Adaptive Spatial Wavenumber Estimation: A Computationally Efficient Laser Scanning Ultrasonic Guided Wavefield Imaging for Depth-Resolved Inspection

The ultrasonic guided wavefield imaging (UGWI) has attracted widespread interest in industrial nondestructive testing and evaluation applications because it can noncontactly record full wavefield data with inhomogeneous structural information. By leveraging the thickness-wavenumber dispersion relationship of guided waves and wavenumber spectrum, the size, location, and depth of defect can be quantitatively characterized from full wavefield. However, one major problem encountered in UGWI is the compromise one has to make between anti-interference ability and imaging efficiency. To simultaneously achieve both anti-interference and high efficiency, a computationally efficient approach called adaptive spatial wavenumber estimation (ASWE) is proposed to achieve depth-resolved inspection. By modifying the fixed spatial window to an adaptive spatial window in local wavenumber estimation (LWE), the imaging process is greatly accelerated without compromising depth resolution and robustness. Meanwhile, to improve the depth accuracy of the imaging results, a reference wavefield-based filter is introduced to isolate detection wavefield from other accompanying wavefields, and an inverse procedure of longitudinal and transverse wave velocities is design for calibrating dispersion. The validation experiments are carried on several metallic plate specimens with artificial thinning defects of various shapes, stepped, and linear depth. It is demonstrated that the proposed ASWE method is an accurate and efficient modality to study depth-resolved inspection and promotes the applications of UGWI method in industry.