PSI - Issue 59

128 Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 59 (2024) 125–130 4 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Oleh Venhryniuk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000

crack, even in tests of hydrogen pre-charged specimens, where dominant crack growth could be expected on the lateral portions of the specimen, whose metal should experience the most intense immersion. It is evident that the triaxial stress state factor at the top of the crack contributed to the realization of the hydrogen embrittlement mechanism, which is sensitive to local hydrogen concentration. Therefore, it can be assumed that, unlike the higher integral concentration of hydrogen on the lateral layers of the specimen, its local concentration was higher precisely in the zone of maximum triaxial stress state along the fatigue crack front, i.e., in the central zone of the specimen. Figure 2 presents the results of specimen testing with the construction of J – Δ a curves. Their comparison for non-hydrogenated and hydrogen pre-charged specimens at a displacement rate of 0.5 mm/min showed a significant manifestation of hydrogen embrittlement by the decrease in the critical J 0.2 value, which practically halved. The obtained estimates may result from both particularly intensive metal hydrogen charging and increased sensitivity of the post-operated steel to hydrogen embrittlement, revealed through fracture mechanics approaches. In general, the J -integral method is particularly effective in identifying embrittling factors in the mechanical behaviour of ductile steels. This includes operational embrittlement of pipe steels.

Without pre-treatment

- 1 - 2

150

- 3 - 4

0.5 mm/min

100

after PEH

J , N/mm

0.5 mm/min

50

0.005 mm/min

0.05 mm/min

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0

 а , mm

Fig. 2. J – Δ a curves for non-hydrogenated (without pre-treatment) and hydrogen pre-charged (after PEH) specimens at the different displacement rates (v = 0.5, 0.05, and 0.005 mm/min).

Research on the influence of displacement rate variation on preliminary hydrogen-charged specimens has revealed two important features. Firstly, a decrease in the displacement rate enhances the manifestation of hydrogen embrittlement, aligning with tests using the slow strain rate tension method for metals and alloys in corrosive and hydrogenating environments, using both smooth and pre-cracked specimens. This implies that, as in other similar cases, the assessment of the impact of hydrogen charging on the mechanical properties of structural steels, including the fracture toughness of pipe steels, depends not only on the metal structure and hydrogen charging conditions but also on the displacement rate of experimental specimens. Thus, with a two-order reduction in displacement rate, the fracture toughness of hydrogen pre-charged specimens decreased by almost half, and when compared with the results of tests on non-hydrogenated specimens, it decreased by almost four times. Another feature is the higher sensitivity of the J 0.2 parameter compared to the J 0 parameter as also a critical value of the J -integral characterizing the fracture toughness of the metal, including preliminary hydrogen-charged specimens. Table 2 provides the values of these parameters for the investigated testing conditions, confirming this characteristic. It indicates a higher sensitivity of fracture resistance at the stage of static crack growth than at the stage of crack initiation from the top of a pre-existing fatigue crack. This conclusion is consistent with assessments of the impact of long-term operation of pipe steels on the components of impact toughness, reflecting the resistance to crack initiation from a concentrator and the resistance to its propagation. It has been found that the crack