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This paper is inspired by an earlier work [1], [2], which introduced a fluid-coupled waveguide for guided waves in the frequency range from 1.25 MHz to 5 MHz. It consists of two parallel solid plates separated by a liquid layer. Experiments indicate that this type of waveguide is capable of sustaining waves over large distances without significant losses. Possible applications include non-destructive testing and position measurement. In this paper, the wave modes propagating in the waveguide are studied by means of finite element simulations and experiments. First, the simulation model is described and validated for the case of pure plate waves. The obtained dispersion diagrams are in good agreement with the well-known theory for Lamb waves. Furthermore, the first simulation and experimental results for the complete waveguide setup are presented. Sample dispersion diagrams are provided, and it is demonstrated that the waveguide sustains slowly propagating waves. Finally, the effect of the incidence angle of the wedge transducer on activation of various wave modes is briefly discussed.

Some applications require structural health monitoring in inaccessible components. This paper presents a technique useful for Structural Health Monitoring of double wall structures, such as double wall steam pipes and double wall pressure vessels separated from an ultrasonic transducer by three layers. Detection has been demonstrated at distances in excess of one meter for a fixed transducer. The case presented here is for one of the layers, the middle layer, being a fluid. For certain transducer configurations the wave propagating in the fluid is a wave with low velocity and attenuation. The paper presents a model based on wave theory and finite element simulation; the experimental set-up and observations, and comparison between theory and experiment. The results provide a description of the technique, understanding of the phenomenon and its possible applications in Structural Health Monitoring.