Detection and characterization of discontinuities in stainless steel by the laser ultrasonic synthetic aperture focusing technique

The performance of a laser ultrasonic system applied to the detection and characterization of stress corrosion cracks and welding discontinuities in stainless steel is reported. A 50 mJ long pulse laser coupled with a confocal Fabry-Perot interferometer and the synthetic aperture focusing technique are used to enhance weak discontinuity related signals. First, stainless steel plates containing stress corrosion cracks with widths of less than 0.03 mm (1.2 x 10(-3) in.) and depths ranging from 0.5 to 5 mm (0.02 to 0.2 in.) are tested. Both longitudinal and shear waves are used in synthetic aperture focusing technique processing to detect and characterize these cracks from the surface opposite cracking. Images at the breaking surface reconstructed by shear waves provide very detailed structures of branched tight cracks, which are comparable to results obtained by liquid penetrant testing. This excellent imaging is linked to the presence of crosslike features appearing in B-scans that are interpreted and explained. Secondly, a welded stainless steel plate is tested and various weld discontinuities with sizes of about 1 mm (0.04 in.) are detected. Although the material strongly attenuates ultrasound in this case, laser ultrasonics combined with the synthetic aperture focusing technique successfully detects, locates and sizes these discontinuities. These results are compared with those obtained by X-ray computed tomography.