3D imaging of Sub-Millimeter defects using Laser-Generated broadband surface waves and Multi-Band Space-Wavenumber...

To address the challenges associated with the 3D imaging of submillimeter-scale defects and submillimeter crack propagation monitoring, this paper proposes a novel 3D imaging method based on laser generated broadband surface waves and multi-band space-wavenumber analysis. A finite element model is established to investigate the interaction between surface waves and defects of varying depths. The simulation revealed that increased defect depth extends the attenuation effect of surface waves to lower frequencies. Building on this insight, the proposed method first applies a two-dimensional Fourier transform(2D-FFT) to the data matrix to quantitatively assess the defect depth using the transmitted wave cut-off frequency, defined as the frequency where the wave energy on the dispersion curve reduces to half of its peak value. Secondly, the data matrix is reprocessed via Fourier transform to extract the effective transmission frequency band. Subsequently, a sliding window is applied along the frequency axis, and two window functions together with a 2DFFT are employed for local wavenumber estimation of the single frequency wavefield. Following binarization and thresholding, the 3D imaging results are ultimately derived. Experimental results demonstrate that the method can accurately reconstruct the 3D images of defects with dimensions as small as 2 mm × 0.2 mm × 0.1 mm and 4 mm × 0.2 mm × 0.05 mm. Clear 2D morphologies are achieved for defects with lengths ranging from 2 mm to 4 mm and inclination angles of 15° to 45°. The resulting images are virtually artifact-free with a maximum length measurement error of less than 5%, validating the method's potential for monitoring submillimeter crack propagation. The method effectively suppresses noise, enables high-precision 3D reconstruction and multi-scale characterization of submillimeter-scale defects, and substantially enhances detection resolution.