Proceedings of the SEM Annual Conference on Experimental and Applied Mechanics
June 4-6, 2001, Portland, Oregon
Impact Damage Mechanisms in Fiber-Metal Laminates
B. M. Liaw, Y. X. Liu, and E. A. Villars
The City College of The City University of New York
Department of Mechanical Engineering
and
The CUNY Graduate School and University Center
Convent Avenue and 138th Street
New York, NY 10031
Abstract
configuration, thickness, and bending rigidity. For instance,
experimental studies consistently report that very often
delamination occurs at the interface between plies with
different fiber orientation. On the other hand considerably
fewer studies have been conducted on impact damages [4-5]
and other thermomechanical behaviors [6-7] of fiber-metal
laminates. The main objective of this study is to develop
preliminary understanding of damages caused by low-energy
impact in these relatively newer engineering composites. A
typical example of our results can be seen in Fig. 1, which
depicts drop-weight impact damages of panels made of (a)
0.063”-thick 2024-T3 aluminum alloy and (b) 0.056”-thick
Glare 2, a fiber-metal laminate formed by alternating 2024T3 aluminum sheets with unidirectional glass-epoxy laminae
(fiber volume fraction, Vf =50%). When subject to an impact
energy of 30 J at room temperature, the thicker and heavier
aluminum panel incurs severe damage, including partial
punching through; whereas the thinner and lighter Glare 2
panel suffers only a minor indentation with a crack running in
parallel with the fiber direction.
Impact-induced damage mechanisms in Glare and ARALL
fiber-metal laminates subject to instrumented drop-weight
impacts at various impact energies and temperatures were
studied. Damage in pure aluminum panels impacted by
foreign objects was mainly characterized by large plastic
deformation surrounding a deep penetration dent. On the
other hand, plastic deformation in fiber-metal laminates was
often not as severe although the penetration dent was still
produced. The more stiff fiber-reinforced epoxy layers
provided better bending rigidity; thus, enhancing impact
damage tolerance. Severe cracking, however, occurred due
to the use of these more brittle fiber-reinforced epoxy layers.
Fracture behaviors were greatly affected by the lay-up
configuration and the number of layers, which implies that
effects of both fiber orientation and thickness are significant
for the panels tested in this study. Temperature, on the other
hand, does not play a significant role in this study.
Introduction
Fiber-metal laminates are hybrid materials built up from
interlacing layers of thin metals and fiber-reinforced
adhesives. They were developed in late 1970’s [1-2] and
have gained wide attention in the aerospace and space
industries since late 1980. There are two types of
commercially available fiber-metal laminates: Glare and
ARALL. The former is made of alternating glass-epoxy and
aluminum layers whereas the latter is formed by alternating
aramid-epoxy and aluminum layers. Taking advantage of the
hybrid nature from their two key constituents: metals and
fiber-reinforced laminae, these composites offer several
advantages, such as better damage tolerance to fatigue
crack growth, foreign object damage, and corrosion, etc., for
engineering design.
(a) 2024-T3
(b) GLARE 2
Figure 1. Typical damages in 2024-T3 and Glare 2 panels
subject to a drop-weight impact of 30 J at room
temperature.
The Experimental Procedure
Damage mechanisms due to low energy impact onto
composite panels have been studied extensively (see the
review in [3]). In general they are characterized by
incomplete penetration with damage consisting of
delamination, matrix cracking, and fiber failure. However, the
exact failure mode and damage evolution sequence are
greatly affected by the constituent properties, lay-up
In this study tests were conducted in an Instron-Dynatup
8520 pneumatic-assisted, instrumented drop-weight impact
tester equipped with an environmental chamber (−60 to
350°F) and a pneumatic break to avoid multiple strikes.
Square specimens with dimensions of 100 mm x 100 mm (or
4” x 4”) were first clamped circumferentially along a diameter
of 76 mm (or 3”) in the specimen fixture. The fixture-
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For example, both Glare 1 and ARALL 3 are laminates
composed of alternating 0.012” 7475-T76 aluminum alloy
sheets with unidirectional glass-epoxy and aramid-epoxy
layers, respectively (both with Vf =50%). In these tests the
Glare 1 and ARALL 3 panels have a thickness of 0.056” and
0.053”, respectively. Figure 2 shows impact damage patterns
in panels made of these two composites subject to different
impact energies at room temperature. It is clear that Glare 1
possesses higher impact damage tolerance than ARALL 3.
specimen assembly was then placed in the environmental
chamber. Temperature in the chamber was raised or lowered
to the desired setting with the fixture-specimen assembly
being soaked at that temperature for about 20 minutes to
ensure no initial stress resulting from temperature change. A
16 mm- (or 5/8”-) diameter spherical impactor with a weight
of 6.1 kg (or 13.4 lbs) was then dropped from a
predetermined height to cause damage. Time histories of
impact load were recorded by a PC-controlled high-speed
A/D converter. After impact the specimen was then carefully
removed from the environmental chamber for post-mortem
inspection and photography.
