PSI - Issue 68

Akihiko Fukunaga et al. / Procedia Structural Integrity 68 (2025) 1059–1065 Akihiko Fukunaga / Structural Integrity Procedia 00 (2025) 000–000

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specimen which is almost equilibrium surface hydrogen content at 150 °C in 70 MPa gaseous hydrogen, or uncharged specimens in air at room temperature. The RRA values were calculated from the ratio of relative reduction in area in gaseous hydrogen to air at each strain rate. The fracture surface of the specimen after SSRT test was observed using a scanning electron microscope (SEM). The vertical cross-section near the fracture surface was polished by 3, 1, 0.3, 0.05 μm α-alumina after #60, 240, 400, 600, 800, 1000. 1500. 3000 sand paper, and the inverse pole figure and KAM map were observed with 0.25 μm step using EBSD (JEOL: JAMP-9500F) to determine the KAM value. 3. Results and discussions 3.1. SSRT tests for A286 in 70 MPa gaseous hydrogen

Fig.2. (a), (b), (c) Nominal stress − nominal strain curves, (d) Strain dependence of RRA valves, and (e) Fracture surface morphology for SSRT test of A286 in 70 MPa gaseous hydrogen [10].

Figs. 2 (a) − (c) show the nominal stress − nominal strain curves of A286 at 150 ℃ in air and 70 MPa gaseous hydrogen for each strain rate. Up to the yield points and maximum stress points, the stress-strain curves in air (black lines) and in 70 MPa hydrogen (red dotted lines) are almost same. After the maximum stress, the curves in hydrogen are shorter than those in air for slower strain rate of two specimens (Fig. 2 (b) and (c)). Fig. 2 (d) shows the strain rate dependence of the RRA values obtained from the SSRT test. When strain rate becomes slower the RRA values significantly decrease from 80% to 32% The fracture surface of the specimen at the highest strain rate had visible dimples, as the strain rate slowed down, showed quasi-cleavages (QCs) and an intergranular fracture surface at the slowest strain rate of 1.25 ×10 -6 s -1 as shown in Fig. (e). This change in fracture surface morphology is consistent with the change in RRA value.