PSI - Issue 47

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 47 (2023) 190–194 Hryhoriy Nykyforchyn, Olha Zvirko, Oleksandr Oliynyk et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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strength (curve 1 in Figure 2) and, accordingly, in increasing in fatigue limit (curve 2 , Figure 2) at stage I of degradation. As it was demonstrated by Kang et al. (2007) and Zhao et al. (2018), preliminary plastic deformation led to an increase in fatigue limit of cyclically strengthened materials. Stage I will be completed sooner than it could last under the operation of steels without cyclic loading, since cyclic loading intensifies the development of dissipated damages, and, therefore, it accelerates the onset of stage II of the operational degradation. Development of damages in steels under long-term operation (stage II, Figure 2) leads to a decrease in brittle fracture resistance (impact toughness and fracture toughness) and other mechanical properties (strength, fatigue limit and effective fatigue threshold). The development of damages in steels can be facilitated by hydrogen penetrated into a metal as a result of its interaction with aggressive environments, which can accelerate the onset of stage II of the operational degradation of steels (curve 3 in Figure 2). Decreasing fatigue strength characteristics (fatigue limit) during operation was demonstrated by Kossakow ski (2013), Beltrán - Zúñiga et al. (202 3), and others.

Fig. 2. Scheme of steel degradation stages under mutual action of cyclic loading and hydrogenating environment: stage I – deformation aging (strengthening); stage II – development of dissipated damages. Parameters: 1 – yield strength  YS and ultimate strength  UTS ; 2 – fatigue limit  -1 without hydrogen; 3 – fatigue limit  -1 with hydrogen; 4 – impact toughness KCV, effective fatigue threshold  K th eff , and fracture toughness K IC . In Figure 3 dependence between impact toughness and stress range determined for specimens made of 0.2% C carbon steels from marine portal crane after approximately 33 years of operation is presented. The stress range was measured on the crane surface under loading similar as the operational one using the strain gauge method described by Nemchuk et al. (2019) and Pustovyi et al. (2022). The lowest resistance to brittle fracture was observed for the highest stress range (Figure 3). Similar results were reported by Wen and Li (2021) for static and dynamic loading.

Fig. 3. Dependence of impact toughness KCV on stress range σ e determined for longitudinal (1) and transverse (2) specimens made of 0.2% C carbon steels from marine portal crane after 33 years of operation.