PSI - Issue 54

4

Akihiko Fukunaga et al. / Procedia Structural Integrity 54 (2024) 115–122 A.Fukunaga / Structural Integrity Procedia 00 (2023) 000 – 000

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We investigated how the RRA value of A286 changes when the strain rate is slowed down from 5 × 10 -5 s -1 at 150 °C. As shown in Fig. 3[6], the RRA values decreased significantly from 80%, which is considered to be unaffected by hydrogen, to 51% and 32%, respectively, when the strain rate was decreased from 5 × 10 -5 s -1 to 7.5 × 10 -6 s -1 and 1.25 × 10 -6 s -1 .

Fig. 3. RRA depending on strain rate of SSRT at 150 ℃ for A286 in 70 MPa hydrogen gas [6].

By SEM observation the fracture surface morphology at the outer layer including the initial crack varied greatly as follows: Specimen A at strain rate of 5 × 10 -5 s -1 : shallow dimples; Specimen B at strain rate of 7.5 × 10 -6 s -1 : QCs and partial dimples; Specimen C at strain rate of 1.25 × 10 -6 s -1 : partial intergranular fracture (IG) and QCs. The core area also showed Specimen A: typical dimples that are a ductile fracture surface; and Specimens B and C: QCs and dimples. Then it was confirmed that, in spite of a little difference in nominal stress – nominal strain curves, RRA dependance on strain rate of SSRT test was large and that the change in the RRA values corresponded with the change in fracture surface morphology. 3.2. Hydrogen embrittlement of elastic deformation and plastic deformation regions The RRA values obtained from this fracture test represent the behavior after plastic deformation. On the other hand, metallic materials used in high-pressure gas facilities such as hydrogen refueling stations are used in the elastic deformation region below the allowable stress. Therefore, it is necessary to clarify whether these values are due to the effect of hydrogen during elastic deformation region or during plastic deformation region where cracks are formed on the surface. In this study, as shown in Fig.4, the SSRT test was paused in the middle of the test, the load was released, the test environment was switched from 70 MPa gaseous hydrogen to air via nitrogen for safety reason, and the test was restarted until failure [7].