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ABSTRACT

Semak P. M. Evaluation the condition of the material surface layers by
measuring the velocity of surface Rayley waves with different frequencies -
Manuscript.
The thesis is submitted for Ph.D. degree in technical sciences, speciality 05.02.10 -
diagnostics of materials and constructions - Karpenko Physico-Mechanical Institute,
Lviv, 2020.
The dissertation work is aimed at solving important scientific and technical
problems of materials and designs diagnostics, namely – development of methods for
estimating the inhomogeneous near-surface layer of material for thickness based on
measuring the propagation speed of Rayleigh waves with different frequencies.
Degradation and almost all types of material processing begin with the modification
of the surface layer, and then change the properties of the material on the internal volume
of the sample (we are talking about mechanical processing, in particular hardening,
thermal, flooding, coating). Naturally, the urgent task is to estimate the parameters of the
inhomogeneous near-surface layer of material in thickness. To solve this problem, it is
adequate to use surface acoustic Rayleigh waves at different frequencies. This is due to
the fact that the depth of penetration of the surface acoustic Rayleigh wave depends on
the frequency of the probing signal, and the speed, attenuation and "structure" of the
Rayleigh wave are associated with mechanical, thermal and other characteristics of the
surface layer of material in which it (wave) is propagating. Therefore, it is possible to
obtain information about the state of the surface layer of the material in thickness based
on the speed, attenuation and other parameters of Rayleigh waves at different
frequencies.
An analytical review and systematic analysis of literature sources about modern
methods and means of measuring the parameters of surface acoustic Rayleigh waves was
done. Restriction on the use of surface acoustic Rayleigh waves of different frequencies
to determine the parameters of inhomogeneous near-surface layers in thickness was set.
It is estimated the thickness of the material layer, which can be characterized by
determining the velocity of the Rayleigh wave of a certain frequency. This analysis is
based on the depth distribution of the Rayleigh wave energy. A criterion is proposed
according to which wave velocity is determined by a layer of material with a thickness of
0.6 λ (λ is the wavelength).
To ensure the required resolution of measuring of changes in the surface acoustic
Rayleigh wave parameters in thickness, it is justified the use of long-term probing radio
pulses, which are narrowband. The width of the allowable spectrum of Rayleigh wave
acoustic pulse and its connection with depth separation are analyzed. It is shown that for
high separation it is necessary to use probing acoustic pulses with duration one and half
orders of magnitude for the period of high-frequency signal filling.

A method for measuring the velocity of surface acoustic Rayleigh waves based on
the use of long-term probing radio pulses has been developed. Rigidly connected
piezoelectric transducers are used to measure the speed of Rayleigh surface acoustic