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SHOCK INITIATION OF BUBBLE SENSITIZED COMMERCIAL EXPLOSIVES
SHOCK COMPRESSION OF CONDENSED MATTER 1991 S.C. SCHMIDT, R.D. DICK, J.W. FORBES, D.G. TASKER (editors) 1992 Elsevier Science Publishers B.V.
691
SHOCK INITIATION OF BUBBLE SENSITIZED COMMERCIAL EXPLOSIVES K.K. FENG*, D.E.G. JONES* and S.K. CHAN** ♦Canadian Explosives Research Laboratory, CANMET, Energy, Mines & Resources Canada, 555 Booth St., Ottawa, Ontario, K1A 0G1, Canada. **ICI Explosives Canada, McMasterville, Quebec, Canada This paper describes a study of the low pressure shock initiation of slurry and emulsion explosives. Slurry and emulsion explosives under low amplitude shock exhibit two different fundamental hot-spot initiation mechanisms. Slurry explosives are sensitized by gas bubbles whereas emulsion explosives are sensitized by microspheres. The results are also used to compare the Hugoniot of the reacted and unreacted explosives. The results indicated that the mechanism of the hot-spot initiation in slurry explosives is different from that in explosives containing glass microspheres.
1. INTRODUCTION
adiabatic compression as a mechanism in such
It is well known that the sensitivity of an explosive to
initiation.
initiation is significantly increased by the presence of gas bubbles1.
The fundamental mechanism of the
action of such gas bubbles is, however, still highly
2. INITIATION UNDER IMPACT AND DDT CONDITIONS
uncertain, particularly for solid explosives2. Slurry and
When a gas bubble is present in a liquid explosive,
emulsion explosives have characteristics of liquids as
such as slurry and emulsion explosives, the impact
far as their initiation mechanisms are concerned. The
sensitivity is increased significantly1.
initiation mechanisms operable in liquids are much
manufacturing, handling and transporting of these
simpler than those which occur in solids.
These
explosives more hazardous. In order to determine the
mechanisms operate largely independently, at different
effect of gas bubbles on the impact sensitivity of these
ranges of compression rate and allow an improved
explosives, the ASTM Impact Test tool3 was used to
understanding of the process of initiation of these
test various slurry explosives. This tool was used to
explosives.
study the response of an EGMN based slurry explosive.
This makes
At low shock pressure, the adiabatic compression of
An impact weight of 5 kg was used. Positive results
bubble gases in slurry explosives becomes the most
were obtained above a drop height of 0.36 m. There
predominant hot-spot mechanism.
The presence of
seems to be little doubt that the initiation mechanism is
glass microspheres in emulsion explosives is probably
the ignition of the explosive by the hot compressed gas
not effective for the adiabatic compression mechanism.
bubbles4.
This paper presents some data on the ASTM impact,
Another hazard test carried out was the DDT test.
deflagration to detonation transition (DDT) and low
Two EGMN based slurry explosives which showed
shock pressure initiation of slurry and emulsion
DDT behaviour were tested.
explosives. This data demonstrates the importance of
igniter end pressure wave velocity records for a test
Figure 1 shows the
692
K.K. Feng et ai
with slurry explosive initiated by the igniter mixture.
(normalized by the initial atmospheric pressure) and
The transition to detonation can be clearly seen from
delay to explosion data is shown in Figure 2.
the wave velocity record which has a steady velocity of 0) CD
o c o o
.6 .4
CO to
a.
.2 0
10
3 I
0 .2
.4
.8
.6
1
1.2 1.4
Time , ms I n t e r f ace
0 ■ Ig n i t o r
nO 6 "5"
.2 770 m/s
o II
N
.4
CL
E
Transition
.6
c A3 to
.8
Q
1
>
0 .2
*
i
.8
1
x o
Explosive A Explosive B
>4
Impact data
o _ ,X^.^ r 2 -o 0 0.0 0.3 0.6 0.9 1.2 1.5 Delay time to explosion / I O-^m^
\ \ \ \
4.25 km/s
CJ
\ \
1.2
1.4
FIGURE 1 Ignition end pressure and wave velocity records.
FIGURE 2 Correlation of the pressurization rate with the delay time to explosion.
