Zero Group Velocity and backward Lamb modes

Claire Prada (1) , Franck Philippe (1) , Dominique Clorennec , Maximin Cès (2) , Daniel Royer (2)

1 IL - Institut Langevin - Ondes et Images (UMR7587)
2 LOA - Laboratoire Ondes et Acoustique (UMR 7587)

Laser ultrasonics techniques are very useful to investigate Lamb modes in plates. In this talk, the remarkable properties of zero group velocity (ZGV) and backward Lamb modes will be discussed and illustrated through measurements done with nanoseconds laser ultrasonics. These modes originate from the repulsion between two dispersion branches having close cut off frequencies, corresponding to a longitudinal and a transverse thickness mode of the same symmetry. The lowest branch exhibits a minimum corresponding to the ZGV mode and a negative slope associated to backward propagation. The frequency spectrum of the plate elastic response to a local impact is entirely dominated by ZGV resonances and for isotropic plates the set of resonance frequencies provides an accurate measurement of the local Poisson's ratio. When either the transverse or the longitudinal acoustic velocity is known, plate thickness can be precisely determined. Backward modes, having negative phase velocities, exist in the vicinity of a ZGV mode. It will be shown that mode conversion from the S2 forward mode to the S2b backward mode can be achieved at a plate thickness change. Then, negative refraction and focusing can be obtained with a simple flat lens.

Mots clés: Lamb mode; zero group veloctiy; resonance; backward wave

Domaines: Acoustique [physics.class-ph]

Claire Prada, Franck Philippe, Dominique Clorennec, Maximin Cès, Daniel Royer. Zero Group Velocity and backward Lamb modes. Acoustics 2012, Apr 2012, Nantes, France. ⟨hal-00811250⟩