Image Reconstruction of Corrosion under Coating Film by Dynamic Shear Strain Analysis of Lamb Waves

Most of artificial structures are made by carbon steel plates, and are facing the possibility of corrosion. Paint coatings are applied to the surface of the steel plate structures in order to resist rust and corrosion. However, because of long-term usage, rust progresses from small scratches of the coating film and is leading to debonding of structure. In last decades, Lamb waves have been studied intensively as sensitive tools to detect subsurface damages in large areas. However, these media are often characterized by multimodal and frequency dispersive behavior. This implies that the received signals can be expressed as a sum of wave modes that travel at different frequency dependent velocities. Consequently, these characteristics make it difficult to reconstruct images of damages by Synthetic Aperture Focusing Technique (SAFT) or Time of Flight Diffraction Technique (TOFD). Thus for non-destructive testing using a guided wave, it is an important issue to construct an imaging technology for extracting the information of the flaw from the observed signals. This paper proposes a novel method for reconstructing image of the corroded region under coating films. The proposed method has an ability to characterize the corroded region through evaluating the linearity between the orthogonal pair of out-of-surface shear strains. Acoustical experiment has been carried out to validate the outcome of the method. The squared shape specimen is made of S45C steel. Employing laser interferometer, single A0 mode Lamb wave has generated to excite the model structure having a single hollow shaped corroded region under aluminum film.