NON-CONTACT INSPECTION OF COMPOSITES USING
AIR-COUPLED ULTRASOUND
J. Peters1, V. Kommareddy1, Z. Liu2, D. Fei3, and D. Hsu1
'Center for NDE, Iowa State University, Ames, IA 50011
Chinese Academy of Space Technology, Beijing, China
3
Caterpillar, Inc., Peoria, IL 61629
2
ABSTRACT. Conventional ultrasonic tests are conducted using water as a transmitting medium.
Water coupled ultrasound cannot be applied to certain water-sensitive or porous materials and is more
difficult to use in the field. In contrast, air-coupled ultrasound is non-contact and has clear advantages
over water-coupled testing. The technology of air-coupled ultrasound has gained maturity in recent
years. Some systems have become commercially available and researchers are pursuing several
different modalities of air-coupled transduction. This paper reports our experience of applying aircoupled ultrasound to the inspection of flaws, damage, and normal internal structures of composite
parts. Through-transmission C-scans at 400 kHz using a focused receiver has resolution sufficient to
image honeycomb cells in the sandwich core. With the transmitter and receiver on the same side of a
laminate, Lamb waves were generated and used for the imaging of substructures. Air-coupled scan
results are presented for flaw detection and damage in aircraft composite structures.
INTRODUCTION
Ultrasonic testing has been used for years as a method for examining parts and
components in production, as a means of verifying product quality and for inspecting in
service parts for damage and defects. Conventional ultrasonic testing does have
drawbacks, however. Many components, such as certain ceramic matrix composites, have
surfaces that could be compromised by coming into contact with typical coupling agents
such as gels, mineral oil, or water. Also, because the best time to perform an inspection is
often on the production floor, the use of squirters or a large scanning system can be
impractical. The use of air-coupled ultrasound combined with a simple encoding system
appears to show promise as a means of overcoming some of these obstacles.
Air-coupled ultrasonic testing (ACUT) has its disadvantages also. The most
obvious of these is the large loss of signal due to the high impedance mismatch between
air and most materials. For example, in the situation of a through transmission inspection
of steel with air-coupled ultrasound one would expect a loss of 160 dB of signal due to the
energy loss at the interfaces. If the through transmission ultrasonic inspection was
performed in water the energy loss at the interfaces would be only 20 dB. [1]
Improvements in transducer design and electronics have made strides in recent years in
making this obstacle less daunting. A second difficulty related to ACUT is that two
transducers are required for most conventional applications. Because of this, pulse echo
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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FIGURE 1. Beam profile of an unfocused 120 kHz probe.
testing is not feasible. This implies that to inspect a component with ACUT there must be
access to both sides for through-transmission testing or some sort of surface wave test
must be performed. Another disadvantage of ACUT lies in the fact that the technique is
limited to fairly low frequencies. While air coupled transducers of frequencies of 5 MHz
or higher have been produced, the best performance seems to be obtained at frequencies of
1 MHz or less. Finally, the alignment of transducers is critical for obtaining good results
using either through-transmission or one-sided Lamb wave methods.
This paper reports some current work being done to overcome these difficulties and
to produce a practical system for performing air-coupled ultrasonic testing in the field.
EQUIPMENT
The equipment used to obtain the results presented in this paper was the Sonda007CX AirScan from QMI, Inc. Conventional C-scan images were produced by using a
commercially available raster scanning system from Sonix, Inc. [2] The transducers used
were piezo-ceramic and ranged in frequency from 50 kHz to 400 kHz. The transducers are
driven by a tone burst and have a fairly columnar beam profile as shown in figure 1. [3]
THROUGH TRANSMISSIOM RESULTS
Through-transmission experiments were performed on a variety of materials. Cscan images made of composite honeycomb structures indicate ACUT in throughtransmission can be used to detect defects as small as 10 mm in diameter with 400 kHz
transducers, as shown in Figure 2. In solid laminates, defects as small as 3mm can be
clearly imaged at 400 kHz. A section of a 1V6" thick filament wound rocket motor casing
was successfully inspected using a pair 120 kHz unfocused probes. In addition to
composites, ACUT in through-transmission was also demonstrated to be a useful tool in
sorting timber specimens that may have been affected by bacteria. Early tests show that 50
kHz transducers can be used to easily penetrate 10 inches of reinforced concrete.
Another area that is being studied extensively involves the inspection of repairs of
composite structures on aircraft. [4] Figure 3 shows a C-scan image of a composite
honeycomb panel containing a repair. The dark ring (low transmitted amplitude)
corresponds to a region that was determined by destructive sectioning to have a disbond
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Single
tape, front
Single
tape, back
Crushed
front
Double
tapes, back
0
FIGURE 2. ACUT C-scan of composite-honeycomb sandwich panel. Transducers are 400 kHz
(transmitter - unfocused / receiver ~1" focus)
between the facesheet and the honeycomb core. Air-coupled ultrasonic testing appears to
be a promising method for investigating composite repairs.
A simple transducer modification that addresses two of the difficulties involved
with ACUT mentioned earlier is being developed. By apodizing the transmitting and
FIGURE 3. ACUT C-scan of composite panel containing a repair. The large dark ring is an indication of
a facesheet honeycomb core disbond, the small oval ring at the center is the edge of a plug on the
backside.
