Determination of Stresses and Strains in Elastoplastic Deformed Body from Hardness Characteristics

Abstract

Introduction. At the present time, the problem of determining current values of strains and stresses in the case of plastic deformation of metal is very topical since it is rather difficult to calculate these characteristics by the plasticity theory methods. Such data are required for the determination of stresses in the body surface layers, optimization of the manufacturing process, improvement of quality and for the design of products of optimal weight.To determine stresses in an elastoplastic deformed material in different stages of manufacture of products and to control their mechanical properties during operation, the static and dynamic hardness method is widely employed [1,2]. The hardness method appears to be most suitable in terms of the ease of realization, convenience, low cost, availability of instrumentation and the possibility to carry out research without loss of product integrity.The aim of this work is to further develop nondestructive testing methods, in order to improve the reliability of determination of current stresses in plastically deformed metal from hardness values.Procedure and Test Results. It is known that hardness characterizes such properties of materials as strength and ability to resist plastic deformation or brittle fracture or brittle fracture on local contact interactions with a harder body, indenter [1]. If one has a calibration curve of hardness against stress level, it is proposed to determine stresses in the plastic domain from hardness distribution [3].However, in spite of obvious advantages, the classical version of the hardness method is insufficiently informative since hardness is in most cases only slightly sensitive to many types of structure transformation [4], and the correlation of hardness values with other mechanical characteristics is not always stable and unambiguous [5].Analysis of the results of the research carried out for technical applications using the hardness method shows that it is very difficult to improve reliability of correlation between hardness values and other mechanical characteristics, such as the ultimate and yield strength of material [3,6,7]. Note that the more plastic the material, the less exact will be the derived relationships between hardness and strength.The ambiguity of variation of metal hardness as a function of the strain level reached has been established experimentally from the test results for specimens of steels of different classes: St. 3, 20KhN3, 17G1SA, 40Kh, stainless steels, titanium alloys and V95 aluminum alloy under short-time static, sustained and cyclic loading conditions. The hardness of metals changes only slightly under the above loading conditions; moreover, it may both increase and decrease [8][9][10]. Similar results were obtained when investigating metalworks of lifting machines in the case of high-cycle fatigue [11].It may be noted that hardness as the most common characteristic of material, which determines its quality and the possibility to use it in constructions under different operation conditions, is amb...

Method of Performance Assessment of Lubricant Compositions Containing Nano-Additives Used for Wheel–Rail Pair

“…According to the results, there was no correlation between the increase in mean hardness ∆ and the value of maximum wear (Figure 5, a). This finding confirmed the conclusions of [29][30][31][32][33][34]. At the same time, the ∆ -∆ℎ dependence can be described by the slightly nonlinear smooth function if the data for the only nonmetallic additive (graphite powder) is disregarded in the series of experiments (Figure 5, b).…”

“…The results of experimental studies of damage accumulation processes in metal materials of different grades under cyclic, short-term, and long-term static loading are presented in [29][30][31][32][33][34]. The damage level was evaluated on the basis of the statistical distribution parameters of the hardness measurement data and its mean value.…”

“…Figure 2 shows the relationships between the relative value of the homogeneity coefficient and the magnitude of the deformation of the materials of different grades. The graphs are based on the data provided in [33]. The mechanical properties or structural condition measurements of the samples characterize the current damage capability of the material Π(Τ ) , where Τi represents certain parameters characterizing the level of operational load (in our case it is the length of the sliding path).…”

Express Method for Assessing Performance of Lubricant Compositions Containing Nano-Additives Used for Wheel–Rail Pairs

“…The results of experimental studies on damage accumulation processes in metal materials of different grades under cyclic, short-term, and long-term static loading are presented in [ 35 , 36 , 37 , 38 , 39 , 40 ]. The damage level was evaluated based on the statistical distribution parameters of the hardness measurement data and the mean hardness value.…”

“…However, if there were no phase transformations or changes in the chemical composition or density of the material’s surface layers during thermo-mechanical loading, the average hardness value would change only slightly. At the same time, the scatter of hardness measurements would increase with the growth of the operating time parameter, the choice of which would depend on the type of experiment (in uniaxial static tension—stress or strain values, in cyclic loading—number of cycles and maximum cycle stress, and in long-term loading—stress and strain values and creep values) [ 35 , 36 , 37 , 38 , 39 , 40 ]. The data allowed the authors to substantiate the main idea of the method for an assessment of material damage, which is based on determining the correlation between the statistical dispersion parameters of the hardness measurement during large-volume tests and the structural material’s operating time.…”