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
SPALL STRENGTH OF CERAMIC IN A MULTILAYER SYSTEM
B.A.M. Vaughan, N.H. Murray7, W.G. Proud and J.E. Field
PCS, Cavendish Laboratory, Madingley Road, Cambridge, CBS ONE. UK.
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Now at Corus Group PLC. E-mail: nataliehmurray@hotmailcom
Abstract. Investigations into the dynamic properties of alumina ceramic have been carried out for several
years at the Cavendish Laboratory [1,2,3]- Previous work has demonstrated a reduction in spall strength
with an increased width of the compression pulse using either thicker fliers, or a flier with longer double
transit time for the shock wave. Here the variation in spall strength of an alumina ceramic as part of a
multilayer system is investigated. Results indicate that the spall strength decreases with increasing time for
which the target is under compression. There is some indication that spall strength may decrease faster
under shock ring-up than a single shock taking the sample to the same ultimate pressure.
interaction of rarefaction fans. These release fans
are dispersive, which gives rise to a geometry
dependence.
INTRODUCTION
Ceramics have been used in armour applications
for several decades, first seeing use in the 1960's to
protect American aircrews from small arms fire
[4,5]. Their success is due to their inherent high
compressive strength and low density, and are used
whenever weight is a constraint [6]. Modern armour
configurations are typically multi-layered structures
consisting of ceramic and other materials such as
metals and composites.
Plate impact experiments have been performed
on several types of alumina ceramic [2] to
determine Hugoniots and other properties, including
spall strength [1] and lateral stress. In the case of
lateral stress, studies of ceramics with and without a
cover plate of metal or ceramic between the flier
and the target were performed [3]. The results show
the importance of geometry on measured stress in
the target, and prompted the current study, which is
to investigate the spall strength of 880 alumina
ceramic in several multilayer systems.
The spall strength of a material can be
determined experimentally by impact of plates of
material, which are carried into tension through the
^reload
C/5
Time
FIGURE 1. Schematic representation of a gauge trace showing a
target undergoing spall.
The spall strength can be shown [7] to be given
by
' spall
2ZU
(1)
where ZT and Zw refer to the impedance of the
target material and to the PMMA window
747
independent measurement of the spall process.
Three different target configurations were
investigated and compared with previous studies
[1]. The impact velocity in each case was selected
to give rise to a ca. 4 GPa peak stress in the target.
Table 1 shows the target configurations
investigated.
respectively, areload and amin are the stresses
recorded by the gauge in the PMMA indicated in
Fig. 1. For the target ceramic, the impedance is
taken to be the elastic impedance, Z880 = p0cL,
whereas for the PMMA window, the impedance at
each stress state is determined from 7W = alIu p ,
using data taken from Marsh [8]. Figure 1 shows a
schematic of a stress trace. The solution given in
Eqn. (1) assumes elastic behaviour and is more
appropriate
in
the
case
when
TABLE 1. Target configuration
Configuration
Flier
Cover
(mm)
plate
(mm)
1
1.8 Cu
1.2 Al
2
1.8 Cu
1.2 Al
3
1.8 Cu
2.4 Al
4
3.0 Al
None
5
3.0 880
None
EXPERIMENTAL
Experiments were carried out using the single
stage 50-mm gas gun facility at the University of
Cambridge [9]. Longitudinal stresses were
measured by means of commercial manganin
gauges (Micro Measurements type LM-SS-210FD050). These were placed in the 'back-surface'
configuration as shown in Fig. 2. The calibration
data of Rosenberg et al. [10] were used to convert
the voltage data into stress. In addition, the pressure
dependence of the impedance of the PMMA
backing has been taken into account when
calculating the stress in the target ceramic. It should
be noted that if the impedance matched stress
profile is used to measure oreioad and &min9 then Eqn.
(1) reduces to
Target
thickness
(mm)
4.0
11.0
4.0
12.0
6.0
RESULTS
With the generic experimental arrangement
shown in Fig. 2 the traces shown in Fig. 3 were
obtained for the configurations 1-3.
The back surface stress is converted to stress in
880 by an impedance matching technique that
accounts for the change in impedance of PMMA
with increasing stress.
spall ~ °' load ~~ °"
In some experiments, VISAR was used to record
the PMMA-ceramic interface velocity, giving an
Cu Al
</>
o
3.
Gauge
location
1 v•4——
Vpr
4——————
t
PMMA
Backing
CD
Q_
0.2
-VISAR
1
880
Ceramic
59.75
FIGURE 2. Experimental arrangement for multiple step
shock. The fliers were 1.8 mm OFHC copper, the target was
880 alumina ceramic and the buffer material was Dural
(dimensions given in Table 1). The backing was 12 mm thick
PMMA.
60.00
60.25
Time (us)
FIGURE 3. Traces obtained for the calculated stress in 880
alumina ceramic. The corresponding VISAR traces are
shown alongside.