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FIGURE 4. Apodization of transmitter and receiver results in easier alignment of transducers and higher
resolution C-scan image. Apodization opening is about 5mm.
receiving transducer (or at times only the receiving transducer) as show in Figure 4, a
higher resolution image can be obtained. The effect of this modification is shown in
Figure 5. Without apodization no detail of the panel's honeycomb substructure is imaged.
After apodization, with the same transducers, much of the honeycomb structure becomes
visible. The resolution of the apodized image is approximately that which can be obtained
with 400 kHz probes (with focused receiver). The modification also makes the transducers
easier to align.
FIGURE 5. ACUT through transmission C-scans performed with 120 kHz unfocused probes. The top
image was obtained using the transducers without modification. In the lower image both transmitter and
receiver are apodized as shown in Figure 4. (Note: The images have different orientations.)
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LAMB WAVE RESULTS
Often through transmission inspection of a part is impractical because both sides of
the component are not accessible. Because pulse-echo ACUT is not feasible, one approach
that is being studied is the use of Lamb waves to inspect parts where there is access to just
one side of a part. There are, however, several difficulties to using this method. One
problem is that the large specular reflection from the surface can mask the desired signal.
A second problem arises because of the physical size of the transducers (~ 1" diameter for
the 120 kHz probes). This requires a physical separation of the transmitting and receiving
probe requiring, at any location, a line segment and not a point is being sampled. The
technique is a "dark field" technique, which can make interpretation of the results
difficult. Another difficulty that occurs when inspecting components made from composite
materials is due to the fact that the signal strength is highly dependent on the ply
orientation of the structure. Finally, as is the case in performing all Lamb wave
inspections, the transducers can be difficult to align and the injection angle must be
controlled carefully.
One of the difficulties mentioned above in performing Lamb wave inspections with
ACUT is related to the large specular reflection from the surface of the part being
inspected. One way of mitigating this problem is by introducing a "sound barrier"
between the transmitting transducer and the receiver. This sound barrier will block any
direct signal between the transducers and any signal reflected off the surface of the part. A
more effective method for discriminating the desired Lamb wave signal from any other
spurious signal can be achieved by attaching "horns" to faces of the transducers as shown
in Figure 6. These horns, with an exit diameter of about 4mm, eliminate the need for a
sound barrier.
FIGURE 6. Apodization of transmitter and receiver in Lamb mode scanning allows for better discrimination
between desired Lamb wave signal, front surface reflection, and other cross talk.
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FIGURE 7. Lamb wave C-scan of reinforced composite plate. Notice dark band at the location of the ribs
due to the energy associated with the wave being absorbed into the rib.
ACUT Lamb wave C-scan images were made of two types of composite
specimens. The first type of specimen is a composite solid laminate with reinforcing ribs
on the backside. The C-scan of this structure is shown in Figure 7. The figure shows that,
at the locations of the reinforcing structure, the amplitude of the Lamb wave signal is
reduced due to the fact that some of the energy associated with the wave is transmitted to
the reinforcing rib. The darker diagonal bands associated with the ribs appear wider than
the ribs themselves due to the fact a line segment, and not a point, is being sampled at each
location.
FIGURE 8. Lamb wave C-scan of a composite honeycomb sandwich panel. Note that the size of the
imaged defect is larger than the actual disbond due to the line segment sampling nature of the scan.
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ACUT Lamb wave C-scan images were also obtained on a suite of compositehoneycomb sandwich structures with engineered triangular disbonds between the facesheet
and honeycomb core of each sample. The suite of specimens included facesheet
thicknesses ranging from 3 to 13 plies thick. The C-scan image obtained for the 5 ply
specimen is shown in Figure 8. The signal amplitude observed in the disbond region is
higher than in the surrounding areas of the panel because no energy associated with the
wave is transmitted into the honeycomb in the area of the disbond.
FUTURE WORK
1.
2.
3.
4.
5.
Work is ongoing in a number of areas. These areas include:
Developing a basic understanding of the physics of ACUT with a particular interest
in air-coupled Lamb wave generation.
Developing methods using ACUT for inspecting porous thermal barrier coatings,
concrete, wood, and other materials where the use of conventional UT methods is
impractical.
Correlate ACUT test results with other techniques such as the Computer Aided Tap
Test as a tool for investigating composite repairs.
Developing image processing tools to overcome the difficulties caused by sampling
line segments rather points when performing ACUT Lamb wave C-scans.
Finally, work is underway to develop a fieldable ACUT inspection system capable
of generating images for use on large aircraft structures. Encoding systems such as
the "Flock of Birds" [5] magnetic positioning system are being considered for use
with a large yoke as seen in Figure 9.
FIGURE 9. Air-coupled transducers in a "yoke" are used in the inspection of a composite spoiler.
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ACKNOWLEDGEMENTS
This work was supported by the NSF Industry/University Cooperative at the Center
for Nondestructive Evaluation at Iowa State University.
REFERENCES
1. Bhardwaj, M.C., Non-contact Ultrasound: The Last Frontier in Non-destructive
Testing and Evaluation, Encyclopedia of Smart Materials, John Wiley & Sons, 2001
2. Sonix, Inc., Springfield, VA
3. QMI, Inc., Huntington Beach, CA, Transducer data provided by manufacturer
4. Hsu, D.K., Barnard, D.J., Peters, J.J., and Dayal, V., in Review of Progress in QNDE,
Vol. 22, eds. D. O. Thompson and D. E. Chimenti, (submitted for publication)
5. Ascension Technology Corporation, Burlington, VT
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