There is good correlation between the pressurization rates and delay time. This Figure shows data for two
0.77 km/s from the igniter to 0.58 m down steam, at
slurry explosives and one data point for an impact test
which position it changes sharply to 4.25 km/s,
on one of these slurry explosives. The impact data fits
corresponding to the detonation velocity of this
in very well with the DDT data indicating the close
explosive. The pressure reaches 0.45 GPa prior to a
relationship between the two initiation tests.
more dramatic pressure increase which seems to be the
suggests that the mechanism for initiation in the DDT
source of the transition to detonation. If the trajectory
test is the ignition of the explosive by the hot
of the detonation wave is extrapolated back to the
compressed bubble gas similar to that occurring in the
igniter location, the resultant time coincides with the
impact test.
This
moment of explosion in the pressure record. The initial pressurization rate was 490 GPa/s and the time from initiation of the igniter to the moment of explosion of was 0.640 ms. A summary of pressurization rates
3. INITIATION UNDER LOW AMPLITUDE SHOCK PRESSURE From the projectile impact results, it is apparent that
Shock initiation of bubble sensitized commercial explosives
693
the results apply to a transition to detonation and not
the particle velocity if there is no reaction. It is also
initiation to deflagration which may not result in
possible to obtain the shock pressure in the Plex-
detonation. For this reason, DDT and modified gap
iglas attenuator as a function of thickness of the
6
tests were used to provide data for low shock pressure
plate. The shock velocity, Us, is calculated from
initiation.
explosive density, d, shock pressure, Ps, and parti-
Waxed RDX, a slurry explosive and an 3
emulsion explosive at density of 1.15 g/cm were used
cle velocity, Up (Us=Ps/(d*Up)). The results are
in this test. The waxed RDX was used as a standard
compared with the Hugoniot of unreacted emulsion
because of the reproducibility of shock velocity from
and slurry explosives in Figures 3 and 4, respec-
donor charges. The slurry and emulsion products were
tively.
typical cap sensitive commercial explosives. The slurry contains air bubbles and the size varies from a few micrometers to a few millimetres.
The emulsion
includes glass microspheres with an average size of 70 /xm. The pressure of the gas inside the microspheres is normally below atmosphere pressure. The Hugoniots of the unreacted explosives were obtained by conducting wedge tests. This technique involves the simultaneous measurement of the shock
1.0
velocity at the free surface and the free surface velocity which equals twice the particle velocity. The procedure possible to obtain the shock velocity in the plexiglas plate. gap test was used to provide data for low amplitude
1.0 CO
charge, a plexiglas attenuator plate, acceptor charge and Argon filled back light. The free surface velocity of the acceptor is recorded as a function of the thickness
OL
<> V
V*
8°'{ 0.0
o
0
\ \
Details of the experimental set up are
provided in the literature8. The test requires a donor
5.0
I
For initiation under low shock pressure, a modified
by Kroh7.
4.0
0 2.<
OL
O
shock initiation. The test is similar to one implemented
3.0
FIGURE 3 Hugoniot of Emulsion Explosive
used in wedge tests is described in the literature6. It is attenuator as a function of thickness of the plexiglas
2.0
Free Surface Velocity / k m / s
o
1.0
— Inert Hugoniot 0 Experiment 2.0
,^^i . . . . i . . . 3.0 4.0
.
Free Surface Velocity / k m / s FIGURE 4 Hugoniot of Slurry Explosive
of the attenuator. The free surface velocity equals twice From Figures 3 and 4, it is concluded that: (1) The shock velocity required for initiation is higher in emulsion than in slurry explosive. (2) At low gap pressures, the difference between the unreacted
5.0
694
K.K. Feng et al.
Hugoniot and free surface velocity is positive for
there is no reaction under low shock pressure. Because
slurry explosives and negligible for emulsion explo-
the adiabatic compression of bubble gases in slurry
sives. The increased particle velocity for slurry ex-
explosives becomes the more effective
plosives is a direct result chemical reaction in the
mechanism, the slurry explosive will exhibit reaction
slurry. Chemical reaction in the emulsion does not
under low shock pressure.
occur until gap pressures exceed 20 GPa. Hence,
REFERENCES
emulsion explosives require high shock pressure for
1. F.P. Bowden and A.D. Yoffe, Initiation and growth of explosion in liquids and solids, 1952, Cambridge University Press.
initiation whereas slurry explosives are initiated at relatively low shock pressures. At low shock pressures it is not possible for sufficient hot spot formation in emulsion explosives. 4. CONCLUSIONS There are two basic hot-spot mechanisms in slurry explosives. Shock heating of materials around the gas bubble is the dominant mechanism if shock initiation events involve a particle velocity above a few hundred meters per second. At lower pressures, the adiabatic compression of bubble gases becomes the more effective hot-spot mechanism. There is good correlation of the results of DDT and impact tests.
