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 510.
A FLASH X-RAY TECHNIQUE TO MEASURE STRAIN
DISTRIBUTION AT INTERFACES SLIDING AT HIGH PRESSURE
AND VELOCITY
R.E. Winter, P. Taylor, D J. Carley, A J. Barlow, H. Pragnell and L. Markland
AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK
In this technique blocks of metal of widely different densities are placed in contact to form a composite
block. Detonating an explosive charge positioned on the edge of the assembly drives the light metal at a
higher velocity than the heavy metal resulting in sliding between the metal blocks. The aim is to
measure the strain distribution in the material adjacent to the sliding interface by radiographing thin
foils embedded in both materials perpendicular to the interface. A high density foil is inserted in the
low density material and a low density foil in the high density material. Use of a high energy flash x-ray
machine allows large metal blocks to be used which in turn allows radiographs to be taken at a time
when significant slip has occurred. The foil fiducials were clearly seen in a pilot experiment fired with
aluminium and steel blocks. The result of this experiment was well matched by a hydrocode. The code
has been used to design an experiment to provide data on sub surface deformation.
INTRODUCTION
adjacent to the interface was measured by
radiographing 407 u,m lead wires embedded into the
aluminium. Four radiographs were taken using a
sodium iodide fluor viewed by four gated CCD
cameras.
Reported results showed that motion of the order
of 0.5 mm had taken place along the interface. The
distribution of the deformation in the aluminium
was deduced from the positions of the embedded
wires.
Although the work of Hammerberg et al. provided
good data it suffers from the drawback that the total
relative motion of the sliding metals is small
compared with the resolution of the diagnostic.
The objectives of the work described here is to
use a configuration, similar in principle to
Hammerberg's, but which employs explosive
loading, together with significantly larger
components, to increase the amount of relative
motion of the two metals. Larger components may
be used because a high energy flash x-ray
diagnostic is available. The main aim of the first,
pilot, experiment was to determine the feasibility of
Many problems in shock physics and metal
forming involve the sliding of one surface over
another (1-3). Analysis of such situations often
requires the slip at the interface between two metals
to be calculated. The deformation field in the
vicinity of the interface between two metals sliding
against one another at high velocity and pressure,
will depend on the forces generated at the sliding
interface and on the strength properties of the two
materials. Measurement of this deformation field
provides data for evaluation and optimisation of
calculational models.
Hammerberg et al (1) used a pulse power
technique to implode a cylindrical metal liner onto a
cylinder made up of two aluminium wedges and
two tantalum wedges. The cylindrical target had a
diameter of 30 mm. The impact drives shock waves
inwards into the two materials. Since the shock and
particle velocities are higher in the aluminium than
in the tantalum the Ta/Al interface experiences
differential slip. The deformation of the material
507
the left side of the system as shown. The explosive
was detonated by a line initiator.
the foil technique. For example would the foils be
visible in both high density and low density
materials and would the foils remain sufficiently
planar to allow useful measurement? A second aim
was to determine our ability to compute the
macroscopic response of the system.
0
1
2
3
Explosive
Aluminium
S/Steel
4
Time, us
FIGURE 1. Relative motion of foil estimated from 1-D
treatment
FIGURE 2. Configuration used to assess visibility of foils
One of the concerns with this experiment was that
releases from the front and rear faces of the
assembly (as seen by the x-ray machine) would lead
to curvature of the foils which would in turn make
them difficult to resolve. The 2D Eulerian
Upper limits to the shock and particle velocities
imparted to the stainless steel and aluminium blocks
may be deduced assuming one-dimensional
conditions. Particle velocities of 2.0 and 1.2 mm/u,s
are derived for aluminium and steel respectively
and for the shock velocities, 8.1 and 6.3 mm/jo,s
respectively. The relative velocities of the two
materials after the aluminium and steel shocks are
Lead
2.0 and 0.8 mm/u,s respectively. The corresponding
relative displacement vs time relation for a foil
positioned 4 cm from the explosive surface is
shown in Fig. 1. This simple analysis suggests that
appreciable relative slip will occur within a few
microseconds of the shock reaching the foil
fiducial, (although in practice shock attenuation will
significantly reduce these displacements).
Aluminium
Lead
FIGURE 3. A 2-D Eulerian calculation showing the bending of
the fiducial in the aluminium. The lead blocks are included to
improve the radiography by reducing the difference in dose
through the aluminium and steel.
EXPERIMENT
The feasibility experiment is shown in Fig. 2. A
0.1 mm tantalum foil is mounted between two
aluminium blocks of dimensions 40x50x80 mm and
60x50x80 mm. The blocks are clamped together
using steel bolts. This unit is mounted on top of a
similar unit in which 0.1 mm aluminium foils are
clamped between stainless steel blocks. The contact
faces were flat to within better than ±5 |im. 25 mm
thick blocks of lead, (not shown in Fig 2), were
placed at the front and rear of the assembly to
reduce the dose difference through the two blocks.
A block of HMX based explosive was mounted at
calculation in Fig. 3 shows that some curvature of
the fiducials is expected. However, although the
amount of curvature is sufficient to significantly
reduce the line of sight mass of the foil seen by a
perfectly aligned x-ray beam, it has the merit of
increasing the amount of angular misalignment that
could be tolerated.
