Scanning laser source technique for detection of surface-breaking and sub-surface cracks

Kromine, A. K.; Fomitchov, P. A.; Krishnaswamy, S.; Achenbach, J. D.
May 2000
AIP Conference Proceedings;2000, Vol. 509 Issue 1, p335
Academic Journal
A new approach is presented for ultrasonic detection of small surface-breaking and subsurface cracks using laser-based techniques. This approach does 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 monitors the changes in the generated ultrasonic signal as the laser source passes over the area of inspection. Changes in amplitude and frequency of the laser generated ultrasound are observed resulting from the changed conditions under which the ultrasound is generated over uniform and defective areas. These changes are quite readily detectable using existing laser detectors. The main advantages of the proposed scanning laser source (SLS) technique are: (i) enhanced signal-to-noise performance compared to the conventional pitch-catch mode of operation, (ii) ability to detect smaller defects, (iii) ability to detect defects of various orientations with a respect to the scanning direction, (iv) compatibility with either laser interferometric or conventional PZT transducer detection, and (v) ability to inspect surfaces of complex geometry. Results are presented for detection of fatigue cracks using a complete laser based ultrasonic system developed at Northwestern University. This system includes a fiberized laser ultrasonic source based on a portable Q-switched Nd-YAG laser (λ=1.06 μm, energy per pulse ≤200 mJ). This laser source can be combined either with a conventional PZT transducer or a fiber optic Sagnac interferometer capable of detecting ultrasonic signals on rough and curved surfaces. Experiments were carried out on a variety of specimens including turbine disk specimens provided by Allied Signal Engines. © 2000 American Institute of Physics.


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