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
DEFORMATION AND DAMAGE OF TWO ALUMINUM ALLOYS
FROM BALLISTIC IMPACT
Charles E. Anderson, Jr. and Kathryn A. Dannemann
Southwest Research Institute, Engineering Dynamics Department
P.O. Drawer 28510, San Antonio, TX 78228-0510
A series of impact experiments were conducted on 4.76-mm-thick aluminum plates to investigate the
deformation and damage behavior of two aluminum alloys, 6061-T6 and 7075-T6. The Sierra 165 leadfilled bullet was used to load the plates. Impact velocities were varied from approximately 260 m/s to
370 m/s. The flow stress for 7075-T6 aluminum is approximately twice that for 6061-T6 aluminum;
however, the ballistic limit velocities differ by only 10%. The 7075-T6 aluminum plates exhibit less
deformation than the 6061-T6 plates at the same impact velocity, but at some critical velocity, a
through-thickness crack appears in the 7075-T6 plate, ultimately leading to plate perforation. In
contrast, the 6061-T6 plates continue to deform and fail by ductile tearing. These differences in
damage/failure result in the two alloys having much closer ballistic limit velocities than expected based
on differences in strength.
INTRODUCTION
EXPERIMENTS
The aluminum alloy 7075-T6 is approximately
85% stronger than aluminum 6061-T6. The initial
yield stress is 505 MPa versus 275 MPa for the Al7075-T6 and A1-6061-T6 alloys, respectively.
Ultimate tensile stress values are 570 MPa and
310 MPa, respectively. However, the two alloy
plates have almost the same ballistic limit velocity.
(For the purposes of this paper, no distinction is
made between the ballistic limit velocity, and the
velocity at which 50% of the projectiles will
perforate the target, i.e., V50). We determined, for
4.76-mm-thick plates, that the ballistic limit
velocity for the A1-7075-T6 is 366 m/s; that for the
A1-6061-T6 alloy is 330 m/s. An experimental
study was initiated to investigate the response of
these two alloys to ballistic impact, using a range of
impact velocities up to approximately the ballistic
limit velocity. The objective was to understand the
differences in response of these two alloys, and
obtain high-fidelity experimental data that might be
used to assess the accuracy of numerical
simulations.
The projectile used for the experimental study
was the Sierra 165 ball round. This lead-filled,
0.30-cal bullet is 29.65-mm long and has a total
mass of 10.7 grams (165 grains). A thin copperalloy jacket surrounds the bullet, except for the nose
area (spire point).
Approximately 10 targets each were fabricated
from 6061-T6 and 7075-T6 aluminum plate. The
plates were 20 cm x 20 cm, and were 4.76-mm
thick. The plates were held in a target frame and
clamped at the edges. A grid pattern was photoetched on the backside (opposite the impact side) of
each plate to facilitate deformation observations
measurements. The impact velocity was varied
between approximately 270 m/s and 370 m/s.
RESULTS: General Observations
The bullet, because it is quite soft, deforms
considerably, flattening into a pancake-like shape.
A photograph of the copper jacket and lead filler
1298
The remaining thickness of the plates at the
impact location (there is thinning of the plate
beneath the impact point) was also measured using
a dial indicator gage. Each plate was situated on a
reference surface with the bulge downwards (i.e.,
adjacent to the flat surface; the opposite orientation
from the deformation measurements). A dial
indicator reading was obtained for the reference
surface.
Another reading was taken on the
impacted surface. The remaining thickness was
determined by taking the difference between the
reference and impacted surface readings.
The plate deformations for the 10 experiments
conducted on the A1-6061-T6 plates are plotted in
Fig. 2. The extent of deformation increases with
impact velocity. At an impact velocity of approximately 317 m/s, a through-thickness crack appears
in the A1-6061-T6 plate. The horizontal dashed line
in the figure denotes the onset of cracking. Figure 3
shows the deformation and through-thickness crack
for the 321-m/s experiment.
12.0
FIGURE 1. Post-test photograph of (a) copper-alloy jacket;
(b) lead filler.
335 m/s
327 m/s
323 m/s
321 m/s
-»- 318 m/s
-D-- 315m/s
-+- - 305 m/s
302 m/s
286 m/s
270 m/s
are shown in Figs. l(a) and l(b), respectively.
The deformation of the aluminum plates
increases as the impact velocity increases. In
general, the degree of deformation, at the same
impact velocity, is greater for the 6061-T6 plates
compared to the 7075-T6 plates. At some critical
velocity, a through-thickness crack develops. The
impact velocity at which the through-thickness
crack develops is approximately the same for the
two alloys (~321 m/s). However, the maximum
deflection of the A1-6061-T6 is approximately three
times that of the A1-7075-T6 plate when the
through-thickness crack develops.
