Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 9 charged for a second time with almost the same amount of hydrogen as in the first charging, as was previously confirmed. At the beginning of the repeat fatigue test, d a /d N was 5.2 × 10 −11 m/cycle, which is slightly higher than the 4.1 × 10 −11 m/cycle threshold value, indicating crack propagation. However d a /d N then decreased to 1.0 × 10 −11 m/cycle and crack propagation was not clearly observed; this was accompanied by a decrease in hydrogen content to less than 4.8 mass ppm. Hence, it was presumed that hydrogen enhances the growth of a non-propagating crack. However, in the case involving hydrogen content of less than 4.8 mass ppm, the non-propagating crack was not caused to propagate by the hydrogen. The explanation for this behavior is that, as the hydrogen was released into the atmosphere during the test, there was insufficient contained hydrogen for enhancement of the fatigue crack growth. At the end of the S25C steel fatigue test, crack propagation occurred at a lower hydrogen content. It is possible that small cracks initiated by the introduction of the torsional prestrain grew in locations other than the main crack site, and all of these cracks then coalesced. For a hydrogen content range of 4.8 to 1.4, the main crack did not propagate at the surface, however it was presumed that the small cracks propagated and coalesced with the main crack at a subsurface. Then, the main crack finally exhibited propagation at a lower hydrogen content, due to coalescence between the small cracks and the main crack. Thus, in the case involving hydrogen content of less than 4.8 mass ppm, the hydrogen itself did not enhance the main crack growth. A similar fatigue test of torsional prestrained (  pre = 45.0 deg/mm) S10C steel was conducted at  a = 150 MPa. A non- propagating crack also occurred at the edge of the artificial small hole in the absence of hydrogen. The fatigue crack growth rates were d a /d N = 4.5 × 10 −11 m/cycle at N = 8–9 × 10 6 cycles and d a /d N = 2.3 × 10 −11 m/cycle at N = 9–10 × 10 6 cycles, which is almost identical to the defined d a /d N threshold for a non-propagating crack for S25C steel. In the S10C steel case, secondary hydrogen charging was not conducted, because the hydrogen content was high (5.8 mass ppm), even after 5 × 10 6 cycles of the fatigue test. At the beginning of the fatigue test of the hydrogen-charged S10C specimen, d a /d N was greater than 10 −10 m/cycle, indicating crack propagation. Fig. 10 shows that d a /d N was decreased by increases in both time and N , while Fig. 12 shows photographs of the crack for N = 1.10 × 10 7 and 1.24 × 10 7 cycles after hydrogen charging of the S10C steel. Crack propagation was only clearly observed in this specimen. In the S10C prestrained specimen case, the hydrogen content was higher than that of the S25C prestrained specimen. However, crack propagation was arrested at a hydrogen content value of 15.6 to 10.7 mass ppm. Hence, 15.6 mass ppm hydrogen is the threshold value beyond which hydrogen enhances the propagation of an arrested crack. It has been reported that the fatigue limit of carbon steel with HV ≤ 200 is not reduced by hydrogen (S25C steel; HV = 129; C H = 0.31 mass ppm) [3]. However, in the presence of higher hydrogen content, it is possible for hydrogen to enhance the propagation of an arrested crack even for these metals, as large amounts of hydrogen decrease the fatigue limit. It seems that the difference in the hydrogen content threshold values for S10C (15.6 mass ppm) and S25C (4.8 mass ppm) steels is due to the densities of the small cracks initiated during the torsional prestraining. Because the pearlite content of S25C steel is greater than that of S10C, a greater number of pearlite cracks were initiated for S25C steel than for S10C. This problem requires more investigation, in order to clarify the effects of the small cracks initiated during torsional prestraining on the non-propagating crack. (a) N = 1.10 × 10 7 ( C H,R = 15.6 mass ppm) (b) N = 1.24 × 10 7 ( C H,R = 10.7 mass ppm) Figure 12 : Non-propagating crack (S10C;  pre = 45.0 deg/mm;  a = 150 MPa; f = 30 Hz).  pre : Specific angle of twist. C H,R : Hydrogen content by the equations in Fig. 11.