Internal and external fields of insonified plates along lamb mode branches of zero order

Abstract

A detailed investigation is presented of the reflection and transmission coefficients and of the internal displacement field along the branches of the A 0 and S 0 Lamb modes of a water-immersed, infinite steel plate insonified by a plane wave. The extensive numerical results presented are based on exact elastic theory. The results for the transmission and reflection coefficients exhibit some previously unknown phenomena in the low fd regions of both zero order branches. For example, it is demonstrated that a plate only 0·0001 shear wavelengths thick can behave opaquely over a small angular region near the S 0 branch. A Lamb mode is shown not to imply a fixed pattern of vibration, but one which evolves along the branch. This evolution is interpreted in terms of the concept that the behaviour along such a branch is influenced by other modes besides the local one. Displacement magnitudes are encountered within the plate which are up to the order of 100 times the displacement magnitude of the incident wave.