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
BAR IMPACT TESTS ON ALUMINA (AD995)
James U. Cazamias1, William D. Reinhart2, Carl H. Konrad3, Lalit C. Chhabildas2,
Stephan J. Bless4
1
LLNL, L-414, PO Box 808, Livermore, CA 94551
SNL, MS 1181, PO Box 5800, Albuquerque, NM 87185-1181
3
Bechtel, 3900 Paradise Road, Suite 183, Las Vegas, NV 89101
4
IAT, 3925 W. Braker Ln., Suite 400, Austin, TX 78759-5316
2
Abstract. Dynamic strength may be inferred from bar impact tests, although interpretation of the data
is affected by the time-to-failure of the target bar. To clarify the mechanics, tests with graded density
impactors were conducted on bare and confined bars, 12 and 19 mm in diameter, cut from blocks of
AD995 alumina. Manganin gauge and VISAR diagnostics were employed. Larger rods displayed
higher strength. In some tests the "true" yield stress of -4.5 GPa was achieved.
INTRODUCTION
rium, and it may fail before equilibrium is reached,
and (3) the samples are small. In contrast, in the bar
impact experiment, the impactor does not need to
remain elastic, it is a single wave experiment, and
larger specimens can be used which eliminate small
scale effects and allow direct observation of the failure process.
The bar impact experiment is fairly well understood for ductile metals. A few diameters from the
impact face, a ID stress wave with a maximum
amplitude of YO = GHEL(l-2v)/(l-v) travels down the
bar.
In brittle materials, the picture is a bit more complicated. In the first couple of diameters, the transmitted wave is transformed from ID strain (impact
condition) to ID stress (steady-state solution).
Damage in this transition region can be compressive
and/or tensile [4-5]. The maximum stress that
propagates down the bar is determined by how
much of the wave can get out of the impact zone
before the strength of the damaged material limits
the amplitude of the stress. Therefore, the
compressive strength measured in a brittle bar
impact experiment can possess a dependence on
tensile strength and is seldom equal to the ideal
unconfmed compressive strength.
Polycrystalline alumina (AliOs) is the archetypical armor ceramic. It has been studied since the
1960's, and its material and ballistic properties
have been fairly well characterized experimentally. A
specific high purity alumina, Coors AD995, has
been characterized dynamically via a variety of
experimental methods - plate impact [1-2], bar
impact [3-5], spherical wave [6], and penetration [79].
In spite of these efforts, there has been remarkably
little success at predicting impact or penetration
experiments that involved large contrasts in loading
state or strain rate. In this situation, it is important
to focus on material property tests that mimic engineering stress states and rates. Static tests are dominated by the largest flaw in the material, and ID
shock tests are fully confined; however, ID stress
tests allow the measurement of a strength that is
most relevant to ballistic applications. There have
been several attempts to use the split Hopkinson bar
(a ID stress experiment) to characterize ceramics for
ballistic applications; however, there are several
complications in doing so: (1) ceramics typically
indent the striker bar, (2) the sample undergoes several reverberations while approaching stress equilib787
TABLE 1. Bar Dimensions
Sleeve
Alseg-1
Alseg-2
Alseg-3
Alseg-4
Foam
Steel
Foam
Steel
Sleeve
OD (mm)
38.100
38.100
38.100
38.100
Bar OD Long Length Short Length
(mm)
(mm)
(mm)
12..713
76.835
25.959
12..713
76.810
25.959
19..164
112.751
25.476
19,.164
114.427
26.391
Overall
Length (mm)
102 .949
102 .705
138 .275
140 .670
TABLE 2. Projectile Properties
Alseg-1
Alseg-2
Alseg-3
Alseg-4
Vel.
Dnominal
(m/s)
(mm)
TTPX
(mm)
843
851
859
846
50.8
50.8
76.2
76.2
0.54
0.55
0.99
0.98
pTPX
^A1
(gm/cm3)
0.816
0.822
0.829
0.831
(mm>
0.48
0.49
0.95
0.95
In the present work, three sorts of bar impact
experiments have been performed: (1) normal bar
impacts; (2) confined bar impacts, where the rod is
completely sleeved in a material to provide a
confinement pressure and the measured longitudinal
stress is no longer limited to the ID stress value
and may approach the ID strain value (OHEL); and
(3) graded density impactors, where the impactor is
composed of multiple thin layers to spread out the
loading wave and hopefully suppress tensile
damage.