0.0
-60
-40
-20
0
20
40
60
Distance from Impact (mm)
FIGURE 2. Deformation contours for A1-6061-T6 plates.
RESULTS: Quantitative Analysis
Similarly, the results of 8 experiments conducted on A1-7075-T6 plates are plotted in Fig. 4.
The deformations are considerably less at the lower
impact velocities.
A through-thickness crack
appears in the A1-7075-T6 plates at an impact
velocity of approximately 326 m/s, after which the
deformation increases rapidly. Figure 5 shows the
deformation and through-thickness crack for the
340-m/s experiment.
The extent of deformation for each test plate
was measured with a dial indicator gage. The
plates were situated on a flat surface with the bulge
extending upwards. Dial indicator readings were
recorded every 12.7 mm over a distance of 50.8 mm
on either side of the impact location. The
deformation measurements were plotted versus
location for each impact velocity.
1299
FIGURE 5. Post-test photograph of the backside of an Al-7075T6 plate; the impact velocity was 340 m/s.
FIGURE 3. Post-test photograph of the backside of an Al-6061T6 plate; the impact velocity was 321 m/s.
12.0
-60
-40
-20
0
20
40
60
Distance from Impact (mm)
FIGURE 4. Deformation contours for A1-7075-T6 plates.
FIGURE 6. Post-test photograph the backside of an Al-6061T6 plate; the impact velocity was 335 m/s.
The nature of the deformation, after throughthickness cracking, is different for the two alloys.
The differences in deformation magnitudes in Figs.
2 and 4 are approximately 5 mm, except for the
plates with excessive cracking. The deformation of
the A1-6061-T6 plate continues in a ductile
(bulging) mode as the impact velocity increases;
e.g., see Fig. 6. However, for the A1-7075-T6 alloy,
the stress state is evidently more asymmetric after
through-thickness cracking. The stronger 7075-T6
alloy resists bulging deformation initially. But once
the through-thickness crack appears, the crack(s) is
more easily propagated in the 7075-T6 alloy than in
the 6061-T6 alloy. The local deformations around
the cracks become quite large as the plate "tears,"
e.g., contrast Figs. 5 and 7 with Figs. 3 and 6.
The maximum deformations as a function of
impact velocity are plotted for the A1-6061-T6 and
A1-7075-T6 plates in Fig. 8. The onset of cracking
is denoted in the figure, as well as the V50 velocity.
Note that the maximum deformation (as well as the
deformation contours, Figs. 2 and 4) has a more
gradual increase for the A1-6061-T6 plates than for
A1-7075-T6 plates over the velocity range
investigated. However, for both alloys, a change in
slope is readily apparent at the impact velocity
1300
where the through-thickness crack appears. The
change in slope is larger for the 7075-T6 plates.
There is another change in slope at approximately
the V50 velocity for both alloys, although this
change is more dramatic for A1-7075-T6.
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260 280 300 320 340 360 380 400
Velocity (m/s)
FIGURE 9. Remaining thickness vs. impact velocity.
sists penetration, and instead, perforation results
from catastrophic cracking/tearing.
FIGURE 7. Post-test photograph of the backside of an Al-7075T6 plate; the impact velocity was 360 m/s.
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The deformation behavior resulting from
ballistic impact is compared for two aluminum
alloys. One alloy (A1-7075-T6) is approximately
85% stronger than the other alloy (A1-6061-T6).
The deformation of the plates is consistent with the
differences in strength, but the development of
through-thickness cracks, at nearly the same impact
velocity, and the difference in failure processes,
result in relatively similar ballistic limit velocities,
366 m/s (7075-T6) versus 330 m/s (6061-T6).
The stress-strain response as a function of strain
rate has been measured for the two alloys, and
constitutive constants determined [1]. Numerical
simulations have been conducted, and the
simulation results compared with the experimental
datainRef. [1].
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CONCLUSIONS
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260 280 300 320 340 360 380 400
Velocity (m/s)
FIGURE 8. Maximum deformation vs. impact velocity.
The plate thickness decreases with penetration.
The remaining thickness of the plates is plotted
versus impact velocity in Fig. 9; the initial thickness
of the plates was 4.76 mm. The data indicate that
the A1-6061-T6 plate is penetrated relatively continuously (it is noted, though, that there are slope
"discontinuities" at the same velocities as in Fig. 8)
until perforation. In contrast, the 7075-T6 alloy re-
REFERENCES
1. Dannemann, K. A., Anderson, Jr., C. E., and
Johnson, G. R., "Modeling the Ballistic Impact
Performance of Two Aluminum Alloys," TMS
Fall Meeting, Indianapolis, Indiana, 4-8 Nov.
2001.
1301