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
DYNAMIC TENSILE RESPONSE OF ALUMINA-AL COMPOSITES
R. Atisivan,1 A. Bandyopadhyay1 and Y. M. Gupta2
Institute for Shock Physics and School of Mechanical and Materials Engineering
Institute for Shock Physics and Department of Physics
Washington State University, Pullman, WA 99164
Abstract. Plate impact experiments were carried out to examine the high strain-rate tensile response of
alumina-aluminum (Al) composites with tailored microstructures. A novel processing technique was used to
fabricate interpenetrating phase alumina-aluminum composites with controlled microstructures. Fused
deposition modeling (FDM), a commercially available rapid prototyping technique, was used to produce the
controlled porosity mullite ceramic preforms. Alumina-Al composites were then processed via reactive metal
infiltration of porous mullite ceramics. With this approach, both the micro as well as the macro structures can
be designed via computer aided design (CAD) to tailor the properties of the composites. Two sets of dynamic
tensile experiments were performed. In the first, the metal content was varied between 23 and 39 wt. percent. In
the second, the microstructure was varied while holding the metal content nearly constant. Samples with higher
metal content, as expected, displayed better spall resistance. For a given metal content, samples with finer
metal diameter showed better spall resistance. Relationship of the microstructural parameters on the dynamic
tensile response of the structured composites is discussed here.
used to develop the method to create tension in the
specimens. The method to produce tension in the
sample subjected to uniaxial strain loading was similar
to that described earlier by Barbee et al. (5) In this
method, the test sample was surrounded by a tightly
fitted material of nearly the same shock impedance.
The function of the surrounding material was to
minimize the radial release waves from propagating
into the sample. This is termed as 'momentum
trapping'. Unlike quasi-static tests, surface effects
play a negligible role in these dynamic experiments,
since the tension is produced inside the sample.
Because microdamage in brittle materials take place at
very rapid rates, these experiments provide a means of
initiating microcracks and controlling the load
duration such that these cracks do not coalesce into
large-scale cracks and rupture the material. In this
research, the role of metal content and microstructure
on the dynamic tensile response of alumina-Al
composites were studied.
INTRODUCTION
Three dimensionally honeycomb porous mullite
ceramic pre-forms with interconnected pore structure
were fabricated via the indirect fused deposition
process. With this approach, it is possible to control
the size, distribution and connectivity of the pores in
the resulting porous preform. The porous ceramic
structures were then infiltrated with molten Al metal
by pressureless reactive metal infiltration to form
interpenetrating phase alumina-Al metal-ceramic
composites.
The details of processing of these
structured composites has been described elsewhere.
(1, 2, 3) This approach can be used to design a part
using CAD in which microstructural features as fine as
50 microns can be incorporated and fabricated
directly. This paper discusses the dynamic tensile
response of these composite materials.
In this work, the behavior of the composite material
under high strain rate loading was examined using plate
impact experiments. The work of Gupta et al. (4) was
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EXPERIMENTAL PROCEDURE
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Cylindrical alumina-Al composite specimens with 30
mm diameter and 25 mm long were fabricated to
examine the dynamic tensile response. All the samples
were processed under identical conditions. Three sets
of composite samples were fabricated for the dynamic
experiments with metal contents of: 26, 32 and 42
(vol%). These compositions were used to examine the
effect of metal content on the spall behavior of the
composites. Two more sets of samples with nearly
same metal concentration (~39 vol%), but having
different metal and ceramic widths were fabricated.
These samples were used to examine the effect of
microstructure on the spall behavior of the composites.
The samples with 32% metal content were chosen for
the velocity interferometry measurements under two
different projectile velocities, which provided the
sample response under uniaxial loading. The response
for the 32% metal content was used for estimating
stresses in all other samples.
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Time (jj.s)
Figure 1. The particle velocity profiles for the two
the VISAR shots.
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To characterize the sample response under uniaxial
strain, particle velocity measurements were carried out
with VISAR measurement using a laser interferometry
system. (6) These measurements were used to
estimate the initial compressive stress state in the
composite samples, when subjected to dynamic
loading. Measurements were made at two different
projectile velocities, 139 and 197 m/s. Particle velocity
histories for the two experiments are shown in Fig 1.
The horizontal lines in Fig 1 represent the average
particle velocities. The average particle velocity for
experiment 1 was calculated to be 66±2.6 m/s. The
average particle velocity for experiment 2 was 92±5.1
m/s. The average and standard deviation values were
calculated for the time interval between O.OSjis to
0.5ns.
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a = 149.9 u
0.02
0.04
0.06
0.08
0.10
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Particle Velodty (mm/ps)
Figure 2. The P-u diagram for the composite sample.
is assumed to be a homogeneous material, the shock
speed in the composite can be calculated from the
second Rankine-Hugoniot (R-H) jump condition and
is found to be 4.98 mm/jis. Using a linear elastic
deformation assumption, the shock impedance of the
composite was calculated as 149.9 kbar/(mm/|is). As
this value is fairly close to the impedance of Al 6061T6, it was believed that the aluminum holder
minimized the propagation of radial release waves into
the samples. The particle velocity measurements
obtained using samples with 32% metal were also
used for all other samples.