The configuration was radiographed using AWE's
MOGUL D x-ray source. This provides a dose of
153 R at a meter with a 3,5 mm spot size.
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the foils are measurable to ±0.1 mm. Note that at
the time of the radiograph the steel and aluminium
block have separated by about 3 mm.
FIGURE 5. CORVUS calculation of experiment at 40 |is
EXPERIMENT FN1/1
CORVUS CALCULATION
FIGURE 4. Mogul D radiograph showing foil fiducials in the
aluminium (upper) and steel (lower)
The radiograph was taken at 40 (is from initiation
of the explosive block (31 |is from the time at
which the shock reaches the foil in the aluminium.)
At this time the Eulerian calculation predicted that
the relative displacement of the foils was about 4
mm. Radiographing at this late time provides a
stringent test of the radiography since subsequent
experiments are likely to be fired at an earlier stage
when the fiducials will be less distorted. The x-ray
beam was carefully aligned at the expected position
of the fiducials at the radiographic target time by
using a laser beam set to graze the upper surface of
the lower block. Lead collimation was used to
restrict the field of view thereby minimising the xray scatter. In order to provide as much general
information as possible from this pilot experiment it
was decided to include the relative displacement of
the blocks at the right side of the experiment in the
radiographic view.
FIGURE 6. Code vs Experiment Comparison at 40 ^is
CORVUS CALCULATIONS
The pilot experiment was calculated using the 3-D
Arbitrary Lagrangian Eulerian (ALE) code
CORVUS (4). The code allows different frictional
treatments to be incorporated and also allows void
opening. The calculation, which is shown in Fig. 5,
and compared with the experimental outline in Fig.
6, matches the experiment reasonably well.
RESULTS
A selected area of the radiograph is shown in Fig.
4. The foil in the aluminium block is clearly
resolved. The foil in the steel block is less clear but
still resolvable. It is estimated that the positions of
FIGURE 7. CORVUS Calculation of "Sandwich" configuration
at 30 |as
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CONCLUSIONS
10.0
15.0
20.0
A technique for dynamically imaging the
subsurface deformation that occurs when two
metals slide over each other at pressures and
velocities in the hydrodynamic regime has been
described.
In order to image subsurface distortion we need a
system in which at least a few millimeters of slip
occurs. The time at which the radiograph can be
taken is limited by release waves from the outer
surfaces of the assembly. A larger assembly
provides a larger time window. To radiograph
through large blocks of metal we need penetrating
x-rays. In the pilot experiment described here
AWE's MOGUL D flash x-ray machine was used
to obtain a clear image through 80 mm of steel plus
50 mm of lead.
A future experiment in which the two surfaces
will move relative to each other by ~2 mm while
the metal surfaces are held in contact should
provide useful information on subsurface
deformation during sliding.
25.0
Time (MS)
FIGURE 8. From the top:Pressure in Mb; velocity of Al in
cm/us; velocity of steel in cm/us; relative velocity in cm/jis.
REFERENCES
1.
2
Hammerberg, J.E., Kyrala, G.A., Oro, D.M., Fulton,
R.D., Anderson, W.E., Obst, A.W., Oona, H.,
Stokes, J. and Wilke, M.D., "A Pegusus Dynamic
Liner Friction Experiment", in Shock Compression
of Condensed Matter -7999, Edited by M.D.Furnish
et al., AIP Conference Proceedings 505, New York,
4
Displacement of Foil (mm)
FIGURE 9. Fiducial distortions for modified geometry.
In the next experiment to be fired the
configuration will be modified to form a steelaluminium-steel "sandwich". The configurations at
30 jis are shown in Fig. 7. It is seen from the
calculation that sandwiching the aluminium
between two steel blocks holds the two blocks
together for much longer than was the case with the
pilot experiment. Figure 8 shows parameters at the
fiducial position plotted against time in fis. In order
from top to bottom the plots show: the pressure in
Mb; the velocity of the aluminium; the velocity of
the steel and the relative velocity of the steel and
aluminium. Figure 9 shows, on a graph with a
magnified distance scale on the horizontal axis, the
calculated positions of the fiducials at different
times. The solid and dashed lines are generated by
calculations with merged and sliding interfaces
respectively.
2.
3.
4.
1998 pp!217-1220.
Pelak, Robert A., Rightly, Paul, and Hammerberg,
J.E., "Friction in High-Speed Impact Experiments",
in Shock Compression of Condensed Matter -1999,
Edited by M.D.Furnish et al., AIP Conference
Proceedings 505, New York, 1998, p!221-1224.
Boucher, Roy J., and Chhabildas, Lalit C, "High
Velocity Erosion of Metal Interfaces", in Shock
Compression of Condensed Matter -1987, Edited by
S.C.Schmidt and N.C.Holmes, APS Conference
Proceedings, New York, 1988 pp 741-.744
Barlow A.J., and Whittle, J., "Mesh Adaptivity and
Material Interface Algorithms in a two dimensional
Lagrangian Hydrocode", Chem.Phys.Reports, 2000,
19(2),p.233-258
© British Crown Copyright 2001/MOD
Published with the permission of the Controller of Her
Britannic Majesty's Stationery Office.
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