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
For special copyright notice, see page 422.
CHARACTERIZATION OF THE SATURN AIR LENS AND ITS USE
IN FOAM STUDIES
E J. Harris, D.A. Salisbury, P. Taylor and R.E. Winter
Hydrodynamics Department, AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK
In the Saturn air lens a concave brass plate, projected by a cylindrical charge, generates a nearly planar
initiation at the face of a second explosive cylinder. The lens has been characterised using data from
two experiments in which the donor drives metal plates whose surface motion is determined by laser
interferometry. The data from these experiments has allowed a computational model of the lens, based
on a Eulerian code with the Lee and Tarver reactive burn model, to be optimised. It was found that
modelling the spall of the brass plate and the initiation pattern of the second charge was important. The
analysis of previously reported equation of state experiments on low-density foam in which the lens
was modelled using a simple 1-D treatment have been reviewed.
presented of experiments in which the specimens
were replaced by a known material, PMMA.
Additional experiments designed to measure the
drive from the plane wave lens are described in this
paper. The results are used to support the
development of a more accurate 2-dimensional
model of the SATURN PWL than was available to
analyse the foam equation of state experiments
reported in the earlier work (1-3). The model is
based on the AWE 2D Eulerian code PETRA.
INTRODUCTION
A programme is underway at AWE to develop a
capability to model the consequences of shock
loading assemblies containing foam components.
Maw, Whitworth and Holland (1), described a
series of experiments to measure the response of
foams to multiple shocks. Foam discs sandwiched
between aluminium and steel plates were shocked
through
intermediary poly methylmethacry late
(PMMA) and aluminium alloy layers by an air lens
designed at AWE. During the remainder of this
paper the lens will be referred to as the SATURN
PWL (Plane Wave Lens). The transmitted shock
was monitored on axis by velocity interferometry.
Although accurate measurements of the shocks
transmitted through the foam samples were made it
was not possible to measure the input shock
directly. Instead Maw derived the input shock into
the foam sample by assuming that the plane wave
lens could be approximated by a slab of explosive
of a thickness chosen to obtain agreement with a
separate calibration experiment. Subsequently
Salisbury, Winter, Taylor and Harris (2) published
additional data on three different foams using the
same experimental configuration. Results were also
EXPERIMENTS
The construction of the SATURN PWL is
shown in the lower part of Fig. 1. An EDC1 donor
charge initiated by a detonator drives a curved 4mm
brass plate into a charge of EDC32 explosive. The
EDC32 detonates and drives a near planar shock
into the sample.
Two experimental configurations were used to
support the PWL modelling. In the first, shown in
Fig. 1 and designated SI9/22, the response of an
acceptor consisting of 8.2mm of PMMA and 1.4
mm of aluminium alloy (5083) is monitored by
velocity interferometery. In the second experiment
(SI9/23) the acceptor was a 1.5 mm thick copper
(C101) plate.
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__
No spall
. _ _ With spgll
20
TWIN FABRY-PEROT
ALALLOY^
PMMA
\
-
i i
CL
15
I
10
EDC32 -"
AIR
NAVAL BRASS
15.0
15,8
16.0
16,2
16,4
time/us
FIGURE 2. Calculated pressure into inert EDC32 showing
EDC1
15,2
15.4
15,6
effect of spall in the brass.
Calculations of experiments SI9/22 and 23 were
run to determine whether the response of the
EDC32 could be modelled using programmed burn.
A series of detonation points were assigned along
the edge of the explosive and detonated when the
first shock produced a pressure of 5 GPa at these
points. Figure 3 shows the calculated free surface
velocities
for
comparison
with
the
PMMA/aluminium experiment SI9/22. It is seen
that, although there is a qualitative similarity
between code and experiment, the calculated
relative magnitudes of the velocities on and off axis
at the first velocity jump are significantly different
from experiment. It was concluded that the
experimental results could not be matched
adequately using programmed burn.
———— 100mm ————
FIGURE 1. SI9/22 geometry with 8.2mm PMMA and 1.4mm
aluminium alloy discs. SI9/23 was similar but with the PMMA
and aluminium replaced by a 1.5mm sheet of copper.
MODELLING THE SATURN PWL
In the SATURN PWL planarity is achieved by
means of a 4 mm brass plate curved to impact an
EDC32 acceptor charge simultaneously. One of the
issues to be addressed was whether the brass plate
spalls. PETRA calculations showed that when the
brass plate was allowed to sustain a tension the
pressure dropped to a minimum of -9 GPa on axis
near the mid plane and therefore the brass would be
expected to spall. The effect of including the spall
model was determined by calculating the pressure
pulse delivered to EDC32 to which an inert
Equation of State (EoS) had been assigned. Figure 2
shows that when the brass plate was allowed to go
into tension a single pressure pulse of -20 GPa was
observed in the inert EDC32, but when a spall
model was used an initial pressure pulse of-10 GPa
followed by a second step of -17 GPa was seen.
The two step pressure structure obtained with the
spall model was not local to the axis but was still
clearly visible 35 mm off the axis.
It is concluded from the above that it is
necessary to include a spall model to accurately
calculate the shock delivered to the EDC32
acceptor.
