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
FACTORS AFFECTING SHOCK SENSITIVITY OF ENERGETIC
MATERIALS
A. Chakravarty, M.J. Gifford, M.W. Greenaway, W.G. Proud, J.E. Field
PCS, Cavendish Laboratory, Madingley Road, Cambridge, CBS OHE. UK.
Abstract. An extensive study has been carried out into the relationships between the particle size of a
charge, the density to which it is packed, the presence of inert additives and the sensitivity of the charge
to different initiating shocks. The critical parameters for two different shock regimes have been found.
The long duration shocks are provided by a commercial detonator and the short duration shocks are
imparted using laser-driven flyer plates. It has been shown that the order of sensitivity of charges to
different shock regimes varies. In particular, ultrafme materials have been shown to be relatively
insensitive to long duration low pressure shocks and sensitive to short duration high pressure shocks. The
materials that have been studied include HNS, RDX and PETN.
INTRODUCTION
nature of hot-spots that are created in the charge and
the chemistry is important in determining the
response of the material to the presence of the hotspots.
A large number of researchers have attempted to
elucidate the role of hot-spots in the shock initiation
of detonation. The reviews of the field given by
Khasainov et al.^ and Dremin2 give a very complete
account of the state of the literature on this subject.
The present study has focussed on varying the
density and grain size of the charges and the nature
of the imparted shock in an attempt to alter the hotspot parameters and so determine the critical factors
associated with them.
When a shock-wave is incident on an energetic
charge, a number of parameters must be considered
when determining whether detonation is likely to
result. The nature of both the charge and the shockwave are important.
In a very simplistic way a shock can be
described by its pressure and duration (ignoring
shock profile at this stage). For a shock to cause
initiation it must be capable of creating sufficient
chemical reaction to sustain it. Acting against this
chemical reaction, to weaken the shock, are
rarefactions due to the expansion of the material
which, due to the subsonic flow of the material
following the shock, will eventually reach the front.
The relationship between the required pressure and
duration for initiation is such that the shock level
must be high enough to cause sufficient reaction to
sustain the shock before the initial shock decays. If
this criterion is met then a detonation will propagate
in the charge.
The magnitude and duration of a shock required
for a particular charge to be initiated are dependent
on the microstructure and chemistry of the charge.
The microstructure is crucial in determining the
MATERIALS USED
Both the pentaerythritol tetranitrate (PETN) and
cyclotrimethylene trinitramine (RDX) were supplied
in ultrafine and conventional forms by ICI Nobel
Enterprises, Ardeer, U.K. The ultrafine powders
have a primary particle size of ~1 pm and are
produced by a proprietary process.
The
1007
conventional grain material has a particle size of
about 180 urn.
The hexanitrostilbene (HNS) used in these
studies was supplied by DERA, Fort Halstead and
came originally from Bofors AB in Sweden. The
ultrafme form is known as HNS IV and has a grain
size of less than a micron. The HNS IV was
supplied both in a pure form and with pressing
additives. In the case where zinc stearate and
graphite were added to act as pressing agents, the
additives contributed approximately 1% to the total
mass of the material. The coarse grain HNS (known
as HNS II) had a grain size that was typically of the
order of 25 um.
The gaps that were used to mediate the shock
pressure were discs of PMMA placed between the
detonator and the surface of the column. A thin
layer of silicone grease was used between all three
components of the test in order to aid the
reproducibility of the testing. PVDF gauges placed
between the PMMA gap and another piece of
PMMA in the charge position were used to obtain
an indication of the shock pressure during a test. A
typical trace from a PVDF gauge is shown in figure
1.
Both photographic streak recording and brass
witness plates were used to determine whether a
detonation event had occurred during a test.
EXPERIMENTAL METHOD
Short Duration Shocks
Two principal experimental methods were used
during the course of the research described here.
For the imparting of relatively long duration shocks,
a gap testing geometry was used. When short high
pressure shocks were required a system for
generating laser-driven flyer plates was used.
The HNS charges used for the short-duration
shocks were 5 mm long, 5mm diameter cylinders
contained within 25 mm diameter PMMA
confinements. The charges were incrementally
pressed into the confinements. The surface of the
charges was polished with 2500 grade SiC paper to
provide a consistent surface finish. The quality of
the surface finish was checked using a Sloan
DekTak II surface profilometer.
Long Duration Shocks
The charges used in these experiments were
incrementally pressed columns of either RDX or
PETN. The confinements used were 25 mm long 25
mm diameter PMMA cylinders. The explosive
columns were 5 mm in diameter.
The donor charge used during the experiments
was a PETN boosted C8 detonator which was found
to have a reliable output in terms of the shock
pressure produced.
0.3
0
0.5
1.0
Distance along scan (mm)
FIGURE 2. Profilometer traces from the Sloan DekTak II.
The laser-driven flyer launching system is
described fully in previous publications from this
laboratory-*"^ and details can also be found in the
paper by Greenaway et al. in these proceedings.
The system uses a Nd:YAG laser to accelerate
flyers 1 mm in diameter and 5 um thick to velocities
up to 8 mm us'1. On impact these flyers provide
intense shocks lasting approximately 1 ns. The
FIGURE 1. Typical trace from a PVDF gauge.
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energy of the pulse imparted to the flyer is
controlled in order to determine the velocity of the
flyer. Energies between 50 and 400 ml were
accessed during this study.
A Hadland Imacon 790 high speed image
converter camera was used to provide streak
photographs of the initiation events. The camera
was triggered from the signal that fired the laser
with a suitable delay added. These photos allowed
calculation of the position of the initiation event
within the column.
as measured using the PVDF gauges described
previously.
The gap required to prevent initiation of the
RDX charges increased significantly in both the
ultrafme and conventional materials as the porosity
increased, but the ultrafine material was consistently
less sensitive to this form of initiation.
