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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4. Sensitivity of fatigue crack growth parameters to operational degradation of steels The characteristics of crack growth resistance are particularly sensitive to the operational degradation of steels. Zvirko (2021) summarized that most data in this field concerned fracture toughness, often determined by the J-integral method, taking into account the high plasticity of structural steels. Particularly, it is shown that fracture toughness is even more sensitive than impact strength which is widely used for the assessment of the operational degradation. Thus, the analogous parameter for cyclic loading K fc is expected to be of similar sensitivity. However, there are some peculiarities in using the parameters of fatigue crack growth resistance. They include the sensitivity of fatigue thresholds, as reported by Nykyforchyn and Zvirko (2022). It is recommended to evaluate the fatigue thresholds taking into account crack closure; the nominal fatigue thresholds are less sensitive for the assessment of the operational degradation then the effective ones since the crack closure effect is more pronounced in the operated steel. The crack closure effect can be neglected in the middle amplitude range of stress intensities; besides, Nykyforchyn and Zvirko (2022) emphasized that in this region of stress intensities, where power-law behaviour nominally described by the Paris equation, fatigue crack growth rate is weakly sensitive to structural changes in metal and, thus, to the processes of operational degradation. The fatigue threshold for steel with high plasticity increases in a corrosive environment due to crack blunting and enhancing the crack closure effect; therefore, this characteristic is ineffective for assessing its degradation. However, the operational degradation increases the susceptibility of steel to stress corrosion cracking, which results in the appearance of a plateau-like section in the mid-amplitude range of the fatigue crack growth curve where crack growth is independent on ΔK (Polishchuk et al., 2015). Manifestation of the susceptibility to hydrogen induced cracking is also possible, as confirmed by Voloshyn (2020) in his comparative studies on the influence of corrosive environments on fatigue crack growth under cathodic polarization in gas pipeline steel in the as-received and operated states. 5. Concluding remarks The staging of operational degradation of structural steels is supplemented taking into account their cyclic mechanical loading: deformation ageing (strengthening) and development of in-bulk dissipated microdamage. Results demonstrate a variable change in fatigue limit with operation time caused by deformation strengthening from the one side and developed microdamage from the other one. The impact of corrosive environments with hydrogenating capabilities is mainly in intensifying the development of dissipated microdamage, including under the combined action with cyclic mechanical loads. Taking into account the crack closure effect under cyclic loading enhances the sensitivity of fatigue thresholds to operational degradation of steels. The effect of corrosive environments on fatigue crack growth is the most significant for in-service degraded metal in the mid-amplitude region of the fatigue crack growth curve da / dN – Δ K , thus demonstrating also their susceptibility to stress corrosion fatigue. References Beltrán-Zúñiga, M.A., Rivas-López, D.I., Dorantes-Rosales, H.J., González-Zapatero, W., Ferreira-Palma, C., López-Hirata, V.M., Hernández Santiago, F., 2022. Fatigue life assessment of low carbon API 5L X52 pipeline steels retired from long-term service. Engineering Failure Analysis 106769. Cabrini, M., Lorenzi, S., Pastore, T., Pesenti, B.D., 2019. Hydrogen diffusion in low alloy steels under cyclic loading. Corrosion Reviews 37(5), 459–467. Čamagić, I., Jović, S., Makragić, S., Živković, P., Burzić, Z., 2021. Influence of temperature and operation time on the fatigue strength and microstructure of welded joints of A-387Gr.B Steel. Materials Science 57(1), 86–93. Dadfarnia, M., Sofronis, P., Brouwer, J., Sosa, S., 2019. Assessment of resistance to fatigue crack growth of natural gas line pipe steels carrying gas mixed with hydrogen. International Journal of Hydrogen Energy 44(21), 10808–10822. Dziubyk, A.R., Voitovych, A.A., Student, O.Z., Dziubyk, L.V., Khomych, I.B., 2022. Evaluation of the technical state of reinforcement of the concrete-beam span of a bridge constructed at the end of the last century. Materials Science 57(4), 466–474. Han, Y.D., Wang, R.Z., Wang, H., Xu, L.Y., 2019. Hydrogen embrittlement sensitivity of X100 pipeline steel under different pre-strain, International Journal of Hydrogen Energy 44(39), 22380–22393. Ebihara, K.-I., Sugiyama, Y., Matsumoto, R., Takai, K., Suzudo, T., 2020. Numerical interpretation of hydrogen thermal desorption spectra for iron with hydrogen-enhanced strain-induced vacancies. Metallurgical and Materials Transactions A 52, 257–269.