Analysis of Guided Waves in a Bilayered Plate

Abstract

Inspection of coated material is vital in order to ensure the integrity of the protective barrier. In some cases, the inspection process is complicated by the fact that the surface of the protective coating may not be accessible, thus the inspection must proceed with only access to the surface of the opposite side (this will be referred to as the inner surface). One method which can be applied in such a situation is the excitation of guided waves or Lamb waves in the coated material. Lamb waves excited from the inner surface will sense the variation in the coating conditions as well as flaws in the steel plate. Therefore for a correct and unambiguous interpretation of Lamb wave data for corrosion-related flaws, the effects of the coating and how they differ from the effects of corrosion-related flaws must be understood. To this end this paper will concentrate on the effects of the coating and its influence on dispersive characteristics of a soft layer bonded to a steel plate. By calculating the dispersion relations for a bare and coated steel plate and comparing the calculated results, unique new modes are seen to emerge in the coated plate. In addition, the coated plate is also modeled as a thin layer on a halfspace allowing a comparison between the problem of a coated halfspace and a coated plate. The comparison demonstrates that only limited information can be obtained by treating the problem as a layered halfspace. More detailed information can be obtained by treating the problem as a coated plate of finite thickness.