PSI - Issue 81

Olha Zvirko et al. / Procedia Structural Integrity 81 (2026) 41–46

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Here J 0H is fracture toughness value at which crack growth is initiated in steel with a certain hydrogen concentration under active loading, Е el is the elastic modulus, υ is Poisson`s ratio.

Fig. 1. Dependencies of the crack growth parameter J 0Н on the loading rate v for specimens of 38 years operated pipeline steel after their preliminary electrolytic hydrogen charging corresponded to different equivalent hydrogen pressure values Р Н eq 2 = 0.6 and 9.71 MPa. As the limit fracture toughness value of the steel of the operated pipeline, it is proposed to use the threshold fracture toughness required by the current design standard ASME B31.12 for hydrogen pipelines to prevent sustained load cracking (a minimum threshold stress intensity factor К IH of 55 MPa·√m ). A steel reaches its limit state (fracture) when its fracture toughness К Н at the certain concentration of hydrogen С Н absorbed during operation equals its critical fracture toughness К IH cr : К Н = К IH cr = 55 MPa·√m . (2) A critical fracture toughness К IH cr = 55 MPa·√m corresponds in conversion to J 0Н cr = 13.3 N/mm. During operation, the fracture toughness of pipeline steels generally decreases, as demonstrated by Nykyforchyn et al. (2025). A decrease in fracture toughness of a steel, on the one hand, depended on hydrogen concentration in it (intensity of hydrogen uptake), and, on the other hand, on the loading rate. Therefore, for a correct assessment of the influence of hydrogen on the value of fracture toughness of an operated pipe steel, the loading rate of specimens should be taken into account. It can be seen from Fig. 1 that even at the lowest loading rate and high hydrogen charging intensity ( Р Н eq 2 = 9.71 MPa), the J 0Н cr value is not reached for the studied steel, indicating its suitability for further operation under such hydrogenation and loading conditions. The dependence of fracture toughness on loading rate was approximated at lower loading rates (Fig. 1), and it was determined that at high hydrogen charging intensity and a rate of approx. 5·10 -4 mm/min, the fracture toughness (crack growth parameter) could be close to the limit value; for low hydrogen charging intensity, this rate will be lower. Hydrogen acts as both a driving force (gas pressure) and a weakening agent (embrittlement). Therefore, to accurately predict crack growth resistance and set safe limits, it is important to take into account a possible presence of crack-like defects (Dubyk and Seliverstova (2019)) in an operated pipeline, undetected by evaluation techniques. Accounting for the crack-tip stress state under hydrogen pressure is crucial for justifying brittle fracture resistance, because hydrogen reduces material fracture toughness (as reflected in the stress intensity factor, SIF, and, if necessary, the J -integral). According to the proposed approach, a steel of an operating pipeline reaches a limit state when its fracture toughness К Н attains the value К IH cr , which corresponds to the stress intensity factor at the crack tip under a given hydrogen pressure in the pipe. To substantiate the criterion, SIF for a pipe of 1220 x 12 mm was calculated for two values of gas pressure 3.5 and 7.5 MPa in the presence of a semi-elliptical crack on its inner surface with a longitudinal or circumferential orientation (Fig. 2) with a crack depth of a = 1 and 2 mm and a ratio of the crack depth to the half-length a/c = 1/3 and 2/3 (API 579-1/ASME FFS-1 (2016)). To calculate the SIF for semi-elliptical cracks in a cylinder in the longitudinal and circumferential directions, the formulas according to API 579-1/ASME FFS-1 (2016) were used: = 2 2 − 2 [2 0 −2 1 ( )+3 2 ( ) 2 −4 3 ( ) 3 +5 4 ( ) 4 ]√ , (3) =[ 0 ( 2 2 − 2 + ( 2 − 2 ) )]√ , (4)