Rayleigh wave dispersion on vertically laminated composite surfaces

Experiments and calculations on Rayleigh wave dispersion in microstructured materials are reported. The microstructure takes the unusual form of vertical laminations at the wave propagation surface. The measurements have been performed by exploiting the phase matching condition for resonant mode conversion of acoustic waves to Rayleigh surface waves on a fluid-loaded half-space. The dispersive Rayleigh wave speed is found to approach the simple mixture value at low frequency and the velocity of the slower of the two media at high frequency. The experiments have been modeled with a continuum mixture approach, where the spatial dependence of the field variables in the plane of the wave propagation surface, but normal to the lamina interfaces and the sound wave vector, has been approximated by averaging these variables across the lamina thickness. This procedure results in additional frequency-dependent terms in the wave equation and corresponds to a substitution for the microstructured composite by a higher-order continuum that retains the dispersive nature of the original material system. Experiments have been performed over more than a decade in frequency. The results of the calculation show rather good agreement with the data over nearly all of its range, confirming the validity of the approximation for this case.