Barandych K. Technological providing of fatigue life of details at their turning processing

The dissertation is devoted to questions of technological assurance of fatigue life of the material of parts working in conditions of cyclic loads by optimization of their cutting parameters. The analysis of the influence of the surface layer of the parts operating under the influence of cyclic loads on their fatigue characteristics is presented. An overview of the methods for determining fatigue life, which showed limited information about the mathematical dependences of fatigue life of the part’s material from the technological conditions of its production, is complete. In carrying out dissertation researches the steel 40Х ГОСТ 4543-71 was chosen as a processed material. An experimental study of the surface layer’s parameters of the samples as roughness, microhardness and microstructure was carried out. The results allowed to establish that each of these parameters is sensitive to cutting parameters, which in the future affect on the fatigue life of parts under operating variable loads conditions. The mathematical dependences , and from the turning cutting condition were obtained in order to determine the complex influence of the surface layer quality of the samples on their fatigue strength after turning by processing the results of roughness and microhardness studies. These mathematical dependences have allowed us to create a complex index of the surface layer’s state of the samples , which increases with increasing feed rate and cutting speed on the studied range. This allows us to make an assumption about the corresponding increase of fatigue life. In order to study the influence of the technological conditions of the turning process on the change of the fatigue characteristics of the part’s material, samples of a round section of type I (GOST 25.502-79) were used. The turning process was performed on a turning center HAAS ST20 with a cutting tool PVVNN 2525M-16Q with a cutting plate VBGW 160404T00815SE without cooling in the range of cutting speeds from 80 to 180 m/min. and feeds – from 0.08 to 0.12 mm/rev at a cutting depth of 0.3 mm. Fatigue tests were being carried according to GOS T 25.502-79 was used. The mathematical dependence of the fatigue life of the 40X steel part from cutting conditions and cycle stress was developed on the results of experimental studies on fatigue and it’s processing. Analysis of this dependence showed that the fatigue life of turning finished specimens from steel 40X for feed rate values from 0.08 to 0.12 mm/rev, cutting speed from 80 to 180 m/min and the depth of cutting 0.3 mm increases with increasing both feed rate and cutting speed. This corresponds to the obtained mathematical model of the complex index of the part’s surface layer and allows confirming the reliability of the fatigue life mathematical model for the 40X steel part. The productivity of the finishing turning process was used as an optimality criterion in solving the task of technological support fatigue life of parts. The determination of rational values of cutting speed and feed rate was carried out from the range of acceptable solutions. This range is defined by a plurality of constraints on the turning process, and also includes an operating limitation on the number of cycles before fatigue failure of the part. Thus, a turning process mathematical model of the 40X steel parts was created. To extend the using range of the mathematical model of fatigue life on the parts, which are made from structural materials of alloyed chromium steels group, which includes steel 40X, the method of taking into account the characteristics of any material of this group is used to obtain the appropriate mathematical model of fatigue life. In order to solve the task of optimizing the parts cutting conditions, the corresponding software was developed in the C # programming language. The finite-element analysis in the framework of the software complex FEMAP 10.2.0 was carried out to determine the maximum stresses in the material of the component under conditions of its operation. With the purpose of technological providing the necessary fatigue life of the part, taking into account the real conditions of its operation for the maximum productivity of the finishing turning process, the methodical recommendations for determining the rational parts cutting conditions at the phase of technological preparation of production are given. As an example of using the proposed methodical recommendations, the task of the technological support of fatigue life of the "Swing mechanism shaft" part (steel 40Х GOST 4543-71), which operates under cyclic loading conditions, was solved. The main results of the dissertation researches are tested on the example of parts, which are manufactured at PJSC "RPA " Kyiv Automatics Plant", Kyiv.