Three-dimensional SAFT imaging for anisotropic materials using photoacoustic microscopy

Area of Science:

• Materials Science
• Nondestructive Testing
• Ultrasonic Imaging

Background:

• Pulsed lasers generate photoacoustic (PA) waves via thermoelastic expansion.
• Photoacoustic microscopy (PAM) uses confocal laser and ultrasonic waves for imaging.
• Synthetic aperture focusing technique (SAFT) is crucial for 3D imaging of internal flaws.

Purpose of the Study:

• To demonstrate the application of PAM with SAFT for nondestructive testing (NDT) of subsurface flaws.
• To accurately image a subsurface delamination in a carbon-fiber-reinforced plastic (CFRP) specimen.
• To investigate methods for improving the signal-to-noise ratio (SNR) in PA wave detection.

Main Methods:

• Developed a PAM system with a confocal laser and ultrasonic wave.
• Applied the synthetic aperture focusing technique (SAFT) for 3D imaging.
• Accounted for directional group velocity variations in anisotropic CFRP materials within SAFT.
• Coated the CFRP surface with a light-absorbent material.

Main Results:

• Accurately estimated the shape and location of a subsurface delamination in a CFRP specimen.
• Demonstrated the effectiveness of considering anisotropic group velocity in SAFT for precise flaw localization.
• Observed improved PA wave amplitude and SNR by using a light-absorbent surface coating.

Conclusions:

• PAM combined with anisotropic-aware SAFT provides accurate 3D NDT of internal flaws in complex materials.
• Surface treatment to enhance light absorption is a viable strategy to improve PA wave detection sensitivity.
• This approach shows significant potential for inspecting composite materials and detecting delaminations.