Interferences in the re-emission field of multimodal leaky lamb waves propagating in an immersed plate: Analytical modelling, simulation and experimentation

Abstract

The transmission and re-emission of energy by leaky Lamb waves from an immersed plate is being studied in the case of multi-modal propagation. When more than two Lamb modes propagate together, interferences can be generated in the re-emitted field. The latter have to be well understood and mastered because they can distort the diagnosis of a Non-Destructive Testing of an immersed plate. They are experimentally observed with C-scan visualisation, but they are not consistent with the Bertoni and Tamir model which describes the theoretical re-emitted acoustic field by leaky Lamb modes excited by an incident ultrasonic beam. The same interferences are also observed by finite-element simulation. The Bertoni and Tamir model is then modified to take into account the incidence of side lobes generated by an immersed ultrasonic transducer. That way, the same interferences observed are theoretically predicted, confirming that side lobes are the cause of it. Side lobes can generate a second leaky Lamb mode which propagate and interfere with the mode generated by the main acoustic beam. This secondary Lamb mode is non-negligible if it has higher leaky attenuation than the targeted mode. Thus, to avoid multi-modal generation with an incident beam, the best method is to aim at the leaky Lamb mode with the highest leaky attenuation coefficient at the chosen frequency.