PSI - Issue 59

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 59 (2024) 125–130 127 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Oleh Venhryniuk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000 3

A series of specimens were tested: 1) in post-operated state; 2)

after in-laboratory preliminary electrochemical hydrogen (PEH) charging with the use of the following parameters: the specimens were pre-charged in an aqueous sulphuric acid solution (pH1) with adding 10 g/l thiourea at a current density 0.05 mА/сm 2 for 50 hours.

Table 1. Mechanical properties experimentally observed for the studied 17H1S pipeline steel after 38 years of operation. Ultimate strength σ UTS [MPa] Yield strength σ Y [MPa] Reduction in area RA [%] Elongation [%] Impact toughness KCV [J/сm 2 ] 570 390 68 25,0 103

Fracture toughness of the tested specimens was determined following the methodological requirements of standard ASTM E 813, which outlines the use of the J -integral method. In the case of elastic-plastic behaviour, the fracture toughness is best characterized by the J -resistance curve. Single-edge notched beam specimens (SENB) with dimensions of 4x15 mm and pre-existing fatigue cracks were subjected to three-point bending (span between supports 60 mm) in air at room temperature. The loading was performed with the registration of the load displacement diagram, where F represents the force on the specimen and Δ denotes the displacement at the point of force application (a deflection of the specimen). Non-hydrogenated specimens were tested using the test displacement rate v = 0.5 mm/min, which falls within the typical range for such experiments. Hydrogen pre-charged specimens were tested under different displacement rates (0.5, 0.05, and 0.005 mm/min) right after the completion of the hydrogen charging procedure in order to observe the effect of the displacement rate on hydrogen embrittlement. For each displacement rate, a series of 6-7 specimens were tested, loaded to different values of Δ , and then unloaded. This corresponded to the different crack extension values Δa from the fatigue crack front, dete rmined on the specimen fractures after thermal treatment of various zones at a temperature of ~300 ºC for 1 hour. These zones included fatigue crack growth, blunting of the fatigue crack tip, and static crack growth Δa. Fracture toughness values, J 0,2 index, for an effective crack growth of 0.2 mm, were calculated from the J – Δ а curves in accordance with the standard ASTM E 813. Additionally, the J 0 level at the initiation of static crack growth from the fatigue crack tip was determined, allowing for a comparison of the sensitivity of the J -integral parameters to hydrogen embrittlement based on the different criteria for assessing the critical value, J 0.2 and J 0 . 3. Experimental results and discussion Figure 1 illustrates examples of fracture surfaces in both non-hydrogenated and hydrogen pre-charged electrochemically specimens after loading-unloading tests using the J -integral method and thermal shading.

a

b

Fig. 1. Examples of typical fracture surfaces of non-hydrogenated (a) and hydrogen pre-charged specimens (b) after testing using the J- integral method.

No fundamental differences were observed in the nature of static crack growth; it predominated in the central part of the fatigue crack front. This indicated the dominance of the triaxial stress state factor at the top of the fatigue