Fig 2 shows the P-u diagram for the composite that
was obtained from the two VISAR experiments. The
particle velocity used to construct this plot was
calculated by determining the change in the particle
velocity from the projectile velocity. This P-u plot
was used to estimate the initial compressive stressstate in the recovery experiments. The slope of the
P-u plot for the composite gives the shock impedance
of the composite. Since the composite
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Recovery Experiments: A schematic of the
experimental configuration is shown in Fig 3.
Recovery experiments were performed on polished
composite samples cut parallel to the build layers
direction. Each sample had the shape of a frustum with
25 mm as the smaller diameter and a taper of 8°. The
machined samples were 6 mm thick. Shock waves
were generated by impacting 3 mm thick Al-6061
impactors. The Al impactor was mounted onto a
projectile and accelerated to a velocity of ~0.140
mm/|is using a 100 mm compressed gas gun. The
projectile velocities were measured just prior to impact
using a method requiring piezo-electric pins. Samples
failed due to tensile stress that was generated in the
sample. Due to the taper, the sample leaves the Al
holder just after impact and is collected in a recovery
chamber at the end of the gas gun filled with soft rags.
Test sample
Influence of metal content: The metal content in the
samples was increased between 23 to 39 vol%. The
thickness of the metal strands was kept constant at 300
Jim. The ceramic thickness was varied from 330 Jim
to 825 |im to change the metal content in the
composite. The projectile velocity was nearly constant
(~ 140 m/s) for all the experiments. It was found that
except 39% metal content sample, all other samples
split into two pieces. Samples with lesser metal
contents split into two pieces along the diameter.
These results indicate that the tensile stresses were
generated inside the sample as expected. Moreover,
samples with higher metal content have better spall
resistance. This is likely due to the fact that the
toughness of the composite increases with increase in
metal content. The ceramic is inherently less resistant
to spall compared to the metal. As the relative amount
of metal in the composite decreased, the spall
resistance decreased as well. The experiment with
39% metal content was repeated five times to verify
that the results are reproducible. In two experiments,
the samples broke into two pieces but the rest of them
were intact in one piece.
Influence of metal width: In this set of experiments,
the width of the metal strands were varied from 300 to
400 |im, keeping the metal content nearly constant (39
vol%) in the composite. As the metal thickness was
increased, the thickness of the ceramic strand was
increased to keep the amount of metal content
constant, while the number of metal strands in a given
sample size decreased. It was found that except the
composite with 300 Jim metal width, all other samples
split into two parts. This result suggests that finer
microstructure provide higher toughness.
Aluminum 6061
Figure 3. The target assembly with alumina-Al
composite sample
RESULTS
Two sets of experiments were performed. The first set
of experiments examined the effect of metal content on
the tensile response of these composites. In the other
set of experiments, the metal width of the composites
was varied while keeping the metal content nearly
constant. The consequent effect of the microstructure
on the failure behavior was examined.
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Failure Analysis: The recovered samples with 39%
metal were mounted in an epoxy, and cut along the
diameter. The sample was polished, and examined
under an optical microscope. Figure 4 shows the
damage accumulation due to tension created in the
sample. The tensile crack can be observed to have
propagated through the interior of the sample. It can
also be seen that the crack had propagated
predominantly through the ceramic rich region in the
composite. Similar results are observed in other cases
as well.
Figure 5 shows a scanning electron
micrograph of a fracture surface with 32% metal
content that splitted in to two pieces. In general, the
failure analysis suggests that the failure was initiated
primarily in the ceramic phase and then propagated
through the ceramic and the metal phases.
CONCLUSIONS
An experimental method to subject the structured
composite samples under dynamic tension was
established. Using particle velocity measurements, the
shock impedance of the composite sample with 32
vol% metal was calculated to be 149.9 kbar/ (rnm/jis).
The shock velocity was inferred to be 4.9 rnm/jis. The
behavior of composite samples under dynamic tension
Figure 5. An SEM micrograph of a fracture surface for
32% metal content samples that split into two pieces.
ACKNOWLEDGEMENTS
Authors would like to acknowledge financial supports
from the US Department of Energy contract number
DEFG0397SF21388 and the Office of Naval
Research grant number N00014-98-01-0550. We
would also like to acknowledge experimental support
from Kurt Zimmerman and Jens Darsell of Institute for
Shock Physics.
Direction of impact
REFERENCES
1. Bandyopadhyay, A. "Functionally Designed 3-3
Mullite-Aluminum Composites," Advanced
Engineering Materials, 1 [3-4], pp. 199-201
2.
3.
Figure 4. (Top) Fractograph of Sample tested at 145
m/s. (Bottom) The enlarged optical micrograph of the
encircled portion in Figure 4a.
4.
was examined. Composite samples with higher metal
content showed a better spall resistance. For a given
metal content, samples with finer metal width showed
better spall resistance. Fractographic analysis showed
extensive cracking in the ceramic rich region and
relatively less damage was observed in the metallic
phase and at metal-ceramic interface.
5.
6.
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