1HS Offenburg, Gengenbach, Germany
Surface and interface acoustic waves are two-dimensionally guided waves, as their displacement field is plane-wave like regarding its dependence on the spatial coordinates parallel to the guiding plane, while it decays exponentially along the axis normal to that plane. When propagating at the planar surface or interface of homogeneous media, they are non-dispersive. Another type of non-dispersive acoustic waves which is, however, one-dimensionally guided, has displacement fields localized near the apex of a wedge made of an elastic material. In comparison to surface acoustic waves, these wedge acoustic waves have the advantages of being diffraction-less and having lower speeds. In this review, their propagation properties are described and compared with those of surface and interface waves. Also, potential applications will be discussed.
Apart from work done within the geometrical acoustics approximation, analytic results for the calculation of the velocities and displacement field of these waves have been achieved only for sharp-angle wedges. Numerical approaches will be reviewed that are in use for the treatment of arbitrary wedge angles. It is shown how perturbation theory can be used for a quantitative determination of the dispersion of these waves due to modification of the ideal wedge geometry like truncation or rounding of the wedge tip and coating of the surfaces. Likewise, an overview will be given of the experimental methods that have been used to excite acoustic waves at wedges and to detect their associated strain fields. These methods involve piezoelectric and interdigital transducers as well as photoacoustic excitation and scanning techniques.
For SAWs in anisotropic elastic media, surface wave existence theory has established a general theorem that guarantees the existence of a subsonic surface wave for almost all propagation geometries. In the case of wedge acoustic waves, such a theory does not exist. However, numerical studies, which will be discussed in this presentation, indicate that the influence of anisotropy is not less pronounced in the case of wedge acoustic waves. For example, these studies reveal that acoustic waves with clearly detectable edgelocalisation do not exist for many orientations of rectangular edges in anisotropic media. Furthermore, wedge acoustic waves have specific nonlinear properties as compared to surface or bulk acoustic waves, which will shortly be discussed, too.
The potential of wedge acoustic waves for applications has been demonstrated in several fields of technology, including nondestructive testing. In view of the progress seen over recent decades in fabrication techniques for high-quality surfaces and microstructures, it may also be worth reconsidering edge acoustic waves in the context of signal processing devices and sensor applications.