PSI - Issue 68

H. Nykyforchyn et al. / Procedia Structural Integrity 68 (2025) 861–867 H. Nykyforchyn et al. / Structural Integrity Procedia 00 (2025) 000–000

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Analysing the results obtained under lower displacement rates, some regularities can be noticed. Firstly, the crack growth resistance decreases with the reduction in displacement rate v , as illustrated for J cr (Fig. 3 а ) and J 0 (Fig. 3 b ) under testing both as-delivered (lines 1 and 2 ) and operated (lines 3 і 4 ) pipe steels. The parameter β (Fig. 3 c ) also decreases under lower displacement rates indicating the reduction in energy consumption for the fracture process. The obtained regularities on the effect of loading rate on crack growth resistance by the J -integral method are consistent with a commonly accepted idea about slow strain rate loading . Under a lower loading rate, hydrogen has more time to diffuse and accumulate in the vicinity of the crack tip. It should be noted that, in conditions of ex-situ hydrogenation, hydrogen release from a specimen can occur during loading, leading to a hydrogen content decrease in steel. However, according to the results in the considered range of v (0.5–0.005 mm/min), hydrogen accumulation at the pre-fracture zone prevails upon its diffusion towards the surface. 3.3. Fractographic features Fracture peculiarities have been analysed for non-hydrogenated and previously hydrogenated specimens of both steel states (Figs. 4 and 5). Non-hydrogenated specimens in general revealed ductile fracture by the initiation of micropores at non-metallic inclusions and carbides, which serve as stress concentrators in steels, as shown by Sylovanyuk and Ivantyshyn (2024). However, some distinctions were noted. In the as-delivered state, quite big uniaxial dimples were formed on the areas between delaminations due to the stretching of partitions between them until fracture (Fig. 4a). In contrast, the shear mechanism prevailed in the formation of dimples (Fig. 4b) in the operated steel. The shear thus can be considered the fractographic sign of the decrease in J I c values for the steel after operation. Preliminary hydrogen charging resulted in occurring cleavage areas on the fracture surfaces of specimens (Fig. 4с, d). In addition, delaminations and secondary microcracking are manifested more on the operated steel (red arrows in Fig. 5d) indicating a significantly lower energy consumption for the fracture. The J I c decrease is also caused by the rising amount of delaminations and the reducing deformation capability of partitions between delaminations which is manifested by the less opening of delamination edges in the operated steel. Similar results have been obtained by Zvirko 1 et al. (2024) for this steel after the Charpy testing.

a

b

c

d

Figure 4. The fracture surfaces of SENB specimens of non-hydrogenated (a, b) and ex-situ hydrogenated (c, d) steel in the as-delivered (a, c) and operated (b, d) states.

It should be noted that hydrogen, accumulated along the boundaries of non-metallic inclusions with the matrix (energy-beneficial hydrogen traps), facilitated their decohesion and breakage of the partitions between nearest