Characterization and Imaging of Thin, Highly Oriented Layers: Theory and Experiment

Abstract

The adhesion problem has been, for many years now, of great interest to many in the ultrasonic NDT community. One if the great difficulties of using ultrasound in the characterization of adhesive bonds is the fact that in many instances, the weakness in the bond lies in the adhesion region itself [1,2]. Because this region is much narrower than the acoustic wavelength, however, it’s impact on a propagating wave is small and poorly understood. In this work, we seek to shed some light on this situation, and others like it, using both theoretical and experimental models. The model is based upon a general approach which allows one to write down a set of “boundary conditions” treating the layer as a single interface in the framework of continuum mechanics. Although this idea was also developed earlier by, for example, Rokhlin and Huang [3,4], our independent derivation [5] may be somewhat simpler and more intuitive. This approach allows for very general interface properties, including anisotropy and nonlinearity of very thin layers or interfaces to be explored.