The influence of the hardening curve of a material on the results of torsion tests of notched specimens

Abstract

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 development of the method for identifying the hardening curves of materials based on the results of torsion tests of notched specimens

“…To process the experimental data obtained using the notched specimens, it is necessary to establish the dependence of the strain in the minimum cross-section of the specimen on the twist angle εsurf(φ). However, as shown by the results of the earlier study [10], this function depends on the nature of the hardening curve of the material and the specific test conditions. In this case, the use of notched specimens requires a whole series of additional studies to establish these relationships.…”

“…If the material shows softening effects, for example, in a hot state, the deformation non-uniformity increases. At the same time, the inhomogeneity of the strain distribution is stochastic and cannot be taken into account when processing experimental data, since it is not known how inhomogeneity changes during the test [9,10].…”