PSI - Issue 40

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

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

ones. In such extreme conditions, the requirements for the trouble-free operation of many objects of modern technology must be met – elements of gas turbine engines, structures of aerospace equipment, components and parts of energy, chemical, metallurgical and other types of equipment. Unsteady elastoplastic loading is accompanied by a progressive degradation of the material's mechanical properties and the damage accumulation as a result of two processes, low-cycle, and high-cycle fatigue, the mechanisms of which differ significantly. The development of plastic strains and hardening processes due to overloads leads to structural changes in the material that determine the kinetics of fatigue damage accumulation and the resource performance of structural elements (Makhutov 2005, Romanov 2021, Smirnova, Zinin 2021, Stepnov, Zinin 2016). The effects of the combined action of basic cycle loads and low-cycle overloads at different stages of structural operation remain understudied. The main difficulties are related to the need to account for structural changes in the material caused by elastoplastic strain and hardening under overloading. Most of the models presented in the current literature (Romanov 1988, Smirnova 2019, Makhutov 2019, Chaboche 2012) for the combined action of low- and high-cycle loading are mainly based on cumulative models of fatigue damage accumulation without considering the loading history and underestimate the problems of structural material changes under overloading and the resulting changes in fatigue behavior of the material. However, the results of fatigue tests and microstructural studies of structural steels of various types presented in works (Chaboche 2012, Gadenin 2006, Kesaev 1982, Kim 2019, Fatoba 2018) show that the change of loading mode in overloading leads under subsequent normal loading to a change in the intensity of microplastic strain and acceleration or slowing of metal damage compared to the steady state process of cyclic loading. This paper presents the results of experimental and microstructural studies of fatigue behavior of structural steels of two different types under a two-stage loading regime (soft fatigue loading and hard elastoplastic strain) and evaluates the effect of repeated elastoplastic overloading on changes in the structural state of the material, the character of fatigue damage accumulation and lifetime in comparison with the static cyclic loading. 2. Research methodology For experimental estimation of the influence of low-cycle overloads on fatigue damage accumulation, a series of fatigue tests was carried out at different modes of loading of samples made of two kinds of steel – cyclically strengthening steel 10G2S1 of high strength and cyclically softening steel 14Kh2GMR. Strengthening or softening of metal under cyclic loading depends on the value of the ratio of ultimate strength u  to conventional yield strength Y  . When 1.2 u Y    the metal is cyclically softening, when 1.4 u Y    it is hardening, when 1.2 1.4 u Y     it is possible both softening and hardening (Kesaev 1982). According to the results of static tensile tests at normal temperature, presented in Table 1, the ratio for steel 10G2S1 is 1.48, for steel 14Kh2GMR - 1.11.

Table 1. Mechanical properties of structural steels.

Steel grade

Characteristic

Test Method

10G2S1

14Kh2GMR

520 350

700 630

GOST 1497-84, temperature 22 ± 3 о С GOST 25.502-88

u  , MPa

Ultimate strength

Y  , MPa

Yield strength

Relative elongation  , %

21

20

270

420

R  , MPa

Endurance limit

The initial loading mode is standard fatigue tests before fracture in pure bending with rotation with a stress amplitude a  = 315 MPa. Then, two-level program tests were performed, including preliminary low-cycle tension compression with a constant strain amplitude (hard loading) and subsequent multicycle loading (soft loading) until fracture with a stress level equal to the amplitude of the initial tests а  = 315 MPa. The modes of the first stage (low-cycle overloads) were determined by 2 variable parameters – the strain level under hard loading a  and the number of low N "training" cycles at this level.