Mokryy O.M.

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An important problem in geotechnics, seismology, non-destructive testing and fracture mechanics is the evaluation of near-surface material characteristics (such as elastic moduli, porosity and hardness) as a function of depth. This problem is particularly relevant for assessing the quality of various metal treatments, hydrogenation and coating. An adequate approach to solving this problem involves the use of surface acoustic waves of different frequencies. This is due to the fact that surface acoustic waves penetrate at a depth of the order of their wavelength. Therefore, waves of different frequencies cover different thicknesses of the near-surface layers, and multi-frequency probing makes it possible to evaluate the characteristics of these layers as a function of depth. Deductive and inductive methods for estimation of the acoustic properties in the material near-surface layers based on multi-frequency sounding by surface acoustic waves are analyzed. Deductive (rigorous) methods are based on the Lame equations and are developed for media with vertically varying parameters or plane-layered structures. The application of these methods is reduced to the variational problem of adjusting the medium parameters until the theoretically determined dispersion curve of the Rayleigh wave phase velocity coincides, with the required accuracy, with the experimentally obtained one. These methods are applicable only to the simplest medium models and generally cumbersome and ambiguous and do not allow the real-time evaluation of material properties, i.e. during the probing. Approximate (inductive) methods based on harmonic and weighted averaging of the Rayleigh wave phase velocity over depth within the wavelength are developed. Their main advantage is the application simplicity. The specific features of these methods for a single-layer coating are considered. An analytical expression for the dispersion curve in the case of a simplified version of the weighted averaging method is obtained, showing that the dispersion curve of the Rayleigh wave phase velocity for a single-layer surface coating is a straight line. This agrees well with the known regularities describing the influence of different types of surface treatment, degradation and coatings on the dispersion of the Rayleigh wave phase velocity. The sign of the slope of the dispersion curve (positive or negative) indicates the stiffness of the coating relative to that of the substrate. The magnitude of the angular slope of the dispersion curve makes it possible to estimate the thickness of the treated surface layer.

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