PSI - Issue 19

Jean-Gabriel SEZGIN et al. / Procedia Structural Integrity 19 (2019) 249–258 Jean-Gabriel Sezgin et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 3 – Non-propagating cracks observed at a stress amplitude of σ a = 110 MPa on non-charged (a) and H-charged (b) specimens The H-induced degradation of the fatigue life was then discussed. As shown in Figure 2, the H-induced degradation of the fatigue life was notable in the low-cycle regime and moderate in the high-cycle regime. According to previous research on a low-alloy steel with the UTS of ~950 MP by (Yamabe et al. 2015), the fatigue-life of circumferentially notched specimen having the same geometry could be predicted from the FCG properties. In the present case, the fatigue-life was also predictable from the FCG properties. Therefore, the H-induced degradation of the fatigue-life revealed in the present study was attributed to the H-assisted FCG acceleration. Hence, experimental results emphasized the presence of two different effects of hydrogen on the FCG acceleration occurring in the high- and low cycle regimes. To clarify the effects of hydrogen in the different regimes, the fracture surfaces were analysed. Figure 4 shows fracture surface morphologies of the non- and H-charged specimens in the high- and low-cycle regimes obtained by SEM. The SEM micrographs (a) and (c), classified in the high-cycle regime, were obtained at a stress amplitude of 120 MPa and a test frequency of 10 Hz. The micrographs (b) and (d), classified in the low-cycle regime, were taken at a stress amplitude of 400 MPa and a test frequency of 1 Hz. The micrographs (a) and (b) were related to the non-charged specimens and the micrographs (c) and (d) to the H-charged specimens. The non-charged specimens presented striation or microstructure-dependent surfaces whereas the H-charged ones presented some brittle fracture surfaces. At low-stress amplitude levels in the high-cycle regime (c), the specimen failed by QC, although at high-stress amplitude levels in the low-cycle regime (d), the specimen failed by a mixture of QC and IG. The fractographic observations then suggested that different mechanisms of the H-assisted FCG acceleration took place depending on the testing conditions. The introduction of hydrogen led to QC surfaces in the whole testing range. However, the high-stress amplitude in the low-cycle regime promoted the occurrence of another mechanism leading to the IG cracking.

Figure 4 – Fracture surfaces observed on non-charged (a,b) and H-charged (c,d) specimens at: a low-stress level in the high-cycle regime, =120 MPa, =10 Hz (a,c) and a high-stress level in the low-cycle regime, =400 MPa, =1 Hz (b,d)