PSI - Issue 77

Jan Kec et al. / Procedia Structural Integrity 77 (2026) 264–271

269

Following the FCGR tests, the specimens were subjected FT evaluation. Figure 5 compares the force (F) versus LLD curves for samples tested in air and charged with hydrogen. For all tested samples, a stable crack growth can be observed with a gradual increase of the applied force. Variations in the curves were noted due to differences in initial crack lengths resulting from prior FCGR testing. Hydrogen-charged samples demonstrated significantly reduced load bearing capacity (maximum forces <10 kN) compared to air-tested specimens (consistently >10 kN). This marked reduction in maximum force indicates diminished crack growth resistance under hydrogen charging.

(a)

(b)

Fig. 5. Force F and load line displacement LLD curves for samples tested in air (a) and hydrogen charged (b).

Figure 6 presents the J-R curves (J-integral versus crack extension) for specimens tested in both air and hydrogen charged. The experimental data were fitted with power-law regression curves with the construction line derived from Equation (1). An exclusion line offset by 0.2 mm was applied and the J 0.2 parameter was determined from the intersection of the regression curve with this offset line. Notable scatter in the J 0.2 values was observed, attributable to inherent weld heterogeneity and potential electrolyte depletion effects (consistent with FCGR test observations). It can be seen that hydrogen charging caused a significant reduction in fracture resistance as air-tested specimens consistently demonstrated J 0.2 values above 200 kJ/m² while hydrogen-charged samples predominantly fell below this threshold. One exceptional case (CT5-9) showed particularly severe embrittlement with J 0.2 < 100 kJ/m². =3.75 ∆ (1) Fractographic analysis (Figure 7) revealed distinct failure mechanisms between the test conditions. Air-tested specimens exhibited characteristic ductile fracture surfaces with dimples of varying sizes, indicative of microvoid coalescence. Relatively large facets are visible around the MnS inclusions. In contrast, hydrogen-exposed samples displayed predominantly quasi-cleavage fracture morphology with limited ductile dimple areas. The hydrogen-induced acceleration of FCGR was quantified by comparing the crack growth rates of hydrogen charged and air- tested samples in the ΔK range of 15 –30 MPa.m 0.5 . The acceleration factor peaked at 20.7 for ΔK = 15 MPa.m 0.5 , decreasing to 9.7 at higher ΔK = 30 MPa.m 0.5 values, see Figure 8 (a). FT degradation was evaluated through J 0.2 parameter comparison, see Figure 8 (b). Hydrogen exposure reduced the average J 0.2 value by 43% relative to air-tested samples, demonstrating significant hydrogen embrittlement susceptibility. This substantial reduction correlates with the observed transition from ductile to quasi-cleavage fracture mode.