The quasi-static compressive strength of AD995
is 2 GPa [10]. The compressive strength calculated
from the onset of HEL-like softening in plate
impact experiments is 4.3 GPa [1]. It should be
noted that there is some controversy over the
interpretation of HEL experiments for alumina.
While we believe Y to be 4.3 GPa, some
interpretations give Y values of over 8 GPa [11].
Wise and Grady [3] impacted unconfined and Taconflned alumina (10.06 mm OD, 80 mm L) with
6061-T6 Al. For the unconfined bars, they
measured a peak stress of 3.15 GPa at 1.035 km/s
and 2.182 km/s. For the confined bars, they
measured peak stresses of 6.32 GPa (2.14 km/s) and
5.80 GPa (1.051 km/s).
Simha [4] impacted unconfined alumina (12.5
mm OD). With 4340 steel impactors he found that
the peak stress was 3.6 GPa in the 100-300 m/s
impact regime. When he used alumina impactors,
he measured lower peak stresses of 3.4 GPa (175
PAI
(gm/cm5)
2.673
2.658
2.705
2.701
TTI
(mm)
0.35
0.35
0.85
0.84
pTi
(gm/cm3)
4 .384
4 .388
4 .529
4 .516
Tst
(mm)
19.08
19.06
19.19
19.19
Pst
(gm/cm )
7.836
7.837
7.837
7.834
m/s) and 2.8 GPa (278 m/s) which he attributed to
the fact that the impactor also fails.
Chhabildas, et al. [5] impacted confined and
unconfined alumina (19 mm OD) with normal and
graded density impactors. For unconfined alumina
with a steel impactor at 318 m/s, they observed a
two wave structure; the first wave loaded the bar to
2.1 GPa and the second to 3.4 GPa. For unconfined
alumina with a graded density impactor at 318 m/s,
the loading ramped to a peak stress of 3.5 GPa (300
m/s) for one shot and a peak stress of 4.2 GPa,
which relaxed to 3.6 GPa for a repeat shot. For
confined alumina with a graded density impactor, in
a 74 mm long bar (321 m/s) they observed a peak
stress of 5.1 GPa, and in a 151 mm long bar (321
m/s and 322 m/s) they observed a peak stress of 4.6
GPa.
EXPERIMENTS
A set of four experiments using an 89 mm
smooth bore single stage propellant gun were
performed on bare and fully sleeved rods at a
nominal impact velocity of 850 m/s with graded
density impactors at the Sandia STAR facility. The
Coors AD995 bars were nominally 12.5 mm and 19
mm in diameter (see Table 1). The 12.5 mm rods
were cut from 4 inch thick tiles. The 19 mm rods
were cut from 1 inch thick tiles. The bars have a
788
nominal density of 3.89 gm/cc, a nominal bar wave
speed of 9.79 km/s, and a nominal bulk wave speed
of 7.72 km/s.
The bars were instrumented with manganin
gauges nominally six diameters from the impact fece
with a nominally two diameter long backing piece.
The backing piece had W vapor deposited as a
mirror surface for a VISAR. There was no window.
The sleeves were made of 4340 steel. The unsleeved
measurements had a foam sleeve for mounting
purposes. The overall lengths differ from the sum of
the long and short lengths due to gauge insertion
and grinding to assure parallel faces. The graded
density impactors consisted of a TPX (plastic) layer
followed successively by Al and Ti layers, backed
with a 4340 steel plate (see Table 2). Velocity was
measured with self shorting pins.
603
5-
CL
O
4-
CO
CO
3-
CO
2-
Alseg 2
10-
I
8
I
\
10
12
Time (|is)
\
16
14
FIGURE 3. Gauge traces for sleeved shots.
CO
0.25-
"o
_0
0.20-
>
0.15-
0
0
•§
0.10-
CO
0.05-
13
Alseg 4
(scaled)
0.30-1
E
*
Alseg 4
0
0
0.00-
10
6
8
10
12
14
Time ((is)
I
12
[
I
14
16
Time (jis)
I
18
FIGURE 4. VISAR traces for sleeved shots.
FIGURE 1. Gauge traces for unsleeved shots.
The manganin gauge stress traces for
unsleeved/sleeved shots are presented in Figs. 1 and
3. The VISAR free surface velocities for
unsleeved/sleeved shots are presented in Figs. 2 and
4. The scaled traces use a temporal scaling
determined by the diameter (d) ratio: tscaled =
Alseg 3
(scaled)
tlargedsmall/dlarge.
u_
.
