In this study, the nondestructive evaluation (NDE) of an internal defect in the weld metal of an aluminum plate has been demonstrated using a photoacoustic microscope (PAM). The internal defect was fabricated inside of a metal plate by welding. The NDE of the position and size of the internal defect was carried out using the PAM. In addition, X-ray tomographic imaging was also carried out for comparison with the obtained PAM images. The depth direction and size of the internal defects were obtained from photoacoustic (PA) amplitude images and the signal intensity distribution.

The nondestructive evaluation (NDE) of welded steel plates was performed using photoacoustic microscope (PAM). Specimens were prepared by changing the maximum welding current to 5, 6.5, and 8 kA. A noncontact inspection without having to assemble a transducer was performed, and the results were compared with those of destructive inspection using an optical microscope. The agreement of NDE using a PAM and results from an actual shear fracture destructive test was good. The large fluctuation in the PA signal measured for the steel plates welded at the small current suggests the inhomogeneity of the inside welded region. Furthermore, visualization of the welded region suggests the feasibility of obtaining information on the status or uniformity of the inner welded region, which is difficult to measure by the other methods.

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A theoretical formulation of photoacoustic (PA) imaging with a line-focus beam (LFB) for surface and undersurface simulated defect was performed. Equivalence between 2D surface defect photoacoustic tomography (PAT) and X-ray CT was derived. PAT imaging experiment was carried out A second harmonics of a LD-pumped YAG laser was used as a LFB. The laser power was 45mW. The length and width of a laser beam on a specimen was 25 mm times 0.65mm The measured area was 27mm x 27mm, while the reconstructed area was 18mm x 18mm. 64 times 64 resolution image was reconstructed from the rotation and translation scanning. Reconstructed PA image agreed with the PA image obtained with a point-focus PA imaging. The frequency dependence of thermally diffused image agreed well with the theoretical prediction (thermal diffusion length is inversely proportional to the square root of modulation frequency) Under surface PAT image was obtained by a thermal wave diffraction formula, and the simulated image agreed well with the experimental data.

A photoacoustic (PA) imaging apparatus using a laser line-focus beam (LFB) was designed to perform rapid inspection and photoacoustic tomographic (PAT) imaging of surface and undersurface defects. 2D-PAT imaging of surface and undersurface defects was demonstrated based on a formulation similar to the X-ray tomography. The obtained PAT images represented forward-projected PA signals collected along the LFB. The reconstructed images were in close agreement with those obtained from laser point-focus beam (PFB) PA imaging. We achieved rapid non-destructive inspection of a surface-simulated defect using a LFB. The reconstructed PA image of the undersurface defect was consistent with that obtained by a plane-thermal wave diffraction model.