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
INVESTIGATING MULTI-DIMENSIONAL EFFECTS IN SINGLECRYSTAL SAPPHIRE*
W. D. Reinhart1, L. C. Chhabildas1, W. M. Trott1, D. P. Dandekar2
2
Sandia National Laboratories, Department 1610, Albuquerque, New Mexico
U.S. Army Research Laboratory, Weapons and Materials Research Directorate
Aberdeen Proving Ground, Maryland
2
Abstract: Most studies in the past have focused on obtaining uni-axial strain states in shocked materials. In
this study, however, results of symmetric impact gas gun experiments on single-crystal C-cut sapphire are
described to observe edge relief waves as they propagate toward the center of the sapphire target shocked to
high pressures. This is made possible by the recent development of a LINE ORVIS, which measures both
spatial and time-resolved particle-velocity variations in materials. A series of experiments have been
conducted over the impact velocity from -0.25 to 0.8 km/s, and in the elastic regime (except the 0.8km/s
experiment). In these experiments, a new line imaging optically recording velocity interferometer system is
used over a line segment of 13mm. Edge relief waves are unmistakably visible with local variations
following the edge relief wave. Heterogeneous effects following dynamic yielding is also observed.
INTRODUCTION:
A variety of established diagnostic tools are
available to determine the shock Hugoniot of
materials. In particular, a single point VISAR[1]
(velocity interferometer) is used to measure material
velocity, a variable that is necessary to relate the
Hugoniot for the material (care is taken to ensure
that the conditions of uniaxial-strain are met in these
applications; namely, the diameter is large compared
to the sample thickness). In this paper, we have
attempted to investigate multi-dimensional wave
propagation in single crystal C-cut sapphire (i.e
crystallographic C-direction is parallel to the sample
cylindrical axis). The line imaging ORVIS [2,3,4,5]
(Optically Recording Velocity Interferometer
System) is projected as a finite length on the target
surface allowing measurements of particle velocity
not only as a function of time, but also yields spatial
distribution over the length of the line. In this study,
a smaller diameter C-cut sapphire disc impacts a
large diameter C-cut sapphire crystal target.
Experiments were conducted over an impact
velocity range from 0.25 to 0.8 km/s corresponding
to a stress regime of 5.5 to 18.5 GPa (exceeding the
approximately 15.5 GPa). A line
with the length of over 13mm is projected on the
sample This length was specifically chosen so that
multi-dimensional effects emanating from the edge
of the impactor would be recorded as it traverses
towards the center of the target Three experiments
having slightly different experimental configurations
will be discussed. Many interesting details of this
well-characterized homogeneous material are
revealed in this study. Complex and apparently
asymmetric variations in the fringe records
following the edge relief waves indicate anisotropic
behavior. In particular, upon dynamic yielding, the
shock front is no longer planar indicative of a
spatially heterogeneous yielding process. In all of
experiments, in-plane crystallographic
these
orientation was not considered because different
crystallographic orientations in the plane of the
crystal, cannot be distinguished, as the longitudinal
wave speeds vary as little as ¥2% in the orthogonal
directions [6]
elastic limit of
* This work was supported by the U. S. Department of Energy under contract DE-AC04-94AL8500000. Sandia is a multiprogram
laboratory operated by Sandia Corporation a Lockheed Martin Company, for the United States Department of Energy.
791
MATERIAL DESCRIPTION:
Single crystal sapphire (A12O3) discs were used for
the experiments with axis parallel to the
crystallographic C-axis. Sapphire was chosen for
Projectile
Target Cup
Imaging ORVIS
im line segment)
211 or 30 HIM
Diameter Sapphire
Impactor
FIGURE 2. Fringe record from LVS APH-4.
Shock arrival at free surface and lateral unloading
are unmistakably visible
50 mm Diameter
Sapphire Target
aluminum coating to allow a reflected beam to be
monitored during the experiment.
LVSAPH-4: ANALYSIS:
FIGURE 1. Experimental configuration. Three or ten
millimeter flier impacts five-millimeter target with
ORVIS imagee on rear surface or insitu.
The streak camera records obtained from this
experiment performed at an impact velocity of 0.564
these experiments because of the very high
Hugoniot elastic limit [7,8] and also because it is a
well characterized material in the elastic regime [9].
EXPERIMENTAL TECHNIQUE:
The experimental method used in this study is
indicated in Figure 1. The experiments presented
were conducted on a single-stage light gas gun. A
symmetric impact configuration for sapphire was
used in this study where the impactor diameter for
all experiments was intentionally sized smaller than
that of the diameter of the target. Impact velocities
were varied to induce stress states from
approximately 65 to 180 GPa. Three electrical
shorting pins were used to measure the velocity of
the projectiles at impact and four similar pins were
mounted flush with the impact plane to monitor
impact planarity. The insitu and free surface
velocity histories were recorded using the recently
developed Line Imaging ORVIS. The ORVIS was
configured to generate a low magnification image at
the detector and provides a line segment of
approximately 13 mm. Experimental techniques for
generating line-imaging ORVIS [2,3,4,5] have been
previously reported, therefore the principles of
operation will not be summarized in this paper. This
modification is expected to provide macroscopic
spatial variations and continuous monitoring of
wave fronts as they propagate through the material.
