PSI - Issue 51

Olha Zvirko et al. / Procedia Structural Integrity 51 (2023) 24–29 O. Zvirko et al./ Structural Integrity Procedia 00 (2022) 000–000

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The two-stage degradation process would be maintained for structural steels also in the case of their operation under cyclic mechanical loading, demonstrating an ambiguous trend for changes in fatigue behaviour (namely, common fatigue characteristics defined on smooth specimens are considered), similar to the defined changes in strength and hardness. Analysing these changes, one can use fundamental mechanical regularities indicating opposite dependencies on steel strength (yield and ultimate strength), on the one hand, the characteristics of plasticity and brittle fracture resistance, including fatigue crack growth rate (curve 1 in Fig. 2) and, on the other one – fatigue limit (curve 2 in Fig. 2). For materials capable of cyclic strengthening (including carbon and low-alloyed steels), cyclic loading during long-term operation increases the current strength level. This (according to 2 in Fig. 2) causes an increase in fatigue strength maintaining over the operation time at stage I (Fig. 1). Considering this aspect, Kang et al. (2007) and Zhao et al. (2018) showed that preliminary plastic deformation led to an increase in fatigue resistance of cyclically strengthened materials. It is obvious that stage I will be completed sooner than it could last under the operation of steels without cyclic loads, since cyclic loading intensifies the development of dissipated damages, that is, it accelerates the onset of stage II of the operational degradation.

Fig. 1. Schematic representation of two stages of in-bulk operational degradation in steels: stage I – deformation aging (strengthening); stage II – dissipated damaging.

Fig. 2. Scheme of dependences of various steel parameters on its strength.

Intensive development of damage leads to a sharp decrease in brittle fracture resistance and the dominance of dependence 2 (see Fig. 2) in the processes of operational degradation of steels. Accordingly, the dependence of the