In this study, the nondestructive evaluation (NDE) of a compound defect in the weld metal of an aluminum plate has been demonstrated using a photoacoustic microscope (PAM). The compound defect is a blowhole, which was formed in the weld metal during welding. Experiments were carried out at different modulation frequencies to achieve depth profiling by changing the thermal diffusion length. Furthermore, X-ray tomographic imaging and scanning laser microscopy (SLM) were also used for comparison with the obtained PAM images. The PAM measurement, which is useful when only front-surface illumination is available, can achieve a spatial resolution comparable to those of X-ray micrography and SLM measurements for internal and surface defects, respectively.
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Shiraishi et al. (2009) studied this question.