"i——i——r
r
12
18
14
16
Time (jis)
The stress corresponding to the VISAR signal is
nominally amax = 0.5 pcbarUmax giving a max = 4.6
GPa for Alseg 3 and omax = 4.2 GPa for Alseg 1.
The values are higher than the manganin gauges'.
FIGURE 2. VISAR traces for unsleeved shots.
789
DISCUSSION
REFERENCES
1.
Grady, D. E., and Moody, R. L., Shock
Compression Profiles in Ceramics, SAND960551 (1996).
2. Dandekar, D. P., and Bartkowski, P., "Shock
Response of AD995 Alumina," in High-Pressure
Science and Technology - 1993, 1994.
3. Wise, J. L., and Grady, D. E., "Dynamic,
Multiaxial Impact Response of Confined and
Unconfined Ceramic Rods," in High-Pressure
Science and Technology - 1993, 1994.
4. Simha, C. H. M., High Rate Loading of a High
Purity Ceramic - ID Stress Experiments and
Constitutive Modeling, Ph.D. Thesis, U.T. Austin,
1998.
5. Chhabildas, L. C., Furnish, M. D., and Grady, D.
E., J. de Physique. IV., Colloque C3, Suppl JP11I 1,
137-143 (1997).
6. Klopp, R. W., Shockey, D. A., Seaman, L., Curran,
D. R, McGinn, J. T., and de Resseguier, T., " A
Spherical
Cavity
Expansion
Experiment
Characterizing the Penetration Resistance of
Armor Ceramics," in ASME Winter Annual
Meeting Symposium on the Mechanical Testing
of Ceramics and Ceramic Composites, 1994.
7. Subramanian, R., and Bless, S.J., Int. J. Imp. Engng.
17, 807-816 (1995).
8. Anderson, C. E., and Royal-Timmons, S. A., Int. J.
Imp. Engng. 19(8), 703-713 (1997).
9. Skaggs, R., "Review of PHERMEX Confined
Ceramic Armor Tests," in Proc. of the 13th
Ceramics Modeling Working Group Meeting,
1997, pp. 3-51.
10. Coors, Ceramic Armor Products Catalog, 1995.
11. Grady, D. E., "Dynamic Failure of Brittle Solids,"
Fracture and Damage in Quasibrittle Structures
(1994).
12. Bless, S. J., Subramanian, R, Anderson, C. E., and
Littlefield, D., "Prediction of Large Scale High
Velocity Penetration Experiments on Ceramic
Armor," in Proc. 13th Army Symposium on Solid
Mechanics, 1993.
For both confined and unconfmed rods, the larger
diameter rods exhibit a greater strength than the
smaller diameter rods. While this might be a rate
effect (actually, a backward rate effect), the work of
[4] and [5] at 100 - 350 m/s (discussed above) does
not support this explanation since those
experiments were carried out at even lower strain
rates. We think that the most probable cause of the
strength variation is traceable to differences in the
stock material. The large scale rods came from one
inch plate, while the small scale rods came from
four inch plate. The greater performance of small
scale ceramic armors when compared to large scale
ceramic armors [12] is consistent with our strength
measurements. Thus, the difference in strength
might be a true scale effect rather than a rate effect.
Importantly, the bar impact test appears to
differentiate between the two scales. Material
variability might also be attributed to the fact that
the small scale rods were cut perpendicular to the
face of the plate, while the large scale rods were cut
parallel.
For the unconfined tests, the shapes of the rise of
the stress wave are fairly consistent, with peak
stresses of 4.5+-0.1 GPa for the large scale and
3.8+-0.3 GPa for the small scale. The VISAR trace
for Alseg 3 exhibits a plateau before decaying. The
plateau is probably not observed in the gauge trace
due to the greater temporal resolution of the VISAR
signal. Taking into account the lower estimate for
the HEL-derived yield stress of alumina of 4.3 GPa,
this is an indication that "true" yield surface has
been reached in the large bar.
For the confined tests, the small scale sample
exhibits a precursor wave that is not observed at
large scale.
ACKNOWLEDGMENTS
This work was performed at SNL and IAT and
supported by ARL under contract DAAA21-93-C0101 and the DOE. Thanks to C.H.M. Simha.
790