Glare 1
Glare 1, 10 J
Glare 1, 20 J
Glare 1, 30 J
ARALL 3, 10 J
ARALL 3, 20 J
ARALL 3, 30 J
5000
ARALL 3
Load (N)
4000
3000
2000
1000
(a) 10 J
0
0
2
4
6
8
10
Time (ms)
(a) Glare 1 vs ARALL 3
Glare 1,
Glare 1,
Glare 1,
Glare 2,
Glare 2,
Glare 2,
Load (N)
6000
(b) 20 J
4000
10 J
20 J
30 J
10 J
20 J
30 J
2000
0
0
2
4
6
8
10
Time (ms)
(b) Glare 1 vs Glare 2
Figure 3. Comparison of load-time histories under impact
tests at room temperature.
(c) 30 J
Figure 2. Damage patterns in Glare 1 and ARALL 3 panels
subject to a drop-weight impact with different
impact energies at room temperature.
Results and Discussion
In order to study fully impact damages in fiber-metal
laminates, various factors in the selection of materials and
specimen configurations were taken into consideration.
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The corresponding load-times histories of these tests are
shown in Fig. 3(a). As shown in Fig. 2, only plastic
indentation but not fracture may have occurred only in the
test of Glare 1 under the 10 J impact condition. From
Fig. 3(a) one can see that other than some fluctuation in the
initial phase, the load vs. time curve corresponding to this
test is fairly smooth; indicating excessive kinetic energy was
mainly attenuated out through damping. On the other hand,
for the remaining tests, the load was drastically reduced;
indicating the onset of fracture. This phenomenon is similar
to impact damage in aluminum/acrylic sandwich plates [8].
temperature effect, if any, is not significant in impact
damages of fiber-metal laminates.
Conclusions
From this study one can conclude that
• Compared to aluminum alloys, impact damage tolerance is
better in Glare while may be worse in ARALL;
• Impact damage tolerance can be improved by using the
ductile but tougher aluminum alloy (such as 2024-T3);
The second set of tests involved the Glare panels with
different aluminum alloys (the stronger yet brittle 7475-T76
for Glare 1 vs the ductile but tougher 2024-T3 for Glare 2).
Results of their load-time histories are shown in Fig. 3(b),
which indicates that Glare offers better impact resistance
when it incorporates the ductile but tougher 2024-T3.
• Quasi-isotropic lay-up configuration provides the best
impact resistance whereas unidirectional laminate offers
the worst;
The third set of tests considered the effect of lay-up
configuration in the prepreg. The results show that in terms
of impact load and crack formation, the quasi-isotropic
configuration ([0°/45°/−45°/90°]) has the best tolerance for
impact
damage
with
the
cross-ply
configuration
([0°/90°/90°/0°]) being the second while the unidirectional
configuration ([0°]4) performing the worst.
• Unlike aluminum/acrylic sandwich, temperature does play
a significant role in impact resistance.
• The minimum impact energy required to cause cracking is
approximately proportional parabolically to thickness;
10000
Load (N)
The effect of number of layers was assessed in the fourth set
of tests in this study. Cross-ply Glare 5 panels with total
thickness varying from 0.044” to 0.172” were tested at room
temperature and the minimum impact energy required to
caused cracking on the bottom face (opposite to the impact
site) of the panel was measured. The result is shown in
Fig. 4, which indicates that this minimum cracking energy is
proportional parabolically to the thickness of the impacted
panel.
6000
4000
2000
0
0.172
Minimum Cracking Energy (J)
95 C
85 C
82 C
60 C
25 C
-20 C
-40 C
-51 C
8000
0
2
4
6
8
10
Time (ms)
80
Figure 5. Impact load-time histories of Glare 5 panels
subject to a 30 J impact energy at various
temperatures.
0.140
60
0.108
40
0.076
0.044
20
0
The Thickness of Panels (in)
Figure 4. Room-temperature minimum impact energy
required for cracking in Glare 5 panels of different
thickness.
(a) 88°C
(b) −51°C
Figure 6. Comparison of damages of Glare 5 panels subject
to 30 J impact at two different temperatures.
Finally the cross-ply Glare 5 panels were impacted at 30 J
under a temperature range within −51 to 95°C. As shown in
Fig. 5 no appreciable difference in the impact load-time
histories was observed. Figure 6 shows the almost identical
indentations in Glare 5 panels subject to 30 J impact at 88°C
(190°F) and −51°C (−60°F), respectively. It implies that
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Acknowledgments
This study was supported by NASA Faculty Award for
Research (FAR) under Grant No. NAG3-2259 and by
PSC-CUNY under Grants 61429-00 30 and 62466-00 31. Dr.
Kenneth J. Bowles and Dr. John P. Gyekenyesi are the
Technical Monitors of the NASA grant. Part of the equipment
used in this investigation was acquired through Army
Research Office Grant No. DAAD19-99-1-0366.
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