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31 August 2000 Applications of scanning laser source technique for detection of surface-breaking defects
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Proceedings Volume 4076, Optical Diagnostics for Industrial Applications; (2000)
Event: Symposium on Applied Photonics, 2000, Glasgow, United Kingdom
A new laser ultrasonic approach, the Scanning Laser Source (SLS) technique, is presented for detection of small surface-breaking defects. In this approach we do not monitor the interaction of a generated ultrasonic wave with a flaw, as in the case of traditional pitch-catch or pulse-echo methods, but rather monitor the changes in the laser generated ultrasonic signal as the source is scanned over a defect. Changes in the amplitude and frequency content of the laser-generated ultrasound are observed resulting from the changed conditions under which the ultrasound is generated over areas without and with a surface- breaking crack. These changes are quite readily detectable using existing ultrasonic detectors. The SLS system includes a fiberized portable Q-switched YAG:Nd laser, which can be combined with either convention PZT transducers or laser interferometers. Results are presented for detection fo small EDM notches and fatigue cracks on flat and curved specimens and thin plates including real structures such as an aircraft turbine disk. It is shown that the SLS technique has several advantages over the conventional pitch-catch approach, including: (i) enhanced signal-to-noise performance, (ii) detection of defects with size smaller than the ultrasonic wavelength (at least 0.125 mm length and 0.06 mm depth), (iii) ability to detect defects of various orientations with respect to the scanning direction, (iv) inspection of surfaces with complex geometry such as bore holes and turbine disk slots.
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Alexei K. Kromine, Pavel A. Fomitchov, Sridhar Krishnaswamy, and Jan D. Achenbach "Applications of scanning laser source technique for detection of surface-breaking defects", Proc. SPIE 4076, Optical Diagnostics for Industrial Applications, (31 August 2000);

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