PSI - Issue 54

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Akihiko Fukunaga et al. / Procedia Structural Integrity 54 (2024) 115–122 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Fig. 6. Nominal stress – nominal strain curves of A286 at strain rate of 7.5 × 10 -6 s -1 at 150 ℃ in 70 MPa H

2 at 0.8% ⇒ air, after 10

stress cycles from 400 MPa to 200 MPa .

3.4. Hydrogen embrittlement mechanism The total amount of hydrogen at the crack tip of metal (θ t ), which is defined as the sum of the amount of dissolved hydrogen in the lattice (θ L ) and the amount of trapp ed hydrogen in defects (θ B ), can be expressed as follows [9]: (1) where θ t is the total amount of hydrogen at the crack tip, θ L is the amount of dissolved hydrogen in the lattice, θ B is the amount of trapped hydrogen in defects, α is the density of defects that become trap sites, and ΔG is the Gibbs energy change owing to hydrogen-defect coupling. The stress cycles in high-pressure gaseous hydrogen during the elastic deformation region also increase interstitial hydrogen diffusion, resulting in a fracture surface that resembled a homogeneously hydrogen-charged specimen with a large θ L . The RRA value was considered to have decrease d owing to the increase in θ L . When θ B increases due to newly generated dislocations during the transition from the elastic to the plastic deformation region, θ L decreases as a source of hydrogen to θ B , resulting in a temporary decrease in θ t in Eq. (1). As a result, the specimen with gas switching at a nominal strain of 1.6% in Fig. 5 is considered to show a higher RRA value than those at lower strains. This increase in θ B is expected to reduce θ L , as described by Huang et al. [10] for commercial pure iron. In the plastic deformation region, the number of defects increases with increasing strain, and θ B becomes more dominant, resulting in an increase in θ t . RRA values gradually decrease as shown in Fig. 5. Hydrogen behavior in metallic materials is closely related to the hydrogen embrittlement mechanism in which the dissolved hydrogen directly weakens the bonding strength of matrix atoms (lattice decohesion, for example, see [11]) and the mechanism in which hydrogen promotes dislocation motion during plastic deformation (hydrogen-enhanced localized plasticity (HELP, for example, see [12]) or vacancy formation (hydrogen-enhanced strain-induced vacancies (HESIV, for example, see [13]) during plastic deformation. The results of this study indicate that the dominant hydrogen embrittlement mechanism of metal may differ depending on the testing conditions, that is, the service environment. Therefore, it is reasonable to control the amount of dissolved hydrogen θ L to evaluate the suitability of the material for use in hydrogen refueling stations and other applications. For example, the surface equilibrium hydrogen content of A286 in 70 MPa and at 150 °C is 50.7 mass ppm [6]; thus, it is more appropriate to conduct SSRT tests on specimens charged with hydrogen above that level, rather than conducting SSRT tests in a high-pressure gaseous hydrogen, as an evaluation test assuming practical usage. ＝ ＋ = [ １＋ ∆ ]