The Synthetic Aperture Focusing Technique (SAFT) is an imaging algorithm used in laser ultrasonics (LU) to visualise the appearance of defects. However, ultrasound excited by a pulsed laser has the characteristics of wide bandwidth and multi-mode directivity patterns, leading to common problems in the SAFT process, such as low utilisation of ultrasound information and possible artefacts. To solve these problems, a Multi-mode Time-domain SAFT (MMT-SAFT) algorithm is proposed in this paper. The influence of ultrasonic directivity is discussed according to the imaging depth range, and imaging with multiple LU modes is performed to reduce artefacts. Simulations and experimental results prove the feasibility of the MMT-SAFT algorithm, which not only presents a clearer image of the upper part of defects but also improves image quality compared with time-domain SAFT using a single ultrasonic mode. The proposed technique can provide feasible directions for laser ultrasonic defect imaging.

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.

This research focuses on analyzing the frequency characteristics of ultrasonic waves induced by a partially closed surface-breaking crack. When acoustic waves interact with the crack, transmission, reflection, and mode conversions occur and the frequency characteristics of signals perform obvious changes. A pulsed laser line source is used to generate ultrasonic waves in the sample with a partially closed surface-breaking crack, and one can see how the frequency characteristics of detected signals change as the pulsed laser beam scans across the sample surface. The optical deflection beam method is developed to detect the ultrasonic signals experimentally. The fast Fourier transform (FFT) is used to analyze the time-domain data, and the FFT data are visualized by a B-scan plot. A clear disruption in the B-scan can be observed when the laser beam illuminates directly onto the crack, which is due to the changes of frequency characteristics induced by the partially closed crack. A frequency-domain B-scan of numerical simulation results is presented, and the clear disruption can also be observed clearly.