Three-dimensional imaging of subsurface delamination in carbon fiber reinforced plastic using photoacoustic wave method

Ultrasonic wave generated by interaction of the material with pulsed lasers is known as photoacoustic (PA) wave. In this study, the PA imaging is applied to the nondestructive testing for carbon fiber reinforced plastic (CFRP). When subsurface flaws are in CFRP, the detection of the flaws is sometimes challenging because the trailing signal from the surface interferes with the scattered signal from the flaws. Here, we developed a PA microscopy (PAM) to reconstruct the location and shape of the subsurface flaws. The PAM employs a confocal arrangement of optical and acoustic lens to maximize the intensity of generated PA waves. To enhance the lateral resolution of the image, we introduced the synthetic aperture focusing technique (SAFT). In the SAFT, it is necessary to consider direction-dependent group velocities due to the strong anisotropy in CFRP. The performance of the PA imaging combined with the SAFT was checked with artificial delaminations in a cross-ply CFRP specimen. It was shown that the PA wave had a wide frequency band and our method offered a high spatial resolution image.