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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applied to the specimen to initiate a crack under the non-charging condition. After the crack initiation,  a was decreased step by step to identify the threshold condition for crack growth. In this study, when the fatigue crack growth rate, d a/ d N , was being less than 10 -11 m/cycle for more than 10 6 cycles after  a had been decreased in the last step, the crack was defined as a non-propagating crack. The critical stress amplitude for non-propagation was defined as the threshold stress,  th , under the non-charging condition. After a non-propagation condition was fulfilled under the non-charging condition, the environmental condition was switched to the H-charging condition that was controlled by the continuous hydrogen-charging method. The test frequency, f , was changed from 20 Hz to 2 Hz. After the switching of environmental condition, the fatigue test was continued at the same shear stress level that had previously been the threshold stress,  th , under the non-charging condition. When the crack started propagation again under the H-charging condition, the shear stress amplitude was further decreased step by step according to the same procedure that was taken under the non-charging condition. When the crack became a non-propagating crack again, the shear stress amplitude was defined as a threshold stress,  th , under the H-charging condition. The growth behavior of cracks was observed at the scheduled numbers of cycles by the replica method and the surface crack lengths were measured on the collected replicas. 3. Experimental results and discussion It was observed under the non-charging condition that several cracks initiated simultaneously along the axial direction on the surface of a specimen in the first step test at a stress of  a = 900 MPa and then, macroscopically, they propagated steadily along the axial direction without causing Mode I branching. In this study, those cracks were defined as a shear-mode fatigue crack and the behavior of the longest crack in a specimen was successively observed afterward. Fig. 4 shows the relationships between half the crack length, a , and the number of cycles, N , for two shear mode fatigue cracks that were observed in different two specimens. The cracks of Fig. 5(a) and (b) are labeled as Crack 1 and Crack 2, respectively, in this study. The numbers in the figure indicate the shear stress amplitudes,  a , in MPa. As shown in this figure, as  a decreased step by step from the beginning of test, fatigue crack growth rate, d a /d N , decreased. When the shear stress amplitudes reached  a = 470 MPa and  a = 440 MPa as shown in Figs. 4(a) and (b), respectively, d a /d N of both cracks became a value in the order of 10 -12 m/cycle. These two cracks were regarded as being non-propagating at those stress levels and the corresponding stress amplitudes were defined as the threshold stress,  th , for growth of a shear-mode fatigue crack. Thereafter, the mechanical conditions of fatigue test, i.e. cyclic shear stress,  a , and static compression stress,  s , were kept unchanged and only the environment condition was switched from the non-charging condition to the H-charging condition. In the case of Mode I fatigue cracks, it is reported that the effect of hydrogen on the crack growth hardly appears at a frequency in 3.1. Effects of hydrogen on the near-threshold behavior of shear-mode fatigue cracks

Fig. 4 Crack growth curves for which numbers in the figure indicate stress amplitudes,  a in MPa.