Development and Applications of Laser-ultrasonic Testing in Nuclear Industry
A laser-based maintenance system, which includes the las er-ultrasonic testing as a non-destructive testing technology, the laser-peening as a surface stress imp rovement technology and the laser welding as a repair technology, for the in-service maintenance of n uclear reactor internal components is presented. Regarding the laser-ultrasonic testing, surface acoustic wave generated and detected by lasers is used to detect surface-breaking cracks. As the benefits of broad -ban feature of the laser-ultrasonics, this method allows to detect very small cracks having the depth of les s than 0.1mm. Also, a frequency response analysis technique of the surface acoustic wave, which propagates through the crack, is developed to size the depth of the crack. The error of this depth measurement thod is estimated at less than 0.2 mm through a series of experiments using stress corrosion cracki ng. A part of the laser-based maintenance system has been already applied to the actual maintenance works in Japanese nuclear power plants . The developed system and its operation are presented. Finally, m ore recent work to detect and visualize small defects on the weld bead by using leakly-Rayleigh waves is introduced.
Characterization of surface-breaking defects in metals with the use of laser-generated ultrasound
The interaction of ultrasound with surface-breaking slots has been used to characterize the slots, with laser generation of ultrasound providing the short-duration Rayleigh pulses used as the probe. Defect depth has been measured in the range 0.3-5.0 mm. Analysis has shown that surface echoes reflected from the defect have two major components. The first arises from direct reflection of a Rayleigh pulse from the top of the defect, whereas the second arises from a shear pulse originating from the bottom of the defect which mode-converts to a Rayleigh pulse on reaching the metal’s surface at the critical angle. This interpretation offers a diagnostic technique for measuring defect depths.
CRACK DEPTH ESTIMATION USING WIDEBAND LASER GENERATED SURFACE ACOUSTIC WAVES
Following previous studies where we analysed the physics of laser generated acoustic waves in metals, techniques have been developed using these acoustic waves which permit flaw detection in metal surfaces. The laser generated surface waves are used in conjunction with transducers to both locate and estimate the depth of surface breaking cracks. The results show that one of the advantages of the laser-acoustic source is that it generates transient ultrasonic pulses with a wide frequency bandwidth.