Implications of structural discontinuities for water coupled NDT and SHM: experiment and modeling

This study focuses on the leakage and scattering phenomena of Lamb waves in liquids as the result of an insonification of the end of an immersed solid plate by means of a bounded ultrasonic beam at the Lamb angle. Acousto-optical experiments are presented, in both the real space and spatial frequency domain. Due to the principle of mode-conversion, distinct scattered beams are observed, propagating along various leaky Lamb angles. Their geometrical position reveals the local completion of the conversion process. Experimental results obtained in the spatial frequency domain indicate the presence of 'extra' spikes which can be linked to non-linear effects between multiple overlapping leaking fields. The experiments furthermore reveal that the leakage and scattering phenomena 'affect' relative large zones of the immersion fluid, especially when quasi-longitudinal Lamb modes are excited. Because realistic structures always have limited dimensions, this observation is of importance in the field of water-coupled non-destructive testing (NDT) and structural health monitoring (SHM), to avoid possible misinterpretation of the recorded signals