Torsion tests allow studying the rheological properties of materials over a wide range of strain and strain rate values, as well as temperature. A key moment in construction of hardening curves is the interpretation of experimental data, which are usually the torque – angle of twist curves. However, there is a number of independent methods to obtain interpretation of experimental data. In addition, there is no single approach to determining the degree of equivalent strain in torsion test. The aim of this paper is to review existing hardening curves definition methods in torsion test and to examine them with the use of finite element modelling.
The article is devoted to the study of rheological properties of metals and alloys in a hot state. A method for torsion testing of materials is considered. It is known that the strain heterogeneity is observed along the gauge length of cylindrical specimens during torsion. Its value can reach several tens of percent, depending on a material and the quality of specimen preparation. Therefore, torsion tests are often carried out on specimens having a notch of a certain shape. In this work, the method of finite element modeling is used to study the effect of the hardening curve of a material on the strain distribution along the gauge length of specimens with a circular notch, as well as on the strain evolution in the minimum section of the notch. Using 12Kh18N9T steel and VT–16 and VT–33 titanium alloys as an example, three different variants of a hardening curve are considered, namely, monotonic hardening, monotonic softening, and also the curve with a local maximum of stress. The results of the study have shown that the stress-strain behaviour of a material has a significant effect on the strain distribution along the notch length of the specimen. In addition, the dependence of the strain value in the minimum cross-section of the specimen on the twist angle cannot be predicted. It makes difficult to control a test setup when it is necessary to provide accurate strain rate values during the test. The use of notched specimens for torsion testing requires the development of new automatic-control systems for changing the strain rate values in the minimum cross-section of the notch according to a specified time law.
The article is devoted to the determination of equivalent stresses and strains in the minimum cross-section of the neck during tensile testing of cylindrical specimens. To correct the average tensile stresses, the Bridgman theory is used, based on the experimental measurement of the neck. The paper proposes an approach for calculating the radius of curvature of the neck based on the approximation equation that describes the entire profile of specimen. Its accuracy was estimated using the finite element method. A program has been developed that provides the measure of the diameter of specimen in the minimum cross-section of the neck and the calculation of the radius of its curvature from the video recording of the test process based on the proposed equation. Using this program, hardening curves for 09G2S steel were constructed at engineering, true and equivalent stresses.
Torsion tests allow to study the rheological properties of various materials, including properties in hot state, as well as they allow to conduct physical simulations of real material forming processes, including processes of intensive and alternating deformation. However, it is often a question what size the specimen should have to perform torsion test accurately. The article aims to study the influence of cylindrical specimen size on the distribution of deformation during torsion test. For this purpose, computer simulation of the torsion testing process was performed. The influence of the relative length of the specimen gauge on the deviation of the actual values of the effective strain from the calculated values was quantified. It is shown that the error in the calculation of the effective strain, based on the values of the twist angle and the specimen gauge size, is of 4%. In order to verify the results obtained by computer simulation, the physical simulation was performed. It is shown that the distribution of deformation along the length of the specimen gauge is also significantly influenced by the accuracy of specimen manufacture, as well as the specimen material. In some cases, the error in the calculation of the effective strain, based on the values of the twist angle and the specimen gauge size, can reach more than 80%.
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