This suggests that the mechanism for
initiation in the DDT test is the ignition of the explosive by the hot compressed bubble gas similar to that occurring in the impact test. The glass microspheres in emulsion explosives are probably not effective for generating hot-spots under low shock pressure, since there is insufficient gas present in the microspheres the glass would also absorb some of the gas energy and additional energy would be
hot-spot
2. R.E. Winter and J.E. Field, The role of localized plastic flow in the impact initiation of explosives, Proc. Roy. Soc. Land. A. 343, 399-413 (1975). 3. ASTM standards ANSI/ASTM D 2540-70 (Reapproved 1980), Standard test method for drop weight sensitivity of liquid mono-propellants, American Society for Testing and Materials, Philadelphia, Pa., U.S.A. 4. S.K. Chan and K.K. Feng, Hot spot mechanism in bubble sensitized commercial explosives, Minutes of the 24th Explosives Safety Seminar of Department of Defence Explosives Safety Board, Aug. 28-30, 1990 at St. Louis Missouri, U.S.A. 5. S.K. Chan, Deflagration to detonation transition behaviour of water-gel explosives, C-I-L Inc. (now ICI Explosives, Canada), McMasterville, P.Q., Canada. 6. K.K. Feng and P. Katsabanis, Low amplitude shock initiation of commercial explosives, Minutes of the 23rd Explosives Safety Seminar of Department of Defence Explosives Safety Board, Aug. 9-12, 1988 at Atlanta, Georgia, U.S.A. 7. M. Kroh, K. Thomas, W. Arnold and V. Wallenweber, Shock sensitivity and performance of several high explosives, 8th Symposium (International) on Detonation, 1985, pp. 1131-1138.
required to break the microspheres. Comparison of the emulsion explosive under low shock pressure with the Hugoniot of unreacted emulsion explosive suggests that
8. A.W. Campbell, W.C. Davis, J.B. Ramsay and J.R. Travis, Shock initiation of detonation in liquid explosives, Third Symposium on Detonation, Naval Ordnance Laboratory, White Oak, U.S.A.
691
SHOCK INITIATION OF BUBBLE SENSITIZED COMMERCIAL EXPLOSIVES K.K. FENG*, D.E.G. JONES* and S.K. CHAN** ♦Canadian Explosives Research Laboratory, CANMET, Energy, Mines & Resources Canada, 555 Booth St., Ottawa, Ontario, K1A 0G1, Canada. **ICI Explosives Canada, McMasterville, Quebec, Canada This paper describes a study of the low pressure shock initiation of slurry and emulsion explosives. Slurry and emulsion explosives under low amplitude shock exhibit two different fundamental hot-spot initiation mechanisms. Slurry explosives are sensitized by gas bubbles whereas emulsion explosives are sensitized by microspheres. The results are also used to compare the Hugoniot of the reacted and unreacted explosives. The results indicated that the mechanism of the hot-spot initiation in slurry explosives is different from that in explosives containing glass microspheres.
1. INTRODUCTION
adiabatic compression as a mechanism in such
It is well known that the sensitivity of an explosive to
initiation.
initiation is significantly increased by the presence of gas bubbles1.
The fundamental mechanism of the
action of such gas bubbles is, however, still highly
2. INITIATION UNDER IMPACT AND DDT CONDITIONS
uncertain, particularly for solid explosives2. Slurry and
When a gas bubble is present in a liquid explosive,
emulsion explosives have characteristics of liquids as
such as slurry and emulsion explosives, the impact
far as their initiation mechanisms are concerned. The
sensitivity is increased significantly1.
initiation mechanisms operable in liquids are much
manufacturing, handling and transporting of these
simpler than those which occur in solids.
These
explosives more hazardous. In order to determine the
mechanisms operate largely independently, at different
effect of gas bubbles on the impact sensitivity of these
ranges of compression rate and allow an improved
explosives, the ASTM Impact Test tool3 was used to
understanding of the process of initiation of these
test various slurry explosives. This tool was used to
explosives.
study the response of an EGMN based slurry explosive.
