The article describes an omni-directional multi-element transducer for selective Lamb wave mode excitation. It is composed of several coaxial ring-shaped piezoelectric elements actuated by n-cycled sinusoidal tonebursts. Mode selection is achieved by a special choice of the time delays and the amplitudes of the input driving signals. The method for the determination of these parameters is based on strict analytical considerations. In the limiting case of the infinite number of cycles and with a sufficient number of actuators it allows to generate only a single required mode, and to completely eliminate undesirable ones. It is shown that within the range of applicability of the simplest model for the piezoelectric elements, i.e. when the actuating force can be replaced by the concentrated forces applied at the locations of the edges of an element, no time delays are needed, and the required mode of excitation is achieved only by an approprite choice of the amplitudes of the input signals.

An integral equation based model for a system of piezoelectric flexible patch actuators bonded to an elastic substrate (layer or half-space) is developed. The rigorous solution to the patch-substrate dynamic contact problem extends the range of the model's utility far beyond the bounds of conventional models that rely on simplified plate, beam or shell equations for the waveguide part. The proposed approach provides the possibility to reveal the effects of resonance energy radiation associated with higher modes that would be inaccessible using models accounting for the fundamental modes only. Algorithms that correctly account for the mutual wave interaction among the actuators via the host medium, for selective mode excitation in a layer as well as for body waves directed to required zones in a half-space, have also been derived and implemented in computer code.

-A mathematical model of an electromechanical system excited by piezoceramic patch actuators is developed. The model is based on the solution to the dynamic contact problem for a set of flexible strips interacting with a free elastic layer. Unlike the conventional models, which describe the mechanical part by the dynamic equations for beams, plates, or shels, the proposed model, in addition to the first fundamental modes, also takes into account the higher normal modes of an elastic waveguide. Results obtained with the proposed model and with the simplified models prove to be in good agreement in the low-frequency range. Numerical examples illustrate resonance energy radiation associated with higher modes of the laminate strip-layer structure, as well as the possibility to control its directivity.

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The propagation of in-plane (P-SV) waves in a symmetrically three-layered thick plate with a periodic array of interface cracks is investigated. The exact dispersion relation is derived based on an integral equation approach and Floquet's theorem. The interface cracks can be a model for interface damage, but a much simpler model is a recently developed spring boundary condition. This boundary condition is used for the thick plate and also in the derivation of plate equations with the help of power series expansions in the thickness coordinate. For low frequencies (cracks small compared to the wavelength) the three approaches give more or less coinciding dispersion curves, and this is a confirmation that the spring boundary condition is a reasonable approximation at low frequencies.