Short Duration Shocks
The findings of this study into initiation by short
duration shocks have been explained in some detail
in the paper by Greenaway et al. within these
proceedings. The results of this study involving
laser-driven flyer plates are that HNS II could not
be initiated with very short duration shocks at the
energies available in that system, but that the HNS
IV could be readily initiated with a go/no go
threshold of about 250 mJ of laser pulse energy.
The presence of zinc stearate and graphite as
additives in some of the HNS IV acted to increase
the flyer energy required for initiation of the charges
to approximately 350 mJ.
RESULTS
Long duration shocks
Figure 3 shows the results of the experiments
which used long duration shocks in a gap test
geometry. These experiments were carried out on
PETN and RDX in both ultrafme and conventional
grain sizes. As can be seen the density was also
varied in the RDX study in order to determine the
effect that increased porosity has on the sensitivity
of the charges. Although there is some overlap in
the go/no go gaps for some of the densities, in
general the experimental reproducibility was
extremely high.
&
*
*
•^
•
o
•
n
D
D
D
Ultrafine go
Ultrafine no go
Conventional go
Conventional no go
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D
n
...... vVy. . :
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A A
D
D
•
•
V
:
;
:
60
70
O
D
£
0*0o
HNSJV5'
70% TMD
up
In
D
HNSIV+addfives
65% TMD
(Densfes are only approximate)
al
•:
HNS II
•
78%TMD :
FIGURE 4. Results of the laser-driven flyer tests. Filled objects
denote a "go" result.
80
Density (%TMD)
The results of this study indicate a strong
correlation between pressing density and sensitivity.
Unfortunately due to the nature of the pressing
technique employed and the powder, it was difficult
to accurately reproduce a given density of charge.
Within the limits of the study, it can be said that the
charges pressed to a density of 65% TMD appear to
be less sensitive than those pressed to 70% TMD.
FIGURE 3. Results of gap testing on RDX. Thresholds for
PETN are also indicated.
The ultrafme PETN at a density of 90% TMD
had a critical gap of 3.68 ± 0.01 mm compared with
a gap of 5.57 ± 0.02 mm for the conventional grain
size material. These were shown to correspond to
shock pressures of approximately 4.1 and 2.1 GPa
1009
Without performing a larger study, however, it is
impossible to say what the exact nature of the
dependence on density of the sensitivity is for this
form of initiation.
The effect of density on the sensitivity of the
charges is caused by the change in the relative
density of hot-spot nucleation sites compared to the
density of material available for reaction. It appears
from the results of the gap testing of the RDX that it
is the number of available sites for hot-spots that
determines the sensitivity (at least down to the
density of 40% TMD that was used in this study).
It is not so clear from the laser-driven flyer study
that the same is true in this regime. The importance
of good coupling between the energetic material and
the hot-spot is more pronounced due to the short
duration of the shock, so this may account for what
appears to be a higher sensitivity of the more
densely packed charge. Further research would
have to be carried out with more emphasis on
density in order to determine the optimum density
for charge sensitivity in this shock regime.
DISCUSSION
This study together with previous studies carried
out within this group has shown that simple
orderings of materials by sensitivity cannot be done.
It is not even possible to do this for sensitivity to
initiation by shock as has been demonstrated here.
The results of this study have shown that for a
given situation, the sensitivity of the material is
dependent on the chemistry, the grain size, the
density of the charge and the nature of the shock
itself. The way in which all of these variables
determine the likely response of a charge to an
insult can be linked to their effect on the
distribution, nature and form of the hot-spots that
are caused by the shock.
It has been shown7"9 that the effect of increasing
the shock pressure is to change the relative
importance of the jetting and the gas compression in
the process of pore collapse. As small pores are
more effective for the rapid formation of hot-spots
by jetting and large pores are more effective in the
case of gas compression it seems that it may be the
pore size rather than the grain size that is critical. In
the case of the high pressure short duration shocks
imparted by laser-driven flyers, the incident shock is
not sufficiently large for it to cover an entire pore in
the coarse material and so the releases will act to
hinder collapse. These short shocks are, however,
of sufficiently high pressure for jetting to be
significant and this may well be the dominant
mechanism in the ultrafine charges where the pores
are extremely small.
With the longer duration, lower pressure
detonator-supplied shocks, the shock is sufficiently
large to encompass whole gas spaces in both the
ultrafine and the conventional powders. Due to the
reduced pressure, jetting is a less important
mechanism for hot-spot production than gas
compression and as a result the larger pores that are
found in the conventional charges are more
conducive to the creation of hot-spots capable of
causing reaction.
ACKNOWLEDGEMENTS
The authors would like to acknowledge ICI
Nobel Enterprises (Ardeer), U.K. and DERA, Fort
Halstead for their support of this research. Dr. M.
Cook of DERA is particularly thanked for useful
comments that he has made.
REFERENCES
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Chem. Phys. Rep., 15, 987-1062 (1996).
^ A. N. Dremin, Toward Detonation Theory SpringerVerlag, Berlin, 1999.
3 S. Watson, PhD Thesis, University of Cambridge,
1998.
4
S. Watson and J. E. Field, J. Phys. D: Appl Phys. 33,
170-174(2000).
5
S. Watson, M. J. Gifford, and J. E. Field, J. Appl. Phys.
88, 65-69 (2000).
6
S. Watson and J. E. Field, J. Appl. Phys., 88, 3859
(2000).
7
J. P. Dear, J. E. Field, and A. J. Walton, Nature 332,
505-508(1988).
8
N. K. Bourne and J. E. Field, Proc. R. Soc. Lond. A,
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9
J. E. Field, Accounts Chem. Res., 25, 489-496 (1992).
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