Displacement field of an insonified thick elastic plate

The displacement field engendered within an infinite, water-immersed, steel plate, 0·55 shear wavelengths thick, by insonification by a plane wave at arbitary incidence angle, was examined in a previous paper [1]. For that plate the evolution of the interior field with increase of incidence angle from near normal incidence to near grazing incidence was investigated and related to the characteristics of the plate's reflection and transmission coefficients. The behaviour was shown to be especially influenced by the three Lamb modes which can be excited for that plate thickness. The present paper is a report on a similar investigation for a plate thickness of 2·5 shear wavelengths, at which thickness nine Lamb modes can be excited, with consequent increase in the complexity of the evolution of the interior and exterior fields. Diagrams are presented illustrating in detail this evolution of the plate's displacement field, and explanations for the complex changes of structure with incidence angle are provided. At corresponding modes, the behaviour of the displacement field is shown to be very different at the two plate thicknesses. As in the case of the thinner plate, so too for the 2·5 shear wavelengths thick plate is the motion at the coincidence angles found to be neither totally antisymmetrical nor symmetrical, and an explanation for this behavior is given.