Issue 43

M.P. Tretyakov et alii, Frattura ed Integrità Strutturale, 43 (2018) 146-154; DOI: 10.3221/IGF-ESIS.43.11 147 of requirements for their material consumption. In the context of approach development devoted to the forecasting the construction element destruction, which requires solving such problems like structural integrity on the base of deformation process and material destruction modeling processes. One should take into account the mechanical behavior of materials at the postcritical stage, characterized in the experiment by a load decrease with increasing elongation and immediately precedes the moment of sample failure. The necessity and relevance of the fundamental study of various aspects of material behavior at the postcritical stage of deformation is mentioned in papers [1-6]. The material deformation at the postcritical stage of deformation is accompanied by actively proceeding plasticity processes and in most cases leads to the localization of strain as a necking effect in the gauge length of the specimen. This effect in the test significantly complicates the registration, processing and interpretation of the experimental data [7-10]. However, obtaining experimental data in the localization conditions in the form of a neck is of scientific and practical interest in order to understand the processes characterizing the plastic and postcritical deformation and destruction of structural materials, the theoretical description and modeling of the material behavior in structures, and process simulation [11-15]. In the following sources [2, 4, 15] the basis of mathematical theory of stable postcritical deformation processes of softening media is developed. In [16-19], methodological issues of testing were considered and experimental data of postcritical behavior of structural steels and alloys under various types of stress-strain state and temperatures were obtained. A comparison of the criteria for the proceeding of the deformation process to the postcritical stage under various types of stress-strain state was made [20]. Analytical and numerical solutions of boundary value problems, performing carrying capacity reserves and the increase of the survivability of structures and bodies with cracks considering the postcritical deformation of materials are obtained [21-23]. For the further development of theoretical approaches of the regularities of plastic and postcritical deformation of fabrication materials and disruptive conditions, the critical task is to obtaining experimental data on the deformation regularity of materials under various types of stress-strain state under the conditions of strain localization in the neck form. To register inhomogeneous fields of displacements and strains in such tests, it seems appropriate to use optical methods of experimental mechanics, in particular, the modern perspective method of digital images correlation (DIC) [24]. The aim of the presented work is an experimental study of the mechanical behavior regularities of structural steels during postcritical deformation of specimens under the conditions of the strain localization formation in the neck form at tension. M ATERIALS AND EQUIPMENT he specimens of structural steel 40Cr, the chemical composition of which is shown in Tab. 1, were used in the tests. The content of other elements, not listed in Tab. 1, is less than 0.025 % of each. Specimens for tests were made of a rod with a diameter of 16 mm, as-received state, without additional heat treatment. Fe C Cr Mn Si Cu Ni W 97.3 % 0.362 % 0.996 % 0.619 % 0.240 % 0.204 % 0.166 % 0.030 % Table 1 : Chemical composition of steel 40Cr. Tensile tests at room temperature were carried out using the universal servohydraulic biaxial test system Instron 8850 (100 kN, 1000 N·m, 30 Hz) with the joint using of the noncontact 3D optical system for analyzing displacement and strain fields of Vic-3D Correlated Solutions based on the digital images correlation technique. Installation of the test equipment is shown in Fig. 1. The samples were set in the V-shaped jaws of the hydraulic grips. The load in the tests was recorded by the Instron Dynacell load cell with an accuracy of no more than 0.4 % of the measured value. Extension of the specimens was recorded by the built-in displacement sensor of the test system. The stretching, deformation and diameter changing of the specimens in the test part were measured due to the video system data. The estimation of the accuracy of data recording by the video system was considered earlier in [24]. Data synchronization with the use of analog controller machine channels and video system were used during the testing. The registration of the displacement and strain fields was done with the high-resolution cameras Prosilica, with recorded frequency of 2 Hz, with set resolution of 16.0 MP with using of specialized illumination system. The relatively low frame rate is chosen, because of the monotony study process. Features description of the mathematical aspects of the video system, based on digital image correlation technique, as well as methodological aspects of the system application for solving problems related to solid mechanics is given in the following works [25, 26]. T