The use of Lamb waves for the control of thin-walled components and structures of complex shape makes it possible to diagnose at relatively large distances in comparison with bulk waves, and also has other notable advantages. However, in practice we have to deal with objects which often far from the classical elastic layer with free surfaces, for example, gas pipes laying on the sea bottom, covered with a protective asphalt layer thickness not less than one tenth of the thickness of the pipe itself. Differences between the normal waves in such systems from the classical Lamb waves are also important, as we have shown them to be considered. The wave disturbances with displacements in the sagittal plane are called below quasi-Lamb waves (or Lamb-type waves). We have studied the propagation of quasi-Lamb waves in a pack of several elastic and fluid layers. The dispersion properties of the propagating modes are investigated. Particular attention is paid to the case of an elastic layer coated with a layer of compressible viscous fluid; this case is of special interest for nondestructive testing. A modification of the matrix method for calculating wave fields that extends its range of applicability and to take into account the complex rheology of a viscous liquid of protective layer or layers is proposed. Complex dependences of the attenuation of quasi-Lamb modes from frequency (and the ratio of layer thicknesses) are studied. For certain types of surface load the amplitude-frequency characteristics of quasi-Lamb waves are determined. In the frame of the Tiersten - Krylov model we consider the effect of free surface with defects on the amplitude-frequency characteristics of normal modes.
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