Generation and propagation of Lamb waves in multi-plate structures: contribution of time reversal for ultrasonic non-destructive testing of generation IV reactor vessels

The work of this thesis is in the context of a non-destructive inspection of the internal structures of the primary vessel of a generation IV reactor, using leaky Lamb waves generated on the surface of the primary vessel. The goal is to use time reversal to locate defects in two immersed stainless steel plates parallel to each other. The generation and propagation of leaky Lamb waves in a free immersed plate are first studied, as well as different tools allowing their analysis. In a second step, several ultrasonic imaging methods are studied, and more specifically those employing time reversal, including topological energy which is then applied on experimentally acquired signals. Two types of defect (a through crack represented by the edge of the plate, and a notch) are detected and localized in a single immersed plate thanks to topological energy in the time domain. The case of two parallel immersed plates is finally experimented. The topological frequency energy, less expensive in term of computation time, allows to detect the edge of one of the two plates according to the considered mode. (author)