Baseline-Free Orthogonal Imaging for Rapid Corrosion Screening via Laser Ultrasonics

Ultrasonic guided-wave imaging for corrosion inspection is often constrained by the need for baseline measurements or dense spatial acquisition followed by image reconstruction. To address these limitations, this study proposes a rapid, baseline-free damage detection and localization framework for thickness-loss defects in plate-like structures. The method employs two orthogonal sets of propagation paths to screen the inspection region without dense full-wavefield acquisition. Based on the distinct responses of the damage-sensitive A1 mode and the comparatively damage-insensitive A0 mode to localized thickness loss, a relative modal-energy index is defined for each propagation path without requiring a pre-acquired healthy-state guided-wave signal. The resulting path features are processed using Maximum Gradient Separation (MGS) for adaptive threshold determination, followed by a Spatial Connectivity Constraint (SCC) to identify spatially consistent damage-affected paths. The identified damage-affected paths from the two scan directions are then intersected to determine a two-dimensional bounding region of the damage. Numerical simulations and experiments on an aluminum plate confirm that the proposed framework can identify the damage-affected path ranges and localize the thickness-loss region. These results demonstrate the feasibility of the proposed baseline-free orthogonal imaging strategy for rapid preliminary corrosion screening.