PSI - Issue 40

A.V. Zinin et al. / Procedia Structural Integrity 40 (2022) 470–476 A.V. Zinin at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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In order to study the kinetics of fatigue processes of the studied materials, - continuous registration of strains until final destruction was carried out for each loading mode in the test process. The structural state of the tested steel samples at different strain stages was assessed by three different methods – light, X-ray, and scanning electron microscopy. Methods of visible light and X-ray structural analysis were used for nondestructive control of structural damage during testing, both in the initial mode and during program loading. After destroying the samples, microstructural analysis of thin sections and fractures of the samples on an electron scanning microscope at a magnification of 120... 2560 times was performed. 3. Results and discussion 3.1. Damage diagrams The study of the mechanics of elastoplastic cyclic strain, taking into account structural changes due to the effects of different loading levels, is a very complex problem. In this work, such processes are studied by establishing the patterns of occurrence and accumulation of destructive strain damage in the metal (Smirnova, Zinin 2021, Smirnova 2019) The previously developed model of the stages of plastic strain (Romanov 1988, Gadenin 2006, Merenkova 1979) is used, reflecting the course of two main processes of plastic strain – shear formation caused by the interaction of dislocations and destruction determined by a violation of the metal continuity (Rybakova 1980). Within the framework of this model, the process of the cyclic plastic strain of metal can be described by damage diagrams constructed in the coordinates " 1/ 2 S s   ", where S is the true stress in each of the successive half cycles, s  is the arithmetic sum of all previous plastic strains. Detailed studies by various authors (Romanov 1988, Smirnova 2019, Merenkova 1979, Sarkar 2017) on the kinetics of structural changes occurring in the internal volumes of cyclically deformed material have revealed the three-stage nature of the cyclic deformation process with different damage mechanisms at each stage. The first stage I of cyclic strain has a sharply pronounced elastoplastic character and lasts for a short time – about 1% of the duration of the whole process before fracture. The second stage II is the main one and the longest – up to 95% of the duration of the whole process. At this stage, the strain is not completely plastic, as evidenced by the change in the slope of this section of the diagram compared to the first section. In the third stage III, the elastoplastic-destructive strain is localized in the zone of main fracture development, which originates in the surface layer and further spreads inward along the section, leading to the final fracture. Fig. 1 shows strain diagrams of steel specimens obtained in low-cycle fatigue tests in coordinates " 1/ 2 S s   " which clearly show three areas corresponding to different stages of the process of elastoplastic strain with distinct mechanisms of strain development. Summation by stages of residual strains during cyclic strain is essentially a path of plastic strain by the number of loading cycles.

Fig. 1. Damage diagrams and Q -factor material quality factor “S   1/2 ” and “ η   1/2 ” for the hard cyclic strain of steel 10G2S1 (a) and steel 14Kh2GMR (b) at ε a = 0.5%. The strain s  is the sum of two types of strains – plastic p  and destructive d  , corresponding to the processes of shear formation and continuity violation: s p d     