Generation of a backward Lamb wave by means of mode conversion

This experimental work deals with the reflection of a Lamb wave at the free edge of a thin metal plate. A wave packet of the symmetric first mode S1 is generated with a wedge transducer on a plate and mode conversion phenomena at the free edge is observed. Under experiment conditions, the incident wave packet is totally converted in a reflected wave packet of different characteristics. Actually, the reflected wave packet belongs to the same symmetric mode S1 but it is generated in a zone of the dispersion curve in which the group velocity is counter-directed with respect to phase velocity so to become a so called backward Lamb wave. While the energy sent to the edge of the plate is reflected back, the wave vector direction is not inverted by means of the reflection. The efficiency of mode conversion is high and the backward wave packet reflected has an amplitude even greater than if it would be generated under the same conditions directly by the wedge transducer.