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
THE HUGONIOT ELASTIC LIMIT OF ALON
James U. Cazamias1, Peter S. Fiske2, Stephan J. Bless3
1
LLNL, L-414, PO Box 808, Livermore, CA 94551
RAPT Industries, 581 Melanie Ave., Livermore CA 94550
3
IAT, 3925 W. Braker Ln., Suite 400, Austin, TX 78759-5316
2
Abstract. We performed plate impact experiments on the transparent polycrystalline ceramic aluminum
oxynitride (A1ON, transparent alumina). From VISAR measurements, the Hugoniot Elastic Limit,
GHEL, is 11.0-11.4 GPa with a corresponding yield strength of 1.2-1.5 GPa. A transverse gauge gives a
yield strength of 8.7 GPa for a longitudinal stress of 13.9 GPa, which implies that the failed A1ON
possesses at least its GHEL strength.
INTRODUCTION
the material [2]. The densities for the HEL
experiments in [2] were incorrect, and the corrected
results are presented here.
The modeling of transparent ceramics relies
heavily on descriptions of failure under compressive
loads which are generally derived from experience
with opaque ceramics. However, the microstructural
properties that are responsible for transparency also
imply that sites for stress concentrations are much
reduced when a medium is transparent. It is this
increased homogeneity of transparent materials that
is responsible for characteristic differences in shock
response of transparent and opaque materials. For
example, transparent materials such as glass and
sapphire exhibit much higher initial spall strengths
than polycrystalline ceramics.
Recently, there has been increasing interest in
aluminum oxynitride (A1ON, transparent alumina).
A1ON
has spinel crystal structure (i.e., cubic
symmetry) which results in isotropic optical and
mechanical properties. When the ceramic is 100%
dense with no voids, inclusion or grain boundary
phases, it is transparent.
We have recently performed bar impact
experiments which found that transparent A1ON
exhibited both alumina-like and glass-like behavior
with a bar yield strength of 4.0 GPa [1]. We have
also performed plate impact experiments which
examined the GHEL, spall, and elastic properties of
EXPERIMENTS
25.4-mm diameter disks of A1ON were subjected
to planar impacts using a He single-stage gas gun at
LLNL. See Table 1. The diagnostics for Shots 710
and 711 consisited of a VISAR with a fringe
constant of 541 m/s/fringe. For Shot 725 (a repeat
of 711), a transverse gauge (MicroMeasurements C880113-B pulsed by a Dynasen CK1-50-300
piezoresistive power supply) was used to measure
transverse stress in the sample. Impact velocities
were measured using 2 pairs of piezoelectric pins.
DATA
Figure 1 shows the velocity-time histories for
Shots 710 and 711. Table 2 lists derived values
from the experiments.
767
TABLE 1. Experimental Parameters
Impact Velocity (m/s)
Impactor
Impactor Thickness (mm)
Ptarget (gm/Cm3)
Target Thickness (mm)
ci (km/s)
cs (km/s)
c0 (km/s)
710
790
Cu
5.08
3.68
5.11
10.3
5.91
7.71
711
684
Cu
5.08
3.68
5.11
10.3
5.91
7.71
difference in rear surface arrival times of the initial
elastic wave and its first reflection from the inelastic
wave front, the inelastic wave velocity, Cj n , can be
expressed as
725
680
Cu
5.07
3.68
10
10.3
5.91
7.71
1—
c
Since the magnitude of the inelastic wave is
small compared to the GHEL, the inelastic wave front
becomes elastic after interacting with the initial
elastic
release
wave,
which
leaves a
contact discontinuity at the release point. There is a
step structure in the inelastic rise (Fig. 2) which is
associated with this contact discontinuity, although
some of the temporal thickness of the inelastic
signal is associated with the thickness of the
inelastic wave itself. The noise in the VISAR
signal might be due to the fact that the large grain
size of A1ON (> 100 [im [3]) allows twinning in
the grains. The subject of contact discontinuities
influencing free surface velocities is discussed in
more detail in [4], where it is shown that the late
time free surface velocity, Uf, can be expressed as
I
f—i——i——i—r
0,8
1,0
1-2
(1)
P
600-
0.6
Po
in ~ cl ——
800-
0.4
_
c/Af
14
Tint© fos)
Po
FIGURE 1. Velocity-time histories for Shot 710 and Shot 711.
