Interface Characterization by True Guided Modes

Abstract

<p>Guided acoustic waves along interfaces are especially sensitive to specific properties associated with boundary conditions and bond quality since their energy is effectively confined to the region of interest. On the other hand, this inherent advantage turns out to be a significant drawback for generation and detection of such guided waves. There are two basic types of propagating interface modes, which are shown schematically in Figure 1. First, there are leaky modes with higher phase velocity than at least one of the bulk velocities in the surrounding media. These modes “leak” their energy into one or more phase-matching bulk modes as they propagate along the interface and they can be readily excited by these mode-coupled bulk modes at the same incidence angle. In other words, the energy of leaky interface modes is not strictly confined to the boundary region therefore they are relatively easy to generate and detect. Because of their relatively short propagation length, leaky interface modes provide localized information on boundary properties and possible imperfections, which can be taken advantage of in ultrasonic NDE of bonded structures [1].</p>