Ultrasonic Assessment of Impact Damage in Aluminum/Acrylic ...

Proceedings of the SEM Annual Conference on Experimental and Applied Mechanics
June 4-6, 2001, Portland, Oregon
Ultrasonic Assessment of Impact Damage
in Aluminum/Acrylic Sandwich Plates
B. M. Liaw, A. M. Pierides, and Y. X. Liu
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
Ultrasonic studies were conducted to assess damages
generated by instrumented drop-weight impacting onto 6061-
T6 aluminum/cast acrylic sandwich plates adhered by epoxy.
Depending on several parameters, such as impact velocity,
mass, temperature, sandwich construction, etc., various
types of impact damage were observed, including plastic
indentation, radiating cracks emanating from the impact site,
partial and full debonding, and combinations of these
damages. One major goal of this study is to use immersion
ultrasound techniques in both pulse-echo and throughtransmission
modes to detect these failure mechanisms,
which were clearly visible since cast acrylic is transparent.
The developed NDT techniques can then be used to study
damage behaviors in opaque composites, such as the fibermetal
laminates.
Introduction
As part of the continuing effort of a paper presented in the
SEM Annual Conference last year [1], the main objectives of
this study are (i) to establish research capability in ultrasound
techniques at the authors’ institute using sandwich plates
made of two common commercial materials: 6061-T6
aluminum and cast acrylic, and (ii) to develop preliminary
understanding of delamination and fracture caused by impact
in other advanced sandwich panels (e.g., fiber-metal
laminates [2-5]).
Specimen Preparation, Impact & Ultrasound
The materials for making sandwich plates in this study are:
(i) 6061-T6, a ductile aluminum-magnesium-silicon alloy with
artificial tempering for precipitation heat treatment (97.9 wt%
Al, 1.0 Mg, 0.6 Si, 0.28 Mn, 0.2 Cr) and (ii) a brittle cast
acrylic (PMMA). Typical material properties of these two
materials useful for this study are listed in Table 1. Square
panels with dimensions of 100 mm x 100 mm (or 4” x 4”)
were then cut off cast acrylic with a thickness of 6.3 mm (or
0.25”) and 6061-T6 aluminum alloy with a thickness of
1.6 mm (or 1/16”). This study tested either pure acrylic
panels or sandwich plates made of acrylic and aluminum
panels adhered by a two-component epoxy.
Table 1. Material Properties
Property 6061-T6 acrylic
Density, ρ
g/cm 3 (ib/in 3 )
2.70
(0.0975)
1.19
(0.0430)
Tensile Modulus, E
GPa (Msi)
69
(10)
3.0
(0.44)
Poisson Ratio, ν 0.33 0.36
Longitudinal Wave Speed, c 1
km/s (×10 3 in/s)
6.15
242
2.06
(81)
Shear Wave Speed, c 2
km/s (×10 3 in/s)
3.10
(122)
0.96
(38)
⎧
⎪c1
=
⎪
Note: ⎨
⎪
c =
⎩
⎪ 2
21+
( 1−ν
)
( 1+ )( 1−2
)
E
ρ ν ν
E
ν ρ
( )
The specimens were then clamped circumferentially along a
diameter of 76 mm (or 3”) and impacted in an Instron-
Dynatup 8250 drop-weight impact tester by a 16 mm- (or
5/8”-) diameter spherical impactor with a weight of 6.1 kg (or
13.4 lbs).
The damaged specimens were first inspected visually and
then C-scanned in both pulse-echo and through-transmission
modes using a Physical Acoustics Corporation UltraPAC
immersion ultrasonic imaging system. Focused transducers
(0.25”-dia., 1” focus length) of various frequencies (5 to 25
MHz) were used for the pulse-echo operation whereas a pair
of 0.25”-dia., 25 MHz flat transducer were used in the
through-transmission mode. The schematic of a typical
immersion ultrasound system is shown in Fig. 1.
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Results and Discussion
The first case considered is the dropped-weight impact onto
a cast acrylic plate by a 6.1 kg mass from a height of 51 mm
(2”) and the test resulted in a star-shaped cracking pattern.
The ultrasound technique used here is schematically
depicted in Fig. 2. As shown in Fig. 3 the optical image was
reproduced fairly faithfully by the proposed ultrasound
technique with the exception of a few horizontal white
streaks which were mis-scanned points due to the limitation
of the ultrasound system. Regardless the transducer
frequency (say, between 5 and 25 MHz) the ultrasound
images detected the flaw pattern very accurately. In
particular, the higher frequency transducer, which has a
better resolution, produces more details of fracture,
especially around the impact site.
The ultrasound technique shown in Fig. 4 was then used to
inspect the crack pattern and debonding in a damaged
aluminum/acrylic sandwich plate scanned from the acrylic
side. The resulting C-scan images and the optical image are
shown in Fig. 5, which again indicates the high accuracy of
ultrasound inspection.
The advantage of ultrasound technique is for flaw detection
in an opaque material. We also used the technique shown in
Fig. 6 to scan the aforementioned damaged aluminum/
acrylic sandwich plate from the aluminum side. The resulting
optical and C-scan images are shown in Fig. 7. In this case
only the indentation and debonding are detected by the
ultrasound technique. Because of the presence of
debonding, the back-echo signals from the cracked acrylic
plate were “overshadowed” by the strong echoes from the
debonded interface. This is because ultrasound signals
transmit poorly through the “air gap” within the debonded
area. The experimental evidence can be illustrated by a
typical A-scan ultrasound signal shown in Fig. 8.
To further verify the filtering effect of debonding on
ultrasound signals, we adhered a pre-cracked acrylic plate to
an aluminum panel; thus artificially removing the debonding.
The ultrasound technique shown in Fig. 9 was then used for
C-scan from the aluminum side and the resulting image,
along with their optical counterpart, are shown in Fig. 10. In
this case, without the blocking effect from debonding the
cracking pattern (as displayed in the optical image) was
clearly detected by ultrasound. A typical A-scan signal
showing the identifiable back echo from cracked acrylic is
illustrated in Fig. 11.
Finally as shown in Fig. 12, the impacted aluminum/acrylic
plate studied in previously (see Fig. 5) was also scanned in
the through-transmission mode. Just like the case of pulseecho
mode, the resulting ultrasound image, as shown in Fig.
13, could only detect the debond while the cracking was
again shielded.
Conclusions
From this study one can conclude that
• The ultrasound technique can detect crack and debond
accurately in aluminum/acrylic sandwich plates so long
as the back echo signals are not blocked by any
discontinuity;
• In general, all ultrasound transducers with commonly
used frequencies, i.e., 5 to 25 MHz, can detect flaw
accurately;
• In particular, the transducers of higher frequency have
better resolutions and can detect fine details;
• Both pulse-echo and through-transmission modes are
workable for damage detection in relatively flat
specimens.
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.
References
1. B.M. Liaw, G. Zeichner and Y.X. Liu, “Impact
delamination and fracture in aluminum/acrylic sandwich
plates,” Proceedings of the SEM IX International
Congress on Experimental Mechanics, Orlando, FL,
June 5-8, 2000, pp. 515-518.
2. A. Vlot, “Impact Properties of fiber metal laminates,”
Composite Engineering, Vol. 3, No. 10, pp. 911-927,
1993.
3. C.T. Sun, A. Dicken, and H.F. Wu, “Characterization of
impact damage in ARALL laminates,” Composites
Science and Technology, Vol. 49, pp. 139-144, 1993.
4. H.F. Wu, “Temperature dependence of the tensile
properties of ARALL-4 laminates,” J. of Materials
Science, Vol. 25, pp. 1120-1127, 1990.
5. B.M. Liaw, Y.X. Liu, and E.A. Villars, “Impact damage
mechanisms in fiber-metal laminates,” this volume.
Figure 1. Schematic of a typical immersion ultrasound
system.
265

