Focusing of flexural Lamb waves through gradient index phononic crystals
Bernard Bonello (Paris University and CNRS), Jinfeng Zhao (INSP - University Paris 6), Olga Boyko (INSP - University Paris 6), Remi Marchal (INSP - University Paris 6)
Joint session: Mechanics and Dynamics of Periodic Structures and Dynamic Behavior of Materials
Tue 4:20 - 5:40
Salomon 101
We report on a theoretical and experimental study of the focalization of zero-order flexural Lamb waves, through gradient index phononic crystals (GRIN PC). The structures are formed either by a squared lattice or by a rectangular lattice of air holes in a silicon plate. The gradient index results from the variation along one direction of the lattice spacing in the former case or of the size of the air inclusions s in the latter case. In both cases the acoustical refractive index features a hyperbolic secant profile which allows focusing free of aberrations. We used a finite elements method to calculate the displacements field within the GRIN PC. We show that focusing over spots with lateral dimensions close to the diffraction limit (0.5 ) are easily obtained with these systems. This value goes to about 0.8 when focusing outside the GRIN PC. We used a non-contact laser-ultrasonic technique both to excite flexural Lamb waves in the MHz range and to monitor in situ the displacements field within the GRIN PC. An efficient focusing of the waves is observed. The experimental data compare very well with the theoretical prediction for both GRIN PC structures geometries.