Leakage and scattering of a leaky lamb wave by a structural discontinuity: Acousto-optics and FEM

This study focuses on the leakage and scattering phenomena of Lamb waves in liquids as the result of the insonification of the end of an immersed solid plate by means of a bounded ultrasonic beam at the Lamb angle. Both acousto-optics measurements and FEM simulations are presented and discussed. It is shown that the observed complex scattered field is composed of the leaking fields of mode-converted backward propagating Lamb waves. The geometrical position of the leakage indicates that the conversion process is completed very locally. On the other hand, it is observed that large zones of the immersion liquid become ´polluted´ with scattered ultrasound. An analysis performed in the spatial frequency domain furthermore revealed the presence of a nonlinear interaction between multiple overlapping acoustic fields. Since real structures have limited spatial dimensions, the current results indicate a possible source of misinterpretation during water-coupled non-destructive testing (NDT).