PSI - Issue 19

Masanobu Kubota et al. / Procedia Structural Integrity 19 (2019) 520–527 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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crack propagation. Thus, the delay of the crack initiation in the hydrogen environment that was seen in the smooth specimen was not significant. In addition, the acceleration of crack growth more easily occurred since the crack length was the total of the notch depth and crack length. These are the possible explanation for the small difference in the fatigue life of the deep-notched specimen. The results of the staircase method are shown in Figure 9. The fatigue limit of the deep-notched specimen determined by the staircase method was 240 MPa both in air and in the hydrogen gas. When considering the results of the smooth specimen that the fatigue limit in the hydrogen gas was higher than that in air, it could be considered that hydrogen reduced the fatigue limit of the deep-notched specimen. The difference in the stress condition at the crack initiation site between the smooth specimen and deep-notched one and the change in the mechanism achieving the fatigue limit between the smooth specimen (crack initiation) and the deep-notched one (non-propagating crack) changed the effect of hydrogen on the fatigue limit.

Fig. 8. S-N curves of deep-notched specimen.

Fig. 9. Results of staircase testing of the deep-notched specimen; (a) In air; (b) In hydrogen gas.

4. Conclusion

To gain a better understanding of the effect of hydrogen on the fatigue limit, high-cycle fatigue tests were carried out at f = 20 Hz in 0.1 MPa H 2 using a smooth specimen and deep-notched one made of SCM435 high-strength steel with HV 600. (1) The fatigue limit of the smooth specimen increased in the hydrogen gas compared to that in air. (2) The fatigue limit of the deep-notched specimen was the same as that in air. (3) Hydrogen assisted in the development of persistent slip bands.

Acknowledgements

This study was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.