PSI - Issue 77

47 7

A. Hell et al. / Procedia Structural Integrity 77 (2026) 41–48 Author name / Structural Integrity Procedia 00 (2026) 000–000

Fig. 6. SEM fractography with x500 magnification of the fatigue precracks at 4 mm length. (a) H-charged, 10 Hz; (b) H-charged, 5 Hz.

In SEM fractography, analysis of the fatigue precracking zone of uncharged samples shows a mixture of trans- and intergranular failure with fatigue striations on the surfaces and small zones with cleavage markings (Fig. 5). The precrack surfaces of the charged samples in Fig. 6 exhibit an increase of cleavage areas with a flat morphology and of intergranular failure ratio. This becomes more dominant when fatigue loading has been conducted at f = 5 Hz and 27 kN maximum force, where an influence of hydrogen on FCG could be observed in mechanical data. The increased fatigue crack growth rate therefore derives from promoted intergranular cracking and easier transgranular cleavage or quasi-cleavage failure due to hydrogen-related localized plasticity and embrittling mechanisms. The fracture surface of ductile crack extension is characterized by a transgranular dimpled morphology in uncharged and H-charged steel (Fig. 7). Secondary cracks are observed in both cases. The fracture surface seems to be less rugged in hydrogen charged P355NH, but no clear differentiation in dimple size and morphology can be made. No indications of quasi cleavage facets or intergranular failure have been found.

Fig. 7. Ductile fracture surface (x100 mag.) at maximum crack extension from J- ∆a -testing in (a) uncharged and (b) H-charged P355NH.

An influence of hydrogen on void growth and coalescence is most likely to account for the facilitation of stable crack extension, but has to be further examined in upcoming studies. Moreover, as the reduced crack growth resistance in H-charged steel stems from the concentration of internal hydrogen at the crack tip due to lattice diffusion, reduced strain rates and testing with in situ electrochemical or gaseous H 2 -charging might lead to more pronounced effects and