<p>Quantitative guided wave thickness mapping in plate like structures and pipelines is of significant importance for petrochemical and nuclear industries to accurately estimate the minimum remaining wall thickness, as guided waves can provide a fast scanning approach inspecting a large area without needing direct access. A number of inverse algorithms have been studied and implemented in guided wave reconstruction, including ray tomography, diffraction tomography, Hybrid Algorithm for Robust Breast Ultrasound Tomography (HARBUT) and full waveform inversion. In almost all inverse algorithms, a primary assumption is widely used: three-dimensional guided wave inversion of thickness is simplified as a two-dimensional acoustic wave inversion of velocity, with the dispersive nature of the waves linking thickness to velocity. This assumption considerably simplifies the inverse procedure, however, it makes it impossible to account for mode conversion in reconstructions. In reality, mode conversion is quite common in guided wave scattering with asymmetric wall loss (such as single sided corrosion), and compared to non-converted guided wave modes, converted modes may provide greater access to valuable information about the thickness variation, which, if exploited, could lead to improved performance. Geometrical full waveform inversion (GFWI) is an ideal tool for this, since it accounts for mode conversion. It reconstructs the geometry of defects by iteratively distorting the shape of elements at the boundary of the target following the negative direction of geometrical gradient computed from second order finite-element equations. Elastic wave propagation accurately captured in this inverse algorithm, so guided wave mode conversion therefore can be fully evaluated. In this paper, quantitative thickness reconstruction based on GFWI is developed in a plate cross-section and this method is applied to study the performance of thickness reconstructions using mode conversion. A series of defects, including simple Hann shaped defects and complex defects, are reconstructed in numerical simulations by this method, demonstrating strong performance in thickness reconstruction. Experiments are also carried out to show validity of the thickness reconstruction with mode conversion. This method can also be extended to 3D guided wave tomography for higher resolution thickness reconstruction.</p>

Quantitative Guided Wave Thickness Mapping Using Geometrical Full Waveform Inversion

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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