PSI - Issue 54

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

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The nominal stress – nominal strain curves as follows; In 70 MPa hydrogen SSRT tests were conducted up to 0.5% and 0.9% nominal strain in the elastic deformation region, then the load was released and the test was restarted in air. The curves (Specimen H and I) did not show any hysteresis. Specimen J in 70 MPa hydrogen up to 1.6% near the yield point, exhibited slight hysteresis. Specimens K and L in 70 MPa hydrogen up to 10% and 20% in the plastic deformation region, showed significant hysteresis due to plastic deformation. The relationship between the nominal strain exposed to 70 MPa hydrogen at 150 °C and RRA values of A286 at a strain rate of 7.5 × 10 -6 s -1 is shown in Fig. 5 [7]. The following points were found: 1) In the elastic deformation region, the RRA value of A286 decreased rapidly to 71% – 78% when exposed to 70 MPa hydrogen at 150 °C. 2) In the plastic deformation region after the yield point, the RRA value of A286 decreased gradually with the nominal strain of exposure to 70 MPa hydrogen at 150 °C. 3) In the elastic and plastic deformation regions, there was a discontinuity in the trend of the RRA values with respect to the nominal strain in both regions. The RRA value decreased sharply in the elastic deformation region, recovered once near the yield point, and then decreased gradually again. The fracture surface of Specimen G in air had a ductile fracture surface covered with large dimples by inclusions and fine dimples because that did not expose to hydrogen. Specimen H in 70 MPa hydrogen up to only 0.5% nominal strain exhibited QCs fracture surfaces without macroscopic unevenness. Specimen I subjected to 70 MPa hydrogen gas up to 0.9% nominal strain exhibited a mixture of shallow dimples and QC fracture surfaces, similar to Specimen H. Thus, the fracture surface morphology of the A286 specimens exposed to 70 MPa hydrogen at 150 °C under a low limited stress in the elastic region was clearly different from that of Specimen G exposed to air. Meanwhile, Specimen J tested in 70 MPa hydrogen up to 1.6% strain, which was near the yield point, exhibited shallow dimples and QC fracture surfaces. Specimen K tested in 70 MPa hydrogen up to a nominal strain of 10% exhibited shallow dimples and QC fracture surfaces. Specimen L tested in 70 MPa hydrogen up to a nominal strain of 20% exhibited fewer shallow dimples and more QC fracture surfaces than those in Specimens J and K [7]. Specimen B tested in 70 MPa hydrogen until fracture shows QC fracture surfaces and partly dimples. These results indicate that the fracture surface of the specimen exposed to 70 MPa hydrogen contained QCs, regardless of the magnitude of the nominal strain, even if the strain was only 0.5% in the elastic deformation region. This was consistent with the rapid decrease in the RRA values. It was confirmed that hydrogen embattlement of A286 caused not only in plastic deformation region but also elastic deformation region. As confirmed above, A286 specimens exposed to hydrogen in the elastic deformation region were affected by hydrogen, even when the SSRT tests were conducted again in air. Whether the effect of hydrogen exposure is further affected by the stress cycles in the elastic deformation region was then investigated. As before, SSRT test was conducted in 70 MPa hydrogen at 150 °C with a strain rate of 7.5 × 10 -6 s -1 up to a load of 400 MPa (nominal strain of 0.8%), then the stress was lowered to 200 MPa (nominal strain of 0.4%), and subsequently the stress was returned to a load of 400 MPa (nominal strain of 0.8%). This was repeated 10 times in 600 s under 70 MPa hydrogen, as shown on the right-hand side of Fig. 6 [7]. The load was then released, replaced by air, and the SSRT test was conducted at a strain rate of 7.5 × 10 -6 s -1 and 150 °C until failure as Specimen M. The nominal stress – strain curve is shown on the left-hand side of Fig. 6. Specimen M after 10 stress cycles showed an RRA value of 52%, which was lower than those of Specimen H and I (72% and 77%, respectively) [7]. The fracture surface morphology of Specimen M was characteristic faceted surfaces with slip lines and QC fracture surfaces. This fracture morphology was different from that of Specimen H and I in 70 MPa hydrogen at strains ranging from 0.5% to 0.9%. The fracture surface of Specimen M subjected to stress cycles in the elastic deformation region was similar to that of homogeneously charged Specimen with 56 mass ppm hydrogen, which exhibited facets with slip line traces with an RRA value of 49% [8]. These results indicate that stress cycles in the elastic deformation region promote hydrogen embrittlement. 3.3. Stress cycles in elastic deformation region