Mokryj O. Methodological background of diagnostics of the state of materials and structural elements by the velocity distribution of surface acoustic waves

The dissertation is devoted to creation of methods and means of determining the distribution of the surface acoustic waves velocity, suitable for non-destructive testing and technical diagnostics. New methods for determining the velocity of surface acoustic waves and their distribution on the basis of contactless laser and contact piezoelectric primary transducers are developed and created. A new simple method for stabilizing the operation of the Michelson interferometer under conditions of vibration and temperature drift is proposed. The method is based on creating an interference pattern on the surface of the photodetector in the form of parallel interference fringes. Based on the numerical simulation, as well as experimenta data, the effectiveness of the proposed scheme is confirmed. A new technique for determining the velocity of surface acoustic waves with a small measurement base, based on an extended probe laser beam has also been created. The proposed method allows us to reduce significantly the error of determining the velocity of the surface acoustic waves in the case of a measurement base of several mm. The methods of reducing the effect of transducer temperature change on the measurement of the surface acoustic waves velocity are developed. A method to reduce the influence of the surface irregularity on the control object as a source of error when using contact converters is proposed. An approach to optimally determine the velocity of surface acoustic waves distribution by creating a technique for carrying out the necessary measurements with a variable step, using a contact transducer, is developed. The criterion of optimization of the measurement step based on the characteristics of the research material and the parameters of the measuring equipment is formulated. The method of achieving the optimum measurement step is also proposed. Methods for determining the distribution of the surface acoustic waves velocity have been developed and created using an installation that allows measuring the passage time of the surface acoustic waves with an error of several nanoseconds. The developed methods have been tested during determination of the distribution of the surface acoustic waves velocity in plastic deformed and hydrogenated low-carbon steels. It is experimentally found that the velocity of the surface acoustic waves decreases in the case of plastic tensile deformation. Conducted studies of the porosity distribution show that with increasing the degree of deformation the porosity increases, and the surface acoustic waves velocity decreases too.