__ _
23
5i"9/22 experiment
calculation
24
25
26
27
28
time/us
FIGURE 3. Aluminium alloy free surface velocities from
calculations using programmed burn. The on axis trace is offset
0.5 km/ps and the off axis trace is offset right by l(js for clarity.
The same offsets are used in later figures.
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Figure 4 shows the response of the EDC32
computed using a Lee and Tarver reactive burn
model (3) for EDC32 based on published data for
LX04 (an explosive similar in composition to
EDC32). It is seen that the brass plate impacts
slightly earlier at the periphery of the plate
producing a concave detonation wave. Figure 5
shows the complexity of the computed pressure
profile through the EDC32 detonation products at
the time that the detonation wave reaches the
sample. These observations strongly suggests that
1D modelling of the lens will be inadequate.
8
1
- _-
23
24
25
si9/22 experiment
calculation
27
26
time/us
28
FIGURE 6. SI9/22, (PMMA/AI sample) Comparison with
calculation.
Figure 7 shows the match to the Copper
experiment SI9/22. In this case the shock
reverberations are well matched but the initial
velocity jump is too high in the calculation. The
relative timing of the on and off axis probes is
calculated well. Improved agreement with the late
time free surface velocities was obtained by
adjusting the EoS of the detonation products. The
ignition and growth parameters were adjusted to
give a good agreement with the known run distance
of LX04. It is concluded that the 2D model
described in this paper matches the characterisation
experiments more closely than the ID model.
FIGURE 4. 2D plot of pressure gradient showing a curved
detonation wave in the EDC32 acceptor charge.
40
30
f.
%
I
10
EDC32
11.0
11.5
12.0
12,5
13.0
13,5
14.0
FIGURE 7.
calculation.
14.5
dlstonce/cm
23
24
S19/23
(copper
25
time/us
26
27
sample) comparison
with
FIGURE 5. Pressure profile through the detonation products on
axis.
DISCUSSION
Figure 6 shows the agreement obtained to the
PMMA/AI experiment SI9/22 using the model
described in this paper. The match is somewhat
better than was obtained with the ID model (as
shown in figure 3).
Salisbury and co-workers presented the results of
experiments with PMMA (SI9/17) and 0.3g/cc
polyurethane (SI9/18) samples. Figure 8 shows the
computed velocity profiles for comparison with
experiment SI9/17. The two computed profiles
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assume (a) Maw's ID representation of the PWL
and (b) the 2D model described in this paper. It is
seen that the match to the first shock is significantly
better with the 2D than with the ID model.
2,5
Bi9/18 experiment
2.0
ID calculation
2D calculation
?
| 1.5
7,0
f
1.0
S
8.5
9.0
9,5
CONCLUSIONS
0.0
1,0
8.0
time/us
FIGURE 10. SI9/18, ID vs. 2D comparison using Maw and
Whitworth's 0.3 g/cc polyurethane foam EoS (4).
J9/17 experiment
D calculation
D calculation
0,5
7.5
1,5
2,0
2.5
time/us
3.0
3,5
4.0
The SATURN plane wave lens has been
modelled in 2-D using the Eulerian Hydrocode
PETRA. A spall model was used to model the
behaviour of the brass flyer and the Lee and Tarver
ignition and growth model was used to model the
response of the main EDC32 charge. The model
closely matches the results of calibration
experiments. It is shown that the changes to the
model used for the plane wave lens affect the
calculated transmitted shock.
FIGURE 8. SI9/17, 1D vs. 2D comparison.
In Fig. 9 an AWE EoS for polyurethane foam
was used to obtain a similar comparison for SI9/18,
the experiment with the foam sample. In this case
there is only a small difference between the output
calculation with ID and 2D PWL modelling. Note
that in both experiments the initial velocity jump
was calculated slightly too high but this result was
consistent with the result from the two calibration
experiments.
REFERENCES
1.
2.0
2.
si9/18 experiment
ID calculation
2D calculation
3.
7,0
7.5
8.0
time/us
8.5
9.0
9,5
FIGURE 9. SI9/18, ID vs. 2D comparison using AWE EoS for
0.3 g/cc polyurethane foam.
4.
Finally, Fig 10 shows a ID versus 2D
comparison obtained using Maw and Whitworth's
polyurethane EOS (4). It is seen that the 2D
modelling has little effect of early time behaviour
but improves the match at late time (after ~8 (is).
Maw, J.R., Whitworth NJ. and
Holland,R.B.,"Multiple Shock Compression of
Polyurethane and Syntactic Foams", in Shock
Compression of Condensed matter -1995, ppl33136
Salisbury, D.A., Winter, R.E., Taylor, P. and Harris,
E.J., "The Response of Foams to Shock
Compression", Shock Compression of Condensed
matter -1999, edited by M.D.Furnish,
L.C.Chhabildas and R.S.Hixson, ppl97-200.
Lee, E. L. and Tarver, C. M., "Phenomenological
model of shock initiation in heterogeneous
explosives", Phys. Fluids 23(12), pp.2362-2372
(1980).
Maw, J.R. and Whitworth, NJ., "Shock
Compression and the Equation of State of Fully
Dense and Porous Polyurethane", Shock
Compression of Condensed matter -7997, ppl11114.
© British Crown Copyright 2001/MOD
Published with the permission of the Controller of
Her Britannic Majesty's Stationery Office.
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