PSI - Issue 51

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

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O. Zvirko et al. / Structural Integrity Procedia 00 (2022) 000–000

1. Introduction Structural steels are susceptible to operational degradation of their physical and mechanical properties, which negatively affects their serviceability. This becomes evident, as a rule, after several decades of operation of steel structures. After that time, losses of initial properties are especially noticeable, and there is no doubt that they are caused precisely by operational changes in the material and not by data scatter or comparison of materials from different metallurgical batches. Zvirko (2021) summarized that the phenomenon of operational degradation of steels, first of all, is in a decrease of their plasticity characteristics and also their resistance to brittle fracture, which exacerbates the risk of abrupt failures of structures. Therefore, Nemchuk et al. (2019), Nykyforchyn et al. (2019b), Okipnyi et al. (2020), Dziubyk et al. (2022), Pustovyi et al. (2022), Vukelic et al. (2022) and Zvirko et al. (2022) focusing their research on these processes, accentuated the degradation of such characteristics as elongation, reduction in area, impact strength, and fracture toughness. For structures operated under the influence of cyclic mechanical loading, the most important characteristic is fatigue strength and fatigue crack growth rate in a metal as demonstrated by Lesiuk and Szata (2015), Čamagić et al. (2021), Tsyrulnyk et al. (2022). Voloshyn et al. (2015) and Nykyforchyn and Zvirko (2022) noted that if the structures are operated under the influence of corrosive hydrogenating environments, then the characteristics of stress corrosion cracking or corrosion fatigue are also very important, and in this respect Polishchuk et al. (2015), Dadfarnia et al. (2019), Lesiuk et al. (2019), Kret et al. (2020), Syrotyuk et al. (2021) and Zhang et al. (2021) especially emphasized the stage of crack growth. The strength characteristics are less sensitive to the processes of operational degradation in steels; in addition, their change during operation can be ambiguous. Nykyforchyn et al. (2020) considered two main stages of the operational degradation of structural steels, deformation ageing and development of dissipated damages. The first stage combines deformation strengthening as a common factor of plasticity loss and deformation ageing itself, consisting of the formation of Cottrell’s clouds. As a result, strength somewhat increases together with a more significant decrease in plasticity characteristics and resistance to brittle fracture. The second stage of operational degradation is more dangerous since the development of microdamage leads to the formation of a macrocrack as a prerequisite for the failure of a structural component. Thus, the effect of the long-term operation of steels on the degradation of their crack growth resistance is especially relevant. The effect of corrosive environments with hydrogenating capabilities has its own peculiarities in the manifestation of the operational degradation of steels, as shown by Marushchak et al. (2019), Nykyforchyn et al. (2020), Zvirko et al. (2021), Nykyforchyn and Zvirko (2022). On the one hand, hydrogen intensifies the degradation of steel properties, especially at the stage of microdamage development, on the other hand, it accelerates macrocrack growth and in this way reduces the characteristics of crack growth resistance. Cyclic loading, being a significant factor of the plasticity loss, has a special role in the processes of operational degradation of structural steels. Thus, an aggressive action of corrosive hydrogenating environments manifests itself in a special way. In the present research, the regularities of the operational degradation of carbon and low-alloyed structural steels under a combined action of cyclic loading and corrosive environments are analysed. 2. Stages of operational degradation of mechanical properties of steels subjected to cyclic loading The above mentioned two-stage process of operational degradation of steels is schematically illustrated in Fig. 1. Comparing to the previous research by Nykyforchyn et al. (2020), the scheme presents also the curves for fatigue limit with and without hydrogen effect. The first stage, namely, the deformation ageing (strengthening), corresponds to a general trend of steel strengthening with some loss of plasticity and resistance to brittle fracture, not accentuating the fact that characteristics of brittle fracture resistance are more sensitive to operational degradation of steels than plasticity characteristics. Peculiarities of stage II are the following: a possible decrease in strength and hardness, and also an increase in elongation. Such metal behaviour is associated with a specific role of microdamage dissipated in bulk of material which reduces strength and hardness. In addition, Nykyforchyn et al. (2020) and Zvirko (2021) pointed out that the factor of damaging could eliminate the reducing in elongation and cause its increase due to the opening of microcracks during tensile testing when determining the mechanical properties of steels by tensile loading.