PSI - Issue 54

Margo Cauwels et al. / Procedia Structural Integrity 54 (2024) 233–240 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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concentration and active uptake during the test promotes crack growth by quasi-cleavage perpendicular to the loading direction.

Fig. 6. (a) Stretch zone in an uncharged specimen, delineated by dash-dot lines (b) Stretch zone region in the ex-situ tested specimen, showing large split in L direction at the end of the fatigue pre-crack (c) quasi-cleavage at the edge of the split in the ex-situ tested specimen (d) Stretch zone region in the in-situ tested specimen, where the white line marks end of the fatigue pre-crack and blue arrows indicate small secondary cracks The stable crack growth region in the uncharged specimen is characterized by dimples, as shown in Fig. 7 (a). For the ex-situ sample, this is mostly unchanged. However, small quasi-cleavage regions (fisheyes) could be detected, as shown in Fig. 7 (b) and outlined in yellow on Fig. 5 (b). Both the orientation of these fisheyes (elongated along the T direction) as well as the presence of elongated MnS inclusions, indicate that they form at the hard segregation bands. The fisheyes appear relatively remote from the notch tip (up to about 0.9 mm, measured along the S direction). This could be a result of sufficient hydrogen needing to accumulate at these locations before quasi-cleavage can occur, but is likely also due to the coincidental location of the hard segregation bands in the specimen. For the in-situ case, nearly the entire crack grew by quasi-cleavage. There are also some secondary cracks parallel to the loading direction, though most are shallow, e.g. in Fig. 6 (d) and more prominently in Fig. 7 (c), except for the significant split indicated by the white arrow on Fig. 5 (c). This full shift to quasi-cleavage results in the reduced load capacity of the specimen, as seen in Fig. 3. In the same steel grade, Wang (2009) also saw this shift when testing with dynamic hydrogen charging versus pre-charging. They related this to a corresponding change in operative hydrogen