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In the nondestructive evaluation (NDE) of metals, laser-induced ultrasound is efficient in detecting internal cracks because of its flexibility and noncontact capabilities. However, image reconstruction is challenging for characterizing inner cracks due to multimode waves and multiple lobes in the laser ultrasound field. Here, we proposed a two-step method to overcome the challenges. First, we propose an array-domain and beam-domain minimum variance (ABMV) beamformer to produce adaptive apodization weights that match the intrinsic beam pattern of the ultrasound, suppressing the interference and noise terms successfully. Second, images reconstructed from longitudinal, transverse, and mode-converted waves are fused to generate a high-quality image that accurately reveals the location and size of the cracks. Compared to traditional delay-and-sum (DAS) and minimum variance (MV) beamforming methods that generate images with either low resolution or low signal-to-noise ratio (SNR), the proposed beamformer achieves both the high resolution as MV and the high SNR as DAS. Furthermore, the multiwave fused image enables more precise crack detection than images reconstructed from any single mode of the waves. The simulation and experiment results show the superiority of the ABMV method. The proposed method overcomes the challenges of multimode and multilobe interference, holding great potential in laser ultrasound-based NDE.
Fan et al. (Mon,) studied this question.