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
THE MEASUREMENT OF HOT-SPOTS IN GRANULATED
AMMONIUM NITRATE
W.G. Proud
Physics and Chemistry of Solids Group, Cavendish Laboratory, Madingley Road, Cambridge, CB3 OHE,
United Kingdom
Abstract. Ammonium Nitrate (AN) is one of the components of the most widely used explosive in the world
namely, ammonium nitrate: fuel oil mixtures (ANFO). By itself, it is an oxygen positive explosive with a
large critical diameter. Hot-spots are produced in explosives by various means including gas space collapse,
localised shear or friction. If these hot-spots reach critical conditions of size, temperature and duration
reaction can grow. This deflagration stage may eventually transition to detonation. This paper describes highspeed image-intensified photography study in which the number and growth of hot spots in granular AN are
monitored for a range of different impact pressures. The results can be used in detonation codes to provide a
more accurate and realistic description of the initiation process.
an interplay between the components. In the
ammonium nitrate system studied here, some of
the causes of energy concentration would be
compression of gases in the pores of the granular
bed, fracture of the crystals, jetting of fragmented
material, friction between the explosive grains or
shear banding in either single crystals or
compacted material [9-11].
Many of these processes occur on a very
short time-scale and the diagnostics used need to
have sufficient time resolution and sensitivity.
The present study uses visualisation of light
emission from granular beds of ammonium nitrate
(AN) impacted by copper flier plates at velocities
of 200 - 700 m s"1. High-speed photodiodes are
used to track the light emission throughout the
impact process and a DRS Hadland Ultra-8
camera to take high-speed images of the system.
This camera has a maximum framing rate of 108
f.p.s. and takes eight images. At such high capture
rates, the light levels of many systems would be
low, however, this camera is fitted with an image
intensifier.
The experimental study was conducted
with several aims; to assess the contribution of
adiabatic pore collapse, to measure the hot-spot
INTRODUCTION
The ignition of energetic materials is important
for studies of initiation of explosives and their
safe handling and use. Calculations carried out in
the middle of the 20th century indicated that
ignition did not usually occur due to bulk heating
of material but following local concentration into
"hot-spots" [1,2]. If heat losses by conduction,
convection, radiation and self-heating dominate,
the "hot spot" is quenched. However, if the
energy produced by chemical reaction exceeds
the losses reaction will build up and spread
through the material [3]. Under suitable
conditions there may be a transition from
deflagration to detonation.
In liquids, the conditions for critical hot
spot formation have been extensively studied [48] and the size, duration and temperature required
have been reasonably well-quantified. One
advantage of a liquid or a solid crystal of
explosive is the inherent homogeneity of the
system and relative ease of visualisation of the
reaction front. Many explosive systems are
heterogeneous and energy concentration relies on
1081
photodiode output or the Ultra-8 framing picture
shown in figure 2, which is the negative of the
captured image, and records the integrated light
output for 10 ps. A scale on the image indicates
that the field of view encompasses the whole
impact area. There was limited light emission and
the conclusion is that there is no significant
contribution from these effects on a timescale
twenty times that used in the studies with AN.
density for different impact conditions and to
determine if the kinetics of reaction are affected
by the induced shock strength.
EXPERIMENTAL
An impact cell, figure 1, contains a bed of AN 2
mm thick. The front plate is 2 mm thick and the
rear glass window is a 25 mm thick. The optic
fibre is fed into a photodiode sensitive to visible
wavelengths and with a time resolution of 1 ns. A
mirror, at 45° to the rear of the cell, directs light
into the high-speed camera.
Front Silvered Mirror
FIGURE 2. Impact on sugar at 700 m s"1, negative image,
exposure time 10 us, gain 100%.
* Optic Fibre
"Ammonium Nitrate Bed
A series of tests were then conducted on AN beds
of 78% theoretical maximum density (TMD).
These were impacted at velocities ranging from
200 to 700 m s"1. The results of some of these
impacts are shown in figure 3, the vertical column
corresponds to the impact velocity and the
horizontal rows to the time after impact. Each
frame had an exposure time of 500 ns. One
advantage of the Ultra-8 system is that the frames
are independent of each other so images can be
taken with no interframe time. The images have
undergone the same data manipulation and the
frames have been adjusted for variations in
camera sensitivity. In the images 12 mm
corresponds to 600 pixels: each pixel corresponds
to the area of 2 or 3 AN crystals. A long focal
length microscope, such as a Parfocal K2, would
magnify the system so that individual grains
could be resolved. This research is not reported
here.
"Glass Window
*• Plate
"'O"iing
-Steel Support Ring
FIGURE 1. Ammonium Nitrate Cell. Copper flier plate
included to indicate the 12 mm impact diameter.
The cell is mounted in at the end of a 19 mm bore
gas gun. The projectile was a 2mm thick copper
plate mounted on a plastic sabot. The pressure in
the impact chamber can be left at atmospheric
pressure or pumped to a vacuum of 10~3 bar. The
flier plate was aligned to the cell to a tolerance
better than 0.05°.
IMPACT EXPERIMENTS
Experiments were performed using framing
photography to observe the fracture of the glass
window. These showed that a ring crack formed
which developed into a cone crack around the
contact area, The cone was formed within ~4 ps
of impact. The centre of the window remained
transparent beyond 10 ps after impact. Given the
shock speeds in the system the high-pressure state
induced by the impact lasts 1-2 ps.
