PSI - Issue 13

Tretyakov M.P. et al. / Procedia Structural Integrity 13 (2018) 1720–1724 Tretyakov M.P./ Structural Integrity Procedia 00 (2018) 000 – 000

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analysis, modeling and forecasting the failure processes significantly important to take into account the mechanical behaviour of materials at the postcritical deformation stage, characterized in the experiment by a load decrease with increasing elongation and immediately precedes the moment of sample failure [Vildeman et. Al (1997), Bazant et. Al (2004), Struganov (2004), Radchenko et. Al (2014)]. By now, the basis aspects of mathematical theory of stable postcritical deformation processes of softening media is developed [Vildeman et. Al (1997)]. Methodological issues of experimental study of postcritical behaviour of materials under various types of stress-strain state, loading system stiffness and temperatures were considered [Tretyakov et. al. (2016)]. A comparison of the criteria for the proceeding of the deformation process to the postcritical stage at plane stress-strain state was made [Wildemann et. al (2014)]. 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 [Sokolkin et. al (1998), Ilinykh et. al (2011)]. In connection with the fact that the postcritical behavior of plastic materials is accompanied by a developed plastic deformation, the processing and interpretation of experimental data, taking into account the inhomogeneity of deformation in the form of a neck [Davidenkov and Spiridonova (1945), Ahmetzyanov at. al (2004), Bai at. al (2009)], is of particular importance. The aim of this work is to study the methodical issues of providing tests by “specimen from specimen” schem e and to estimate the mechanical properties of steel 40Cr during necking effect. This experimental data is important for the further development of theoretical approaches of the regularities of plastic deformation during postcritical behavior. σ P the stress corresponding to the failure (or beginning of the unloading) at the postcritical deformation stage P B the load corresponding to the ultimate stress (or maximum load) P P the load corresponding to the failure (or beginning of the unloading) at the postcritical deformation stage ε longitudinal engineering strain by DIC system y coordinate corresponding to the longitudinal axis of specimen k P coefficient of the postcritical deformation stage realization HV Vickers hardness number 2. Studied material and test equipment The specimens of structural steel 40Cr were used in the tests. The chemical composition of steel 40Cr is shown in Table 1. The content of other elements, not listed in Table 1, is less than 0.025 % of each. For tensile tests, standard solid cylindrical specimens with a nominal diameter of the test part of 8 mm and with a gauge length of 40 mm were used. Specimens were made of a rod with a diameter of 16 mm, as-received state, without additional heat treatment. Nomenclature σ engineering stress ultimate stress σ B

Table 1. Chemical composition of steel 40Cr. Fe C Cr

Mn

Si

Cu

Ni

W

97,3 %

0,362 %

0,996 %

0,619 %

0,240 %

0,204 %

0,166 %

0,030 %

The structure of the initial material is characterized by the presence of perlite and ferritic grid, which was formed along the boundaries of the former austenite grains during the cooling process. The average cell size of the ferrite grid, over which the size of the austenite grain can be estimated, is 54 ± 3 μ m. Analysis of the microstructure in the initial undeformed state, and also after deformation in the neck zone and in the peripheral zone of the working part of the sample, was carried out with the Olympus GX 51 light inverted microscope at 500 and 1000 times magnification. Microstructure parameters are estimated in department of Metal Technology, Thermal and Laser Processing of Metals of PNRPU.