The Bauschinger effect is one of the fundamental properties of most metal alloys exposed to plastic deformation under non-monotonic loading. Development of the methods for quantifying this effect is one the important issues of the theory of plasticity. Calculation of the parameter characterizing the aforementioned effect is required for determination of the stress state in plastically deformable blanks upon pressure metal treatment. The value of the parameter (determined in standard tensile tests followed by subsequent compression of samples) is defined by the ratio of the conditional yield strength of the sample under compression to the value of the preliminary tensile stress. A series of cylindrical samples (~10 pcs.) is usually taken for tensile-compression tests. According to the traditional procedure, long-size standard specimens are pre-stretched to various degrees of plastic deformation. After that short specimens are cut out from those specimens for compression tests to determine the conditional compressive yield strength with a tolerance of 0.2% for plastic deformation. Such a procedure is rather time consuming and expensive. We propose and develop a new single-model method for estimating the Bauschinger effect which consists in testing of a single long-size specimen for tension followed by compression of the specimen in a special device providing deformation of a previously stretched specimen without flexure under conditions of a linear stress state. The device was designed, manufactured and underwent the appropriate tests. The device contains supporting elements in the form of conical-shaped sectors that prevent flexure of a long cylindrical specimen upon compression, a ratio of the working part length to diameter ranges from 5 to 10. The results of experimental determination of the parameter β characterizing the indicated effect are presented. The results of comparing the values of the parameter β determined by the developed and traditional methods revealed the possibility of determining the parameter β using the proposed method. To reduce the complexity of performing tests related to determination of the parameter β we approximated it in the form of an exponent as a function of the magnitude of plastic deformation and determine the only one value of β0 under plastic deformations exceeding 0.05. In this regard, β0 can be considered a new characteristic of the material. The calculated data are in good agreement with the experimental results. The values of β0 are determined for a number of studied steel grades.

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A design of the device for studying plastic compression of long cylindrical specimens under conditions of a linear stress state is presented. The device is developed to study the properties of metals under plastic deformation in conditions of nonmonotonic loading. The goal of getting the accurate experimental data entails the necessary of carrying out tests using one long-length cylindrical sample, with the calculated length being more than five diameters. To prevent flexure of the long-length sample upon compression, the support conical sectors made by cutting truncated cone shaped blank into 6 or 8 equal parts with a central longitudinal hole having a diameter equal to the diameter of the test sample are used. The sectors are coupled by two pairs of semirings. A transcendental equation is obtained for determination of the taper angle of those sectors on the basis of kinematic analysis of the mobile links. This angle depends on the total weight of the sectors and sliding friction coefficients in the corresponding kinematic pairs of the device. For the considered device, the taper angle of the sectors is 36°. This device is designed and manufactured for compression testing of the samples with a diameter of 16.5 mm and a gaged length of 135 mm. Samples from steel 45 are tested with a goal of the flow curve construction and experimental verification of the strain diagrams under conditions of cyclic tensile-compression. Comparison of the calculated and experimental data proved the satisfactory accuracy of the stress determination, which makes it possible to recommend this device as a testing tool to be used in mechanical laboratories of the universities and research institutes.

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Processing of metals by pressure suggests using of various measuring devices. We developed and manufactured a device for measuring angular deformations upon elastic and plastic torsion of circular specimens. The design is based on the operation principle of the planetary mechanism without a movable solar central wheel. Design of the device is based on the Boyarshinov concept added with a round protractor with a graduated scale from 0 to 360° and rotating needle placed on the axis to indicate the angular displacement. The use of the planetary gear significantly improves the accuracy of measuring the angles of rotation of the cross sections of the sample relative to each other at a distance of the calculated length. To low the weight of the device, the main parts were made of aluminum alloy D16Т. Rated diameters of the central wheel and satellite are D1 = 220 mm, D3 = 20 mm, respectively; the linkage module m = 1 mm; the number of teeth on the wheel and satellite is 220 and 20, respectively. Laboratory tests of the device were carried out on a KM-50 torsional machine using a cylindrical sample with a diameter of 15 mm and a working length of 120 mm made of steel 40Kh. The results provided determination of the elastic shear modulus with a deviation of ~2.5% of the reference value. Experimental data were used to plot the torsion diagram and then to get shear diagram τ = τ(γ) according to P. Ludwik’s formula. This diagram was transformed into the hardening curve σ = σ(e) using von Mises theory of the plasticity. The obtained mechanical characteristics allowed us to draw a conclusion on the essential accuracy of measuring angular displacements using the developed device. Thus, the developed and easy to use device can be recommended as testing equipment for determination of the mechanical characteristics of materials under conditions of shear deformations.