NDT of tilted and wedge-type subsurface defects by photoacoustic microscopic imaging

In this study, basic experiments on nondestructive testing (NDT) of simulated tilted and wedge-type subsurface defects using photoacoustic microscope (PAM) have been demonstrated. The specimens used in the experiments were pure aluminum plates. Two types of subsurface defects were fabricated in a specimen. The tilted subsurface defect (type I) was the slit-type simulated defect. The subsurface defect (type II) was wedge shaped. To obtain the depth-profile information, the photoacoustic (PA) measurements were carried out with changing the modulation frequency. The information on the undersurface structure and/or orientation of the subsurface defects could be extracted from the PA amplitude images obtained by the PAM. From the obtained PA signal intensity distribution of the type I specimen, it was found that the distribution was lacking symmetry in bilateral direction. And it is possible to confirm the tilt of the subsurface defect from the asymmetry of the obtained signal intensity distribution. On the type II, the distance between which the heat diffusion during one period of modulation encounters the top of the specimen, can be estimated from the amplitude PA images.