Samples were coated with a thin reflective
FIGURE 3. Geometric Analysis used for experiments for
determination of lateral unloading.
km/s are shown in Figure 2. The ORVIS line is
deliberately positioned on the rear surface of the
target to observe the multi-dimensional effects from
the outer diameter of the impactor as the waves
propagate towards the center of the target. For this
low magnification, the distance that corresponds to
one fringe cycle is 550-(im and the entire line image
from the target spans approximately 13mm. The
FIGURE 4. 3mm line segment near the center of the target
suggests that there is a minor perturbation in the particle
velocity measurements caused by the arrival of the edge release
792
published equation of state parameters given by the
empirical relation, Us = Co + sup, is used for shock
velocity determination. Us is the shock velocity, at
a given particle velocity up, and Co (11.91km/s) and
s(l) are the parameters in the linear shock-velocity
vs particle-velocity relations for sapphire. Simple
geometric analysis is used to estimate lateral
unloading wave speeds from the edge. As indicated
in Figure 3, the initial fringe measurement at the free
surface indicates shock arrival, and the change in
particle velocity provides an accurate indication of
the Hugoniot state. The lateral unloading emanating
from the edges of the impactor/target interface is
now traversing through a compressed medium, and
is clearly visible in the fringe record at the free
surface. The velocity-time profile for a 3mm line
segment near the center of the target (Figure 4)
suggests that there is a minor perturbation in the
particle velocity measurements caused by the arrival
of the edge release wave. Geometric analyses of the
experiment suggest the lateral release wave velocity
is estimated to be 12.7 km/s.
The fringe records indicates impact time at t=0, and
also an edge release as depicted by the slanted line.
There is a perturbation in the fringe record at a time
of 445 ns after impact that is caused by the arrival of
the elastic shock front at the free surface. The
subsequent change in particle velocity at 544 ns in
the fringe record can be attributed to the release
wave emanating from the impactor/TPX interface.
This is shown in Figure 5b, the Lagrangian x-t
diagram, depicts the (one-dimensional) waveinteractions occurring in the impactor and the target.
The interaction of the two release waves at 717ns
within the sample is clearly seen in the fringe record
(Figure 5a). A gradual release followed by a drastic
change in light intensity is observed after 717 ns.
This is due to spallation of the sapphire target
sample. It is interesting that the leading edge of the
edge release wave appears to unperturbed in the
fringe record. It is not surprising because the edge
wave front is monitored at the impact interface while
the interactions indicated above are occurring within
the sample. An analysis of the edge release waves
in this experiment yields a velocity of 11.0 km/s.
LVSAPH-6: ANALYSIS:
CVSAPH-5: Analysis
Using a conventional Visar system, and an
experimental configuration illustrated in Figure 6,
the lateral unloading was observed. The impact
The same general experimental configuration was
used on this experiment; however, the OVIS line
segment was projected through the material to
monitor the impact surface. This test now would
eliminate most of the wave interactions that is
caused by the shock front arrival at the free surface.
In this experiment, a 19-mm diameter, 3-mm thick
sapphire impacts a 50-mm diameter 5-mm thick
target sample at an impact velocity of 0.55 km/s.
Arrival of Lateral Release Via\
o°ot
0.20
0.40 0.60
Time (us)
0.80
1.0(
FIGURE 6. Conventional Visar experiment. Lateral
unloading velocity of 12.9 km/s.
989ns
velocity for this experiment was 0.2 km/s imparting
a stress of approximately 4.5 GPa. Two Visar
probes were located 3-mm apart and 5 and 8-mm
from the target edge respectively. The velocity
profiles in Figure 4b, show that lateral unloading
wave velocity to be 12.9 km/s.
445ns
a).
Impact
FIGURE 5. Lagrangian x-t diagram depicts the (onedimensional) wave-interactions occurring in the
impactor and the target.
793
cu i
Shock
. ,
A
Arrival
AA445 ns
Dynamic
_/. _ _.
Yielding
___ **
775ns
km/s. It appears that this wide variation is related to
the anisotropic nature of the crystal. Even though it
has been reported that the anisotropy in the two
orthogonal crystal axis is quite small, it is quite
likely that the pressure dependence of the elastic
constants is quite significant. This needs to be
pursued further by monitoring edge release waves in
well-defined crystallographic directions. The results
also indicate spatially inhomogeneous yielding
process, even though the studies are performed in
single crystals.
This explains why there are
considerable differences in the Hugoniot elastic limit
measurements in the literature.
b).
FIGURE 7. Experiment conducted above the HEL of
sapphire. Indication of spatial inhomgenity is apparent.
REFERENCES:
1.
LVSAPH-3: ANALYSIS:
In this experiment, we deliberately imparted a stress
above the dynamic Hugoniot elastic limit. A
nominally 30mm diameter, 10mm thick impactor
impacts a 50mm diameter, 5 mm thick target. The
ORVIS line segment monitored the free surface of
the target. In addition, a single point VISAR (Figure
7b) monitored a point (overlapping a segment of the
ORVIS line) on the free surface. As seen in Figure
7a, the fringe record shows that there is an elastic
wave front arriving at the target free surface at 445
ns after impact. The elastic strength is sustained for
approximately 230 ns after which there is a decrease
in particle velocity indicative of a precursor decay
aassociated with the dynamic yielding process. It is
interesting that spatial inhomogenity is observed
during yielding. The edge release feature is also
observed, however due to the light intensity loss it
cannot be monitored once the sample has undergone
the yielding behavior.
2.
3.
4.
5.
6.
7.
RESULTS AND CONCLUSIONS:
In this paper, results of experiments conducted to
monitor multi-dimensional effects in single crystal
sapphire are reported. This is an ongoing study and
these are some of the first measurements of its kind
in single crystal sapphire. There is, however, a wide
variation in edge release wave velocities (~11-13
km/s) measurements. A similar experiment using
conventional velocity interferometry gives 12.9
8.
9.
794
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