~
HEL
(2)
TABLE 2. Experimental Measurement
UHEL (m/s)
QHEL (GPA)
Y (GPa)
cin (km/s)
uin (m/s)
ain (GPa)
710
290
11.0
7.2
8.51
428
15.4
where uin is the particle velocity behind the inelastic
wave. The stress behind the inelastic wave is then
Gin = PQCIUHEL + P^in (uin - UHEL),
giving peak
stresses of 15.4 GPa for Shot 710 and 13.9 GPa for
Shot 711.
Figure 3 shows the change in manganin gauge
resistance, AR/Ro, versus time for Shot 725. The
gauge was nominally placed in the center of the
target. The region of interest is the plateau which
represents the stress behind the inelastic wave; here
AR/Ro = 0.174, giving a transverse stress of a t =
5.2 ± 0.3 GPa (usings Rosenberg's [5-7] analysis)
with a corresponding yield strength of Y = GI - ot =
8.7 GPa ± 0.3 GPa. Increase of the flow stress
above the GHEL is not unexpected since alumina also
exhibits this behavior [8, 9].
711
301.5
11.4
7.5
8.45
378
13.9
Letting UHEL equal one-half the velocity of the
plateau after the elastic wave, we can calculate OHEL
= POCIUHEL (11.0 GPa for Shot 710 and 11.4 GPa for
Shot 711) with a corresponding yield strength of Y
- 2(cs/c,)2oHEL (7.2 GPa for Shot 710 and 7.5 GPa
for Shot 711).
For further analysis, we assume that the inelastic
wave is steady. With po/p =1- UHEL/CI (conservation
of mass), L as the plate thickness, and At as the
768
AlON's GHEL of 11.0 to 11.4 GPa compares well
with those of other brittle materials (6.7 GPa for
AD995 [11], 14-20 GPa for z-cut sapphire [12], 12
GPA for SiC [13], 4.2-5.8 GPa [1st GHEL] and 9- 17
GPa [2nd OREL] for TiB 2 [14], and 6 GPa for soda
lime glass [9]). The strength under shock loading
(7.2-7.5 GPa) is significantly higher than under bar
loading (4 GPa [1]), presumably due to the
suppression of microcracks by pressure.
Assuming cin = c0 + s u;n gives s on the order of
1.9-2.0. This agrees with sapphire (s = 1.95) [15],
but is greater than polycrystalline alumina (s = 1.3)
[8]. At low pressures, (dK/dP)T ~ 4.4 [16]. Since s
= 0.25((dK/dP)o,s + 1), approximating (dK/dP)0,s by
(dK/dP)T gives s = 1.35, implying that the
hydrostatic behavior of AION should resemble that
of polycrystalline alumina. Our measurements occur
near the GHEL and may be affected by changes in the
strength of the material [17]. Consequently, this
large value of s is another indication that the
strength is actually increasing.
8QQ45
650
eoo550-
0,3
I
04
I
I
I
0,5 0,6 01
Tima {ps)
I
08
FIGURE 2. Expanded view of velocity-time histories.
0 20 -i
CONCLUSION
015 -
We performed plate impact experiments on AION
using a V1SAR and transverse manganin gauges.
010 -
The Hugoniot Elastic Limit, GHEL, is 11.0-11.4
0.05-
GPa with a corresponding yield strength of 7.2-7.5
GPa. A transverse gauge gives a yield strength of
8.7 GPa for a longitudinal stress of 13.9 GPa,
which implies that the failed AION possesses at
least its GHEL strength.
000
ACKNOWLEDGMENTS
14
This work was performed under the auspices of
the DOE by LLNL under contract number W-7405ENG-48 and ARL by I AT under contract DAAA2193-C-0101. Thanks to T. Hartnett of Raytheon
Corporation for supplying samples. Thanks to C.
H. M. Simha for assembling the transverse gauge.
FIGURE 3. AR/Ro-time history for Shot 725.
DISCUSSION
We do not have a good explanation for the
dynamic overshoot of the elastic wave. It might be
a rate effect. Dynamic overshoot is observed in 840,
which also has extremely slow plastic waves. These
behaviors are attributed to a dramatic loss of
strength above the OHEL [10]. We have shown that
AION does not collapse to the hydrostat, so this is
not an explanation for the overshoot behavior.
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