Figure 2. Ultrasound technique for inspecting a cracked
acrylic plate.
Figure 4. Ultrasound technique scanning a damaged
aluminum/acrylic sandwich plate from the acrylic
side.
Optical image
C-scan (5 MHz)
Optical image
C-scan (5 MHz)
C-scan (10 MHz)
C-scan (15 MHz)
C-scan (10 MHz)
C-scan (15 MHz)
C-scan (20 MHz)
C-scan (25 MHz)
Figure 3. Optical and ultrasound C-scan images of cracking
in an cast acrylic plate impacted by a dropped
weight of 6.1 kg from 51 mm (2”) high.
C-scan (20 MHz)
C-scan (25 MHz)
Figure 5. Optical and ultrasound C-scan images of
debonding and cracking scanned/photographed
from the acrylic side of a n impacted
aluminum/acrylic sandwich plate.
266

Figure 6. Ultrasound technique scanning a damaged
aluminum/acrylic sandwich plate from the
aluminum side.
Figure 9. Ultrasound technique for inspecting a
pre-cracked acrylic plate shielded by an
aluminum panel.
Optical image
C-scan (25 MHz)
Figure 7. Optical and ultrasound C-scan images from the
aluminum side of a damaged aluminum/acrylic
sandwich plate.
Optical image
C-scan (25 MHz)
Figure 10. Optical and ultrasound C-scan images of a
pre-cracked acrylic plate shielded by an aluminum
panel.
Overdhadowed
back echo
Identifiable back echo
Figure 8. An A-scan ultrasound signal showing
overshadowed back echoes.
Figure 11. An A-scan ultrasound signal showing
identifiable back echoes.
267

Figure 12. Through-transmission ultrasound.
Figure 13. Through-transmission (0.25”-dia., 25 MHz flat
transducers) ultrasound C-scan image of a
damaged aluminum/acrylic sandwich plate
scanned from the aluminum side.
268