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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(a) Crack 1 Fig. 5 Shear-mode fatigue crack growth behaviors that were observed before and after changing only environmental condition from non charging to H-charging condition by keeping mechanical condition of  a and  s unchanged. (b) Crack 2

the order of 20 Hz. Therefore, the test frequency was changed from 20 Hz to 2 Hz to ensure the investigation of the effect of hydrogen. Fig. 5 shows the behaviors of Crack 1 and Crack 2 that were observed before and after switching the environmental condition. As shown in Figs. 4 and 5 the cracks that had been a non-propagating crack under the non-charging condition restarted propagation under the H-charging condition. The crack growth rate, da/d N , just after switching to the H-charging condition reached more than 10 -10 m/cycle, which is more than about hundredfold. This result clearly indicates that the growth behavior of a shear-mode fatigue crack is significantly affected by hydrogen, which has commonly been observed for a Mode I fatigue crack in many metallic materials including JIS-SUJ2. In addition, to measure the threshold level under the H-charging condition,  a was further decreased step by step according to the same procedure that was taken under the non-charging condition. Crack 1 and Crack 2 stopped propagation again and became a non-propagating crack at stress levels of  a = 370 MPa and  a = 360 MPa, respectively, as seen in Fig. 4. These stress amplitudes were defined as the threshold stress,  th , for growth of a shear-mode crack under the H-charging condition. 3.2. Effects of hydrogen on the threshold stress intensity factor (SIF) ranges,  K  th , for growth of a shear-mode fatigue crack and its crack size dependency Okazaki et al. (2011, 2014) proposed the following equation of the shear-mode SIF, K  , for a semi-elliptical surface crack subjected to shear stress at infinity: = 0.69 ∙ √ √ (1) where  is the remote shear stress and √ is the square root of crack area. This equation gives the maximum value of either Mode II SIF or Mode III SIF along the front of a three-dimensional crack, which is represented by a single parameter of K  in place of K II and K III . The threshold SIF ranges,  K  th , were calculated based on half the surface lengths of non-propagating cracks, a , and the corresponding threshold stress amplitudes,  th . These experimental values B (  th = 440 MPa (n n)  th = 360 MPa (H))