SONIC IR IMAGING AND VIBRATION PATTERN STUDIES
OF CRACKS IN AN ENGINE DISK
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 experiments in sonic infrared (IR) crack detection in an F-l 10 fan
disk. This technique uses a single short pulse of sound to cause cracks to heat up and become visible
in the infrared. A low frequency (15 to 40 kHz) ultrasonic transducer fills the sample with sound
that causes frictional heating at crack interfaces. We show that the technique can be applied to quite
large and irregularly shaped objects like an aircraft engine disk without exciting acoustic mode
patterns which might hide cracks in nodal regions.
INTRODUCTION
The hybrid ultrasonic/infrared crack detection technique1'2 that we have described in
previous QNDE conferences has been used, together with a laser Vibrometer, to study the
dependence of the sonic IR effect of the relative positions of the sound source and the slot
being inspected, and on the amplitude of the vibration at the position of the slot. The
reason for performing such a study was to determine whether potential acoustic nodes in a
turbine disk could possibly serve as "hiding places" for cracks. One might easily imagine
that an unfortunate choice of frequency in a sonic IR inspection could excite an acoustic
mode that would have nodes at which no significant vibration would occur. Cracks in such
locations could then be missed. Since the integrity of turbine disks is of the utmost
importance in preventing aircraft accidents, the consequences of missing a crack could be
catastrophic.
EXPERIMENTAL TECHNIQUE
The basic experimental set up is shown in Fig. 1. A sonic source in the 15 to 40kHz
frequency range is used to flood the sample with sound. Friction inside any cracks or
delaminations causes these defects to heat, and the resulting temperature field is imaged by
an infrared (IR) video camera. The camera location may be anywhere that gives a line of
sight view of the area to be inspected, either directly or through mirrors. The laser
vibrometer used to record vibrations is not shown in this photograph.
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© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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FIGURE 1. Photograph of the experimental setup for sonic IR imaging of cracks in a turbine disk. The
ultrasonic gun is seen on the right. The camera is focused at a small angle with respect to the tangential
direction in order to see into the slot. The relative positions of the gun and camera were systematically
changed during the study so as to detect any possible dead spots in the vibration pattern of the disk. The laser
vibrometer is not shown in this photograph.
RESULTS
Our procedure involved keeping the ultrasonic source in a fixed position, with a given
crack (in slot #21) being imaged repeatedly as the F-110 fan disk was indexed around one
slot at a time until we had turned the disk through 180°. In this way we always had an
identical crack as a target, but were able to change the relative position of the crack and the
ultrasonic source. This, of course, necessitated repositioning the camera after each rotation
of the disk, so that the angle and focus of the camera changed slightly between shots.
However, we found that the images of the crack, as well as the amplitude and general shape
of the acoustic waveform adjacent to the crack, were remarkably consistent throughout the
entire range, with absolutely no indication of nodal dead spots. Sample images and
acoustic waveforms for a few more or less equally spaced gun-to-crack displacements are
shown in the figures 2a-5b.
ill!
FIGURE 2a Acoustic waveform of the sound at Slot
21 with the ultrasonic horn at Slot 23.
FIGURE 2b Image of the crack in Slot 21 with the
ultrasonic horn at Slot 23. The bright spot at the
upper right is the position of the vibrometer beam.
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FIGURE 3a Acoustic waveform of the sound at Slot
21 with the ultrasonic horn at Slot 30.
FIGURE 3b Image of the crack in Slot 21 with the
ultrasonic horn at Slot 30. The bright spot at the
upper right is the position of the vibrometer beam.
FIGURE 4a Acoustic waveform of the sound at
21 with the ultrasonic horn at Slot 37.
FIGURE 4b Image of the crack in Slot 21 with the
ultrasonic horn at Slot 37. The bright spot at the
upper right is the position of the vibrometer beam.
FIGURE 5a Acoustic waveform of the sound at Slot
21 with the ultrasonic horn at Slot 44.
FIGURE 5b Image of the crack in Slot 21 with the
ultrasonic horn at Slot 44. The bright spot at the
upper right is the position of the vibrometer beam.
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ACKNOWLEDGMENTS
This work was supported in part by the US Air Force through Universal Technology
Corporation, under Contract Number F33615-97-D-5271, Task Order 0002-030,
Subcontract Agreement 01-S437-002-30-C1, and in part by the Institute for Manufacturing
Research, Wayne State University.
REFERENCES
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
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