PSI - Issue 68

Eduard Navalles et al. / Procedia Structural Integrity 68 (2025) 1105–1114 Eduard Navalles / Structural Integrity Procedia 00 (2025) 000–000

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pearlite bands and micro-voids coalescence dimples in the ferrite. Figure 6c and Figure 6d display similar fracture behaviour, with larger quasi-cleavage brittle areas, unclear features, secondary cracks and micro-voids coalescence dimples. Regarding the bainitic steel, a similar trend is observed. This steel shows ductile fracture surface appearance for argon specimen with great necking deformation and more brittle like failure with some small dimples as well as micro-voids and secondary cracks for the hydrogen tested specimen. 3.2. Influence of hydrogen gas on low cycle fatigue properties Figure 7 shows total strain amplitude versus number of cycles to leakage plots for both steels. Specimens tested with argon are represented by the yellow squares, while the green triangles represent specimens tested with hydrogen. Both graphs follow a similar trend for the two highest total strain amplitude, with significant reduction in fatigue life for the specimens tested in high pressure hydrogen environment compared to those tested in argon. The dotted vertical lines indicate the virtual barrier between the number cycles that represent the low cycle and high cycle fatigue.

Figure 7. Strain range amplitude against number of cycles until leakage of both materials a) ferritic-pearlitic and b) bainitic steel.

Table 3 shows the calculated values for reduction in fatigue life for the specimens tested in hydrogen compared to those tested in argon. At a total strain amplitude of 1.2%, hydrogen gas had a pronounced impact, reducing the number of cycles until leakage by approximately 10 times compared to the argon environment for both steels. The fatigue life of the ferritic-pearlitic and bainitic steels decreased by 88% and 85%, respectively. When the strain amplitude was reduced to 0.6 %, both steels continued to show significant reductions in fatigue when tested in H 2 environment. Fatigue life decreased by 74 % for the ferritic-pearlitic steel and 60 % for the bainitic steel.

Table 3. Number of cycles until leakage and reduction of the LCF life of the hydrogen specimen against the argon one by percentage.

Num of cycles to leakage

Fatigue life reduction

Total strain range amplitude

Tested environment

F-P

Bainitic

F-P

Bainitic

Argon

2198

1435

1.2%

88 %

85%

Hydrogen

273

220

Argon

12267

10897

0.6%

74 %

60 %

Hydrogen

3196

4348

Argon

123340 262528

299000 299000

Failed above LCF regime

No failure

0.3%

Hydrogen

When the total strain amplitude was further reduced to 0.3 %, negligible plastic deformation was induced in the specimens. The bainitic steel showed no observable effect on fatigue properties in either environment. The test exceeded the expected number of cycles for low-cycle fatigue, forcing the experiment to be discontinued. In contrast,