RECENT DEVELOPMENTS IN SONIC IR IMAGING
Xiaoyan Han1, L.D. Favro2, and R.L. Thomas3
Department of Electrical and Computer Engineering
Physics Department and Institute for Manufacturing Research,
3
Physics Department and College of Science
Wayne State University, Detroit, MI 48202, USA
2
ABSTRACT. We describe recent developments in sonic infrared (IR) imaging. The sonic IR
imaging NDE technique uses a single short pulse of sound to cause cracks to heat up and become
visible in the infrared. Results of recent experiments on metal aircraft structures and composite test
samples with natural and simulated defects are presented in this paper. Examples of defects in metals
include cracks originating under fasteners and cracks in mechanical parts. Examples of defects in
composites include fatigue cracks, pillow inserts, pull-tab disbonds, skin-to-core disbonds, etc. Both
20kHz and 40kHz ultrasonic excitation sources were used for studying the frequency effect.
Vibrational behavior of surfaces near the defects was monitored and correlated with the IR signals.
INTRODUCTION
The sonic Infrared (IR) imaging NDE technique, which combines mechanical
vibrational excitation and IR imaging, has been used for detection of defects such as cracks,
disbonds/delaminations on a variety of different materials and structures.1' 2' 3 This
technique is wide-area, fast, non-invasive, and truly dark-field since only the defects
respond to the excitation. We present recent developments in this technique.
EXPERIMENTAL TECHNIQUE
The experimental set up is shown in Fig. 1. A 20kHz ultrasonic excitation source is
used to infuse mechanical vibration into the sample, which is supported by two posts.
Mechanical energy dissipated in the defects causes local heating, and the resulting
temperature distribution on the surface of the sample is then imaged by an IR video camera,
which is focused on the sample. The location of the sonic source is not critical for
observation of the defects. A laser vibrometer is used to monitor the surface vibrational
behavior near the defect region. Our experiments used a laser Doppler vibrometer, which
measures the surface velocity parallel to the direction of the laser beam.
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. The experimental set up. A 20kHz ultrasonic excitation source is used to infuse mechanical
vibration into the sample, which is supported by two posts. An IR video camera images the resulting
temperature rise due to mechanical energy dissipation in the vicinity of the defects. A laser vibrometer is used
to monitor the vibrational behavior near the defect region.
RESULTS
We have used sonic IR imaging to study aircraft structures. In Figure 2, we show
cracks around rivets and corrosion on the B737 testbed aircraft at the FAA's Airworthiness
Assurance NDI Validation Center (AANC) in Albuquerque, NM. Figure 2a is an IR image
taken shortly after the sonic excitation was turned on. The cracks from the two rivets are
evident, in addition to the beginning evidence of heating from corroded areas on the rear
surface of the aircraft skin. Figure 2b is an IR image taken some time later during the
excitation. The corroded regions are comparatively brighter at this later time.
FIGURE 2a. An IR image taken shortly after the
sonic excitation was turned on. The cracks from the
two rivets are evident, in addition to the beginnings
of thermal evidence for rear-surface corrosion.
FIGURE 2b. An IR image taken later during the
sonic excitation. The rear-surface corroded regions
are much more pronounced.
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Figure 3 shows an IR image of a boron-fiber-reinforced composite aircraft repair
patch that contained implanted defects on the same B737 testbed. The implants are clearly
visible in this image. Note that the lower left corner is brighter. This results from the fact
that the acoustic energy was injected at a point out of the field of view on the lower left. In
the image, the energy is partially blocked by the underlying frame and stringer. Note also
that despite this "blockage", the defects are clearly visible in other regions of the image.
In Figure 4a, we show an image of a bevel gear from a NAVAIR helicopter which has
two EDM notches, beneath one of which is a crack. A sonic IR image is shown in Figure
4b, where the crack is clearly evident. Note that no heating occurs in the vicinity of the
second EDM notch, which is directly below the crack in this figure.
In order to get a clearer picture of the origins of the sonic IR effect, we have
undertaken a study of the vibrational behavior of samples undergoing sonic IR inspections
through the use of a Polytec Doppler laser vibrometer. The vibrometer beam is reflected
from the surface of the sample in the vicinity of the defect, and the surface velocity of the
FIGURE 3. An IR image of a boron-fiber-reinforced composite aircraft repair patch that contains simulated
defects.
FIGURE 4a. An image of a bevel gear from a
NAVAIR helicopter which has two EDM notches,
only one of which is associated with a subsurface
crack
FIGURE 4b. An IR image in which only the crack
is excited sonically.
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sample in the direction parallel to the laser beam is recorded in the memory of a computer.
A recorded waveform of an uncoupled ultrasonic gun, in this case one designed to produce
nominal 40kHz vibration, is shown in Figure 5a. The linear ramp-up and the final ringdown of the vibration are the only deviations from a constant amplitude pulse. However,
when the transducer is coupled to a sample through a nonlinear coupling material, the
vibration in the sample shows more a much more complicated form, as shown in Figure 5b.
The difference between these two systems (one being the isolated transducer, the other
being the combined system of transducer/coupling material/specimen) can be better seen
through the corresponding spectra of the waveform. Figure 5c, the Fourier Transform of
the waveform in Figure 5a, shows a single 40kHz response, indicating that horn is
producing this pure frequency only. However, Figure 5d, which is a Fourier Transform of
the waveform in Figure 5b, shows many harmonics of the 40kHz fundamental. Thus, the
vibration at the site of the crack may be quite different from what one might have expected
from the known output of the horn. We have studied many such waveforms, and even more
complicated temporal and spectral structures than those shown in Figures 5b and 5d have
been observed. To obtain a detailed understanding of the sonic heating effect, one must
take these complications into account.
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Time (ms)
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Time (ms)
FIGURE 5a. Vibration delivered by the Branson
40kHz ultrasonic power supply/horn system when it
is not coupled to anything else. The waveform is
constant in amplitude after it reaches a steady state.
The source was set for 800 milliseconds.
FIGURE 5b. Waveform when the transducer is
coupled to a target through a compliant coupling
material. The vibration in the sample shows a more
complicated form. The source was turned on for
200 milliseconds.
JL
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40
Frequency (kHz)
80
120
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Frequency (kHz)
FIGURE 5c. Fourier Transform of the waveform in FIGURE 5d. Fourier Transform of the waveform in
figure 5a, shows the single source frequency
figure 5b, shows the harmonic frequencies of the
fundamental one.
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ACKNOWLEDGMENTS
This work was supported by in part by the DOT/FAA William J. Hughes Technical
Center's Airworthiness Assurance Center of Excellence (AACE), under Contract Number
DTFA0398D-00008, Award Number DTFA0300PIA037, in part by the U.S. Navy, NSWC,
under P.O. Number N00167-00-M0498, in part by the Office of Naval Research under
Award Number NOOO14-02-1-025 9, and in part by the Institute for Manufacturing
Research, Wayne State University.
REFERENCE
1. L.D. Favro, Xiaoyan Han, Zhong Ouyang, Gang Sun, Hua Sui and R.L. Thomas, Rev.
Sci. Instr., 71, 2418-2421, June, 2000.
2. U.S. Patent No. 6,236,049, May 22, 2001
3. Xiaoyan Han, L.D. Favro, Zhong Ouyang, and R.L. Thomas, Review of Progress in
Quantitative Nondestructive Evaluation, Vol. 21, pp. 552-557, American Institute of
Physics, 2002.
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