PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 312–319 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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However, such behavior was not revealed by the crack in the H-charged specimen. Furthermore, as the crack-growth entered the Paris’ law regime, the d a /d N of the non-charged and H-charged specimens was approximately identical. In other words, there was no hydrogen-induced crack-growth acceleration in the near-threshold phase of the Paris’ law regime in this alloy. Therefore, it was assumed that the degradation in the fatigue limit and Δ Κ th of the small crack was due to the effect of hydrogen on the crack-growth behavior, but only near the crack initiation site at a low Δ Κ th . The S - N diagram of H-charged and non-charged CG specimens with large defects is showcased in Fig. 6. The artificial defects introduced into the CG specimens were a circumferential notch ( cf. Fig. 2 (f), √ area = 316 µm) and an 800-µm, semi-circular EDM notch ( cf. Fig. 2 (g), √ area = 501 µm). As seen in FG, fatigue limit degradation due to H-charging at 11 MPa was also noted in both smooth and circumferentially-notched CG specimens. The fatigue limit of the smooth specimen was degraded by 20%, whereas that of the circumferentially-notched specimen was degraded by 29%. However, based on observations, H-charging had no detrimental effect on the fatigue limit in the EDM-notched CG specimens. Considering that non-propagating cracks exist in all of the run-out specimens, the fatigue limit as a crack-growth threshold was determined by the Δ Κ th value of each defect size. Fig. 7 demonstrates the relationship between Δ Κ th and the defect size √ area . The Δ Κ th values of the H-charged specimen, determined from the stress amplitude at which

Material: Alloy 718 CG, HV = 447 In air at RT, R = −1

Fig. 6. S - N diagram of H-charged CG specimens with large defects.

Fig. 7 Relationship between Δ Κ th and the defect size √ area (Kitagawa-Takahashi diagram) in Alloy 718.