Non-destructive laser-ultrasonic Synthetic Aperture Focusing Technique (SAFT) for 3D visualization of defects

Chen-Yin Ni; Chu Chen; Kai-Ning Ying; Lu-Nan Dai; Ling Yuan; Wei-Wei Kan; Zhong-Hua Shen

doi:10.1016/j.pacs.2021.100248

Non-destructive laser-ultrasonic Synthetic Aperture Focusing Technique (SAFT) for 3D visualization of defects

Photoacoustics. 2021 Feb 27:22:100248. doi: 10.1016/j.pacs.2021.100248. eCollection 2021 Jun.

Authors

Chen-Yin Ni¹, Chu Chen¹, Kai-Ning Ying¹, Lu-Nan Dai², Ling Yuan², Wei-Wei Kan², Zhong-Hua Shen²

Affiliations

¹ School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.
² School of Science, Nanjing University of Science and Technology, Nanjing 210094, China.

Abstract

The Laser Ultrasonic (LU) technique has been widely studied. Detected ultrasonic signals can be further processed using Synthetic Aperture Focusing Techniques (SAFTs), to detect and image internal defects. LU-based SAFT in frequency-domain (F-SAFT) is developed to visualize horizontal hole-type defects in aluminum. Bulk acoustic waves are non-destructively generated by irradiating a laser line-source, and detected using a laser Doppler vibrometer at a point away from the generation. The influence of this non-coincident generation-detection on the equivalent acoustic velocity used in the algorithm is studied via velocity mappings. Because the wide-band generation characteristic of the LU technique, frequency range selections in acoustic wave signals are implemented to increase Signal-to-Noise Ratio (SNR) and reconstruction speed. Results indicate that by using the LU F-SAFT algorithm, and incorporating optimizations such as velocity mapping and frequency range selection, small defects can be visualized in 3D with corrected locations and improved image quality.

Keywords: Imaging algorithm; Laser ultrasound; Non-destructive Test (NDT); Synthetic Aperture Focusing Technique (SAFT).