A test sample of sugar with grain size
150 - 210 pm size was impacted at a velocity of
700 m s"1. Sugar is a material noted for its
triboluminescence and hence any light seen in this
image is due to sugar fracturing, the compression
of the gas in the pores of the bed or fracture of the
cell materials. No signal was detected on the
1082
400
500
600
600 Vac
0-500 ns
500- 1000ns
1000-1500 us
1500 - 2000 ns
FIGURE 4. The photodiode output from experiments
conducted at velocities between 400 and 700 m s"1. The legend
indicates the impactor velocity in m s4 and "Vac" indicates an
impact performed under vacuum.
2000-2500ns
The intensity is higher and the rise time shorter
with increased impact velocity. The traces could
also indicate that reaction spreads faster at higher
velocities and is more intense due to grain
fragmentation and friction between the fragments.
In figure 5 the traces are normalised
according to maximum intensity while the time
axis is normalised with respect to the velocity of
impact after the sohck pulse has entered the AN
bed.
2500 - 3000 ns
FIGURE 3. The high-speed sequences of impact on a bed of
78% TMD AN. Vertical columns show the intensity of light at
the impact velocity indicated at the top of the column. The
rows show images at the same capture time.
Below 400 m s"1 no light was detected either with
the photodiode or with the camera. At 400 m s"1
there is limited light output and spots of light are
recorded which rapidly fade. At 500 m s"1 no light
is seen in the first 500 ps but intensity starts to
build up from 1 ps onwards. The annular
structure that can be seen towards the end of most
of the sequences is due to interaction between the
edge of the impactor acting on the impacted
copper plate and the glass anvil i.e. a region of
intense shear. At 600 m s'1 two sequences are
shown "600" was performed at atmospheric
pressure while "600 Vac" was performed at a
reduced pressure of 1 mbar. Interestingly there is
very little difference between the two images
except they are at slightly different
magnifications.
The output from the photodiode gives an
indication of the intensity of the light output
figure 4.
0.5
1
Normalised Time
FIGURE 5 Normalised Photodiode Intensity traces.
There is remarkable similarity between
these traces. The possible exception being "600"
though "600 Vac" matches well with the
atmospheric data from both 500 and 700 m s"1.
This is indicative that the kinetics of the process
are self-similar under the range of impact
conditions applied.
1083
DISCUSSION AND CONCLUSIONS
ACKNOWLEDGEMENTS
A system has been developed capable of
allowing hot-spots to be visualised in granular
beds.
Interestingly, ignition for oxygenpositive AN, is independent of adiabatic heating
of the initial gas content in the pores of the
material, though gas produced by fracture and
frictional rubbing may have a role.
The reaction proceeds in a self-similar
fashion once the ignition threshold at -400 m s"1
impact velocity has been exceeded.
The light output in figure 3 for AN
compared with in figure 2 for sugar confirms that
hot-spot ignition has taken place.
The similarities between "600" and "600
Vac" suggest that crystal fracture produes some
gas as suggested by Chaudhri and Field [14].
Comparison with earlier work on slurries and
emulsions of AN to which microballoons have
been added suggests that for the shock process
involved (several GPa) jetting could be a
significant
hot-spot
mechanism.
Crystal
fragmentation would help ignition break out.
Future research will aim to identify the precise
hot-spot mechanism. Studies are now underway
to look at the effect of bed density, additives
which increase or deacrease friction, different
grain sizes and composition of the gaseous phase
in the bed.
J.E.Field is acknowledged for his encouragement.
J. Gilbert of DERA Fort Halstead and I. Kirby are
acknowledged for their support. C. Granstrom, E.
Molin and P. Kalafatis are acknowledged for their
help. R. Marrah of the Cavendish Laboratory is
thanked for his technical aid. EPSRC is
acknowledged for funding of the high-speed
camera.
REFERENCES
1 F. P. Bowden, M. A. Stone, and G. K. Tudor, Proc. R.
Soc. Lond. A 188, 329-349 (1947).
2 F. P. Bowden and A. D. Yoffe, Initiation and Growth
of Explosion in Liquids and Solids (republ, 1985)
(Cambridge University Press, 1952).
3 J. E. Field, Accounts Chem. Res. 25, 489-496 (1992).
4 F. P. Bowden and M. P. McOnie, Proc. R. Soc. Lond.
A 298, 38-50(1966).
5 G. D. Coley and J. E. Field, Proc. R. Soc. Lond. A
335, 67-86 (1973).
6 N. K. Bourne and J. E. Field, Proc. R. Soc. Lond. A
435,423-435 (1991).
7 E. Wlodarczyk, J. Tech. Phys. 33, 35-61 (1992).
8 E. Wlodarczyk, J. Tech. Phys. 33, 133-166 (1992).
9 N. K. Bourne and J. E. Field, Proc. R. Soc. Lond. A
455,2411-2426(1999).
10 J. E. Field, N. K. Bourne, S. J. P. Palmer, and S. M.
Walley, Phil. Trans. R. Soc. Lond. A 339, 269-283
(1992).
11 J. E. Field, G. M. Swallowe, and S. N. Heavens,
Proc. R. Soc. Lond. A 382, 231-244 (1982).
1084