PSI - Issue 14

Y. Akaki et al. / Procedia Structural Integrity 14 (2019) 11–17 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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introduced through the shaft center of a specimen. The surface of specimen was finished by alumina-buffing following polishing with an emery paper up to #2000. Then, electro polishing was conducted to remove a thin residual stress layer. The residual stress measured by the X-ray diffraction method was within ± 30 MPa, which is a negligible value for torsional fatigue.

2.2. Continuous hydrogen-charging method

Hydrogen-charging was continuously performed during fatigue testing by circulation of an aqueous solution of 20 mass% NH 4 SCN in a specimen. The mechanism of hydrogen circulation as shown in Figs. 2 and 3 was developed by Akaki et al. (2017). The hydrogen-charging by this method was started 48 hours prior to the start of fatigue test. The hydrogen-charging solution was replaced every three days to maintain a sufficient concentration of hydrogen in a specimen throughout fatigue test. In this paper, the fatigue testing conditions with and without hydrogen-charging are hereafter termed an H-charging condition and a non-charging condition, respectively.

(mm) 㻌

(a) Specimen with Crack 2

(b) Specimen with Crack 1

Fig. 1 Fatigue specimens with an axial through-hole.

Fig. 2 Cross-section diagram of fixture with H-charging solution circulating mechanism (Akaki et al. (2017)).

Fig. 3 3D illustration of fixture (Akaki et al. (2017)).

2.3. Fatigue test

In order to investigate the effect of hydrogen on the threshold condition for shear-mode fatigue crack growth, the fatigue tests were conducted by the following method. Fatigue tests were carried out at a frequency of f = 20 Hz in air at room temperature using an MTS servo-hydraulic combined axial/torsional fatigue testing machine. The stress ratio was an R of ‒ 1. In addition to cyclic shear stress, a static compressive stress,  s , of ‒ 1200 MPa was axially applied to the specimen to suppress Mode I branching. In the first step of test, a shear stress amplitude,  a , of 900 MPa was