Laser-ultrasonic inspection of surface-breaking tight cracks in metals using SAFT processing

A method is presented for imaging the detailed structure of surface-breaking tight cracks from the surface opposite to the cracks. The technique combines laser-ultrasonics and data processing with an improved version of the Fourier-domain Synthetic Aperture Focusing Technique (F-SAFT). The key point of this study is to use laser-generated shear waves, which interact very effectively with the crack roots. Ultrasonic images of the stress corrosion cracks, having depths of typically 0.5 mm. and widths of less than 0.05 mm, are successfully reconstructed and compared with results from liquid penetrant testing.