tetrazol-5-amino)-s-tetrazine), which is a
high nitrogen compound. This material burns
rapidly, and produces lots of gas at moderate
temperatures. The second is a nano-scale thermite
(Al/MoO3) system that reacts very quickly at high
temperatures, and produces solid products. It is
interesting to contrast and compare the behavior of
these materials in similar experiments.
High Nitrogen (HN) Materials
Some HN materials, such as BTATZ, burn
rapidly with very little luminosity. Burning BTATZ
in an open configuration (simple mound of material)
is shown in Fig. 6. BTATZ burns on the order of
centimeters per second in normal burning. However,
in this configuration the ignition spread rate is two
orders of magnitude faster than the normal burning
rate. In part (a) of Fig. 6 material on the surface of
the mound can be seen to move ahead of where the
1063
Figure 7. This figure shows an image of nanoscale Al/MoO3
burning from right to left.
Figure 8 shows the same material burning in a
Pyrex tube. The spread of reaction can be seen to
accelerate rapidly. Speeds exceeding 800 m/s are
obtained, although the tube is essentially undamaged.
This affect of confinement indicates a mechanism
such as convective burning is dominating, although
other possible mechanisms are currently being
investigated. In contrast, BTATZ burning in a
similar tube propagated reaction exceeding 1000 m/s
and pulverized the tube. By changing the particle
size of these MIC materials the reaction rate can be
adjusted. This could potentially be a valuable tool in
the study of the propagation physics involved in
these materials, and could provide unique model
validation data. Classical energetic materials, of
reviewed and presented. Initial work involving
novel energetic materials was presented. These
materials create conditions that are in a different
parameter space than classical materials and therefore
offer a unique perspective of rapid combustion in
energertic materials.
course, provide no means of adjusting the reaction
rate. Visual access is also easier to obtain because
confinement requirements are less difficult.
Acknowlegments
These experiments were designed, performed and discussed in
close collaboration with several co-workers, including Blame
Asay, Larry Hill, Laine Berghout, Cindy Bolme, Mike Hiskey,
Darren Naud, and Bryan Bockmon. Without their efforts this
paper would not have been possible.
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Figure 8. Burning nanoscale Al/MoO3 in 3.8 mm inner diameter
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t=74 [is, part (d) at t=l 11 us.
14.
SUMMARY
15.
This paper examines the role of reactive cracks
inreaction violence. Recent results are presented that
illustrate the importance of convective burning in
accident scenarios. Recent work in this area was
1064
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