Experimental and theoretical study of Lamb wave dispersion in aluminum/polymer bilayers

This paper deals with the dispersion of Lamb waves in solid bilayers obtained by coating a 0.61-mm-thick aluminum plate with a polymer of thickness between 180 and 500 μm. The theoretical dispersion curves of the bilayers in air are analyzed in terms of the dispersion in the decoupled layers, the aluminum substratum, and the polymer coating, respectively. Because of the smaller acoustic impedance of the polymer coating, it behaves in the bilayer as if it were rigidly held by the aluminum substratum. The side of the polymer facing air is considered to be stress-free. The decoupled aluminum plate is modeled as if it were stress-free on both sides. The approximate boundary conditions for the coating are verified by a separate experiment. The experimental data obtained by means of a double transmission immersion technique, confirm the numerical analysis. Not all theoretically predicted modes are observed. The results show how coupling between the Lamb modes of the aluminum substratum, and those of the polymer coating, makes the bilayer switch between modes. Displacement and stress profiles are used to further elucidate the analysis. In the last paragraph a simple mechanical model composed of two masses and three springs, is analyzed. It exhibits mode coupling and mode switching in a way which is very similar to that found for these solid bilayers.