748
wave speeds in the different targets. The time for
which the sample is under compression until the
release waves from the rear surfaces of the flier and
target interact is denoted tspau. Table 2 contains the
values of tspan for each of the arrangements and Fig.
5 shows a fit for this data.
TABLE 2. Results
Configuration tspall (US)
1
2
3
4
5
0.600 ± 0.2
0.604 ± 0.3
0.573± 0.2
0.942 ± 0.2
0.659 ± 0.2
Spall Strength
(GPa)
0.214 ±0.08
0.127 ±0.156
0.535 ±0.07
0.356 ± 0.08
0.462 ± 0.09
It can be seen from Fig. 4 (configurations 1 and
2) that the thin aluminium buffer allows two
compression pulses to reach the gauge plane before
the release from the rear of the target arrives.
Interactions deeper in the target and flier do not
register on the gauge after the spall signal, and may
be ignored. The situation at the gauge plane is
similar to that of a single shock, as in [7] and cases
4,5. For the thick aluminium buffer (configuration
3), the spall signal reaches the PMMA-ceramic
interface at approximately the same time as the
second compression pulse, and may be thought of
as a single shock of reduced strength.
Those cases, 3-5, being single or approximately
single shocks, have a higher spall strength than the
two-step compression pulses with similar tspati It is
unclear at this stage whether this is a physical
consequence of the loading process, or the result of
experimental scatter in the data. A two step loading
pulse may weaken the material more effectively
before the arrival of the release fan, which may
explain these findings.
5)
880
CONCLUSION
Spall strength in 880 alumina ceramic is
dependent on the geometry of the system under
investigation. We have found a correlation between
the time the material is under compression until the
release waves interact to form a spall plane, and the
measured spall strength. This is consistent with
previous work [1], which attributed the effect to the
accumulation of damage in the form of
microcracks. The longer the compressive stage be-
x(mm)
FIGURE 4. t-x diagrams for each of the configurations 1-5.
The t-x diagrams corresponding to each of the
configurations in Table 1 are shown schematically
in Fig. 4. The release processes will manifest as
release fans and will include waves both faster and
slower than the initial shock wave. The lines shown
are approximate, but show the relative positions of
749
1999, edited by Furnish, M.D., Chhabildas,
L.C., and Hixson, R.S., American Institute of
Physics, 2000 pp. 581-584.
4. Hannon, F. S. and Abbott, K. H., Materials
Engineering, 68, (Sept. 1968) pp. 42-43
5. Rolston, R.F., Bodine, E., Dunleavy, J.,
Space/Astronautics, (July 1968), pp. 55-63
6. den Reijer, P.C., 4On the Penetration of Rods
into Ceramic Faced Armours', Proc. 12th Int.
Symp. Ballistics, 1, (1990) pp. 389-400
7. Grady, D.E. and Kipp, M.E., "Dynamic
Fracture and Fragmentation" in High-Pressure
Shock Compression of Solids /, edited by Asay,
J.R. and Shahinpoor, M, Springer-Verlag, New
York, 1993, pp. 265-322.
8. Marsh, S.P. "LASL Shock Hugoniot Data",
University of California Press. 1980, pp. 446451
9. Bourne, N.K., Rosenberg, Z., Johnson, D.J.,
Field, J.E., Timbs, A.E. and Flaxman, R.P.,
Meas. Sci. Technol. 6, 1462-1470 (1995).
10. Rosenberg, Z., Yaviz, D. and Partom, Y. J.
CO
±L
Spall Strength, aspa|| (GPa)
FIGURE 5. Measured spall strength using Eqn. (2), plotted
against the time until release waves interact for each geometry.
1, 2 are 'double shock' results.
fore spallation occurs, the more the microcracks can
grow, generating larger flaws so that the tensile
strength of the material is degraded. Additionally,
there may be an effect due to the loading history, so
that for a given compression time, tspau9 a single
shock produced a higher spall strength than a steploading pulse caused by ringing up of stress in an
intervening low impedance buffer. Detailed
modelling of these situations using an appropriately
sophisticated code and further experiments will be
required to fully understand the data presented here.
ACKNOWLEDGEMENTS
The research was supported by DERA
(Chertsey). We thank Drs I.M. Pickup and BJ.
James for providing the materials and their
continued interest in this work. D. L. A. Cross
provided technical assistance.
REFERENCES
1.
2.
3.
Murray, N.H., Bourne, N.K., Rosenberg, Z. and
Field, J.E., J. Appl. Phys. 84, No. 2, pp. 734738(1998).
Murray, N.H., "The Response of Alumina
Ceramics to Plate Impact Loading", PhD
Thesis, University of Cambridge (1997).
Murray, N.H., Millett, J.C.F., Proud, W.G. and
Rosenberg, Z., "Issues Surrounding Lateral
Stress Measurements in Alumina Ceramics", in
Shock Compression of Condensed Matter 750