This makes
At low shock pressure, the adiabatic compression of
An impact weight of 5 kg was used. Positive results
bubble gases in slurry explosives becomes the most
were obtained above a drop height of 0.36 m. There
predominant hot-spot mechanism.
The presence of
seems to be little doubt that the initiation mechanism is
glass microspheres in emulsion explosives is probably
the ignition of the explosive by the hot compressed gas
not effective for the adiabatic compression mechanism.
bubbles4.
This paper presents some data on the ASTM impact,
Another hazard test carried out was the DDT test.
deflagration to detonation transition (DDT) and low
Two EGMN based slurry explosives which showed
shock pressure initiation of slurry and emulsion
DDT behaviour were tested.
explosives. This data demonstrates the importance of
igniter end pressure wave velocity records for a test
Figure 1 shows the
692
K.K. Feng et ai
with slurry explosive initiated by the igniter mixture.
(normalized by the initial atmospheric pressure) and
The transition to detonation can be clearly seen from
delay to explosion data is shown in Figure 2.
the wave velocity record which has a steady velocity of 0) CD
o c o o
.6 .4
CO to
a.
.2 0
10
3 I
0 .2
.4
.8
.6
1
1.2 1.4
Time , ms I n t e r f ace
0 ■ Ig n i t o r
nO 6 "5"
.2 770 m/s
o II
N
.4
CL
E
Transition
.6
c A3 to
.8
Q
1
>
0 .2
*
i
.8
1
x o
Explosive A Explosive B
>4
Impact data
o _ ,X^.^ r 2 -o 0 0.0 0.3 0.6 0.9 1.2 1.5 Delay time to explosion / I O-^m^
\ \ \ \
4.25 km/s
CJ
\ \
1.2
1.4
FIGURE 1 Ignition end pressure and wave velocity records.
FIGURE 2 Correlation of the pressurization rate with the delay time to explosion.
There is good correlation between the pressurization rates and delay time. This Figure shows data for two
0.77 km/s from the igniter to 0.58 m down steam, at
slurry explosives and one data point for an impact test
which position it changes sharply to 4.25 km/s,
on one of these slurry explosives. The impact data fits
corresponding to the detonation velocity of this
in very well with the DDT data indicating the close
explosive. The pressure reaches 0.45 GPa prior to a
relationship between the two initiation tests.
more dramatic pressure increase which seems to be the
suggests that the mechanism for initiation in the DDT
source of the transition to detonation. If the trajectory
test is the ignition of the explosive by the hot
of the detonation wave is extrapolated back to the
compressed bubble gas similar to that occurring in the
igniter location, the resultant time coincides with the
impact test.
This
moment of explosion in the pressure record. The initial pressurization rate was 490 GPa/s and the time from initiation of the igniter to the moment of explosion of was 0.640 ms. A summary of pressurization rates
3. INITIATION UNDER LOW AMPLITUDE SHOCK PRESSURE From the projectile impact results, it is apparent that
Shock initiation of bubble sensitized commercial explosives
693
the results apply to a transition to detonation and not
the particle velocity if there is no reaction. It is also
initiation to deflagration which may not result in
possible to obtain the shock pressure in the Plex-
detonation. For this reason, DDT and modified gap
iglas attenuator as a function of thickness of the
6
tests were used to provide data for low shock pressure
plate. The shock velocity, Us, is calculated from
initiation.
explosive density, d, shock pressure, Ps, and parti-
Waxed RDX, a slurry explosive and an 3
emulsion explosive at density of 1.15 g/cm were used
cle velocity, Up (Us=Ps/(d*Up)). The results are
in this test. The waxed RDX was used as a standard
compared with the Hugoniot of unreacted emulsion
because of the reproducibility of shock velocity from
and slurry explosives in Figures 3 and 4, respec-
donor charges. The slurry and emulsion products were
tively.
typical cap sensitive commercial explosives. The slurry contains air bubbles and the size varies from a few micrometers to a few millimetres.
The emulsion
includes glass microspheres with an average size of 70 /xm. The pressure of the gas inside the microspheres is normally below atmosphere pressure. The Hugoniots of the unreacted explosives were obtained by conducting wedge tests. This technique involves the simultaneous measurement of the shock
1.0
velocity at the free surface and the free surface velocity which equals twice the particle velocity. The procedure possible to obtain the shock velocity in the plexiglas plate. gap test was used to provide data for low amplitude
1.0 CO
charge, a plexiglas attenuator plate, acceptor charge and Argon filled back light. The free surface velocity of the acceptor is recorded as a function of the thickness
OL
<> V
V*
8°'{ 0.0
o
0
\ \
Details of the experimental set up are
provided in the literature8. The test requires a donor
5.0
I
For initiation under low shock pressure, a modified
by Kroh7.
