Characterization of Surface-Breaking Tight Cracks Using Laser-Ultrasonic Shadowing

A laser-ultrasonic method based on the shadowing effect is used for sizing and locating surface-breaking tight cracks in metals. The two laser spots are separated by a fixed distance to detect ultrasound propagating at oblique incidence with respect to the sample surface. The laser spots are scanned along the same line across the path of cracks on the cracking surface or opposite surface. The amplitude of the longitudinal or shear waves specularly reflected from the backwall is extracted from each signal to construct an amplitude profile. In the presence of a crack, the profile shows regions of reduced amplitude due to shadowing of the direct or reflected beam from the backwall. The sharp amplitude drop and the gentle signal recovery observed are well predicted by a model of crack tip diffraction. The crack depth can be simply determined by considering the immediate vicinity of the amplitude drop. Results are shown on a stainless steel sample having a variable depth slot as well as on a sample containing actual surface-breaking tight cracks having widths of less than 30 \u3bcmPeer reviewed: NoNRC publication: Ye