Imaging surface acoustic waves in anisotropic and periodic media.

摘要

A new technique has been developed for studying surface acoustic wave propagation. The technique, which uses non-contact immersion transducers to excite and detect ultrasound with frequencies between 1 and 20 MHz, allows convenient examination of surface waves traveling in arbitrary directions on a sample. Coupling the easy scanning of ultrasound propagation with time- resolved detection of the surface-wave amplitude makes this method particularly suited to examining anisotropic materials.; The first section of this thesis deals with surface waves on crystals and other homogeneous solids. The general theory describing surface-wave behavior on anisotropic materials is adapted to our technique. In anisotropic materials, two types of surface waves, designated Rayleigh surface waves and pseudo-surface waves, satisfy the boundary conditions for a free surface. The experimental design is then described, and images obtained on a variety of crystalline samples are examined. Discrepancies between the calculations and the observed results have led to an investigation of the effects of fluid loading on the surface waves. It is found that coupling between the surface waves and the loading liquid will attenuate the propagating surface waves, with high frequencies showing stronger attenuation. In addition, the loading will affect the velocities of the surface waves, and in some cases will support additional surface-wave modes.; The remainder of the thesis examines surface-wave propagation on a variety of periodic structures. Carbon-fiber/epoxy composites are first examined, and the elastic properties of a homogeneous sample are used to determine the surface-wave velocities of layered samples in the long-wavelength limit. Two systems are then examined in the dispersive regime, where the wavelength of the surface waves is comparable to the periodicity of the sample. The first, a multilayer sample constructed from layers of aluminum and a low-density polymer, has one-dimensional symmetry. Calculations of the acoustic band structure for acoustic waves traveling in this material, and the resulting surface-wave profiles, are compared to the experimental results. Analysis of this sample is complicated by total internal reflection, which occurs at the composite/aluminum interface for a range of propagation directions. The second sample studied is a two-dimensional lattice with hexagonal symmetry. Calculations of surface-wave propagation in this type of sample show that, for certain combinations of materials, a complete acoustic band gap can be obtained. Experiments performed on two samples with this geometry are then compared to the expected surface-wave behavior on these structures.