4.0
0 2.<
OL
O
shock initiation. The test is similar to one implemented
3.0
FIGURE 3 Hugoniot of Emulsion Explosive
used in wedge tests is described in the literature6. It is attenuator as a function of thickness of the plexiglas
2.0
Free Surface Velocity / k m / s
o
1.0
— Inert Hugoniot 0 Experiment 2.0
,^^i . . . . i . . . 3.0 4.0
.
Free Surface Velocity / k m / s FIGURE 4 Hugoniot of Slurry Explosive
of the attenuator. The free surface velocity equals twice From Figures 3 and 4, it is concluded that: (1) The shock velocity required for initiation is higher in emulsion than in slurry explosive. (2) At low gap pressures, the difference between the unreacted
5.0
694
K.K. Feng et al.
Hugoniot and free surface velocity is positive for
there is no reaction under low shock pressure. Because
slurry explosives and negligible for emulsion explo-
the adiabatic compression of bubble gases in slurry
sives. The increased particle velocity for slurry ex-
explosives becomes the more effective
plosives is a direct result chemical reaction in the
mechanism, the slurry explosive will exhibit reaction
slurry. Chemical reaction in the emulsion does not
under low shock pressure.
occur until gap pressures exceed 20 GPa. Hence,
REFERENCES
emulsion explosives require high shock pressure for
1. F.P. Bowden and A.D. Yoffe, Initiation and growth of explosion in liquids and solids, 1952, Cambridge University Press.
initiation whereas slurry explosives are initiated at relatively low shock pressures. At low shock pressures it is not possible for sufficient hot spot formation in emulsion explosives. 4. CONCLUSIONS There are two basic hot-spot mechanisms in slurry explosives. Shock heating of materials around the gas bubble is the dominant mechanism if shock initiation events involve a particle velocity above a few hundred meters per second. At lower pressures, the adiabatic compression of bubble gases becomes the more effective hot-spot mechanism. There is good correlation of the results of DDT and impact tests.
This suggests that the mechanism for
initiation in the DDT test is the ignition of the explosive by the hot compressed bubble gas similar to that occurring in the impact test. The glass microspheres in emulsion explosives are probably not effective for generating hot-spots under low shock pressure, since there is insufficient gas present in the microspheres the glass would also absorb some of the gas energy and additional energy would be
hot-spot
2. R.E. Winter and J.E. Field, The role of localized plastic flow in the impact initiation of explosives, Proc. Roy. Soc. Land. A. 343, 399-413 (1975). 3. ASTM standards ANSI/ASTM D 2540-70 (Reapproved 1980), Standard test method for drop weight sensitivity of liquid mono-propellants, American Society for Testing and Materials, Philadelphia, Pa., U.S.A. 4. S.K. Chan and K.K. Feng, Hot spot mechanism in bubble sensitized commercial explosives, Minutes of the 24th Explosives Safety Seminar of Department of Defence Explosives Safety Board, Aug. 28-30, 1990 at St. Louis Missouri, U.S.A. 5. S.K. Chan, Deflagration to detonation transition behaviour of water-gel explosives, C-I-L Inc. (now ICI Explosives, Canada), McMasterville, P.Q., Canada. 6. K.K. Feng and P. Katsabanis, Low amplitude shock initiation of commercial explosives, Minutes of the 23rd Explosives Safety Seminar of Department of Defence Explosives Safety Board, Aug. 9-12, 1988 at Atlanta, Georgia, U.S.A. 7. M. Kroh, K. Thomas, W. Arnold and V. Wallenweber, Shock sensitivity and performance of several high explosives, 8th Symposium (International) on Detonation, 1985, pp. 1131-1138.
required to break the microspheres. Comparison of the emulsion explosive under low shock pressure with the Hugoniot of unreacted emulsion explosive suggests that
8. A.W. Campbell, W.C. Davis, J.B. Ramsay and J.R. Travis, Shock initiation of detonation in liquid explosives, Third Symposium on Detonation, Naval Ordnance Laboratory, White Oak, U.S.A.