Identification of corrosion damage in submerged structures using fundamental anti-symmetric Lamb waves

Corrosion is a representative modality of damage in metallic structures serving in humid or corrosive environments, and examples include petroleum pipelines immersed underwater or buried underground. To facilitate awareness of corrosion at its initial stage is a key measure to prevent further deterioration and failure of these structures. A damage identification approach capitalizing on the fundamental anti-symmetric Lamb wave mode (A0) and in terms of a pulse-echo measurement scheme was developed for evaluating corrosion in submerged structures. However the presence of a coupled fluid medium and changes in its properties exert influence on the propagation characteristics of the A0 mode in the structures at a phenomenal level, leading to erroneous identification without appropriate rectification. Allowing for this, the effect arising from fluid coupling on the A0 mode was investigated and calibrated quantitatively, whereby rectification and compensation were applied to the identification. The proposed approach was numerically and experimentally validated by evaluating through-thickness hole and chemical corrosion in submerged aluminium plates, with the assistance of a probability-based diagnostic imaging approach. Identification results have demonstrated the necessity of rectification and compensation for the medium coupling effect when applying Lamb-wave-based damage identification to structures with coupled media.