Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 6 between fatigue crack growth rate (d a /d N ) and stress intensity factor range (  K ) for S25C steel. S25C steel exhibits similar tendencies in the acceleration and saturation of fatigue crack growth rate in the  pre range of 0–20.0 deg/mm. Based on these results, Fig. 9 shows the relation between {(d a /d N ) H /(d a /d N ) U } and C H . The fatigue crack growth acceleration rates of S10C and S25C steels do not increase, even when C H exceeds 10 mass ppm. This indicates that {(d a /d N ) H /(d a /d N ) U } has an upper bound estimated to be in the range of about 30 for both S10C and S25C steels. At this point, the effects of hydrogen precharging on fatigue crack growth rate have been reported in the previous studies. In case of 0.08 mass%C ferritic–pearlitic carbon steel ( C H = 1.0 mass ppm), for f ≤ 0.01 Hz, the fatigue crack growth acceleration rate {(d a /d N ) H /(d a /d N ) U } = 10 [1]. Tanaka et al. [7] reported a {(d a /d N ) H /(d a /d N ) U } upper bound of 30 for Cr–Mo steel, JIS-SCM435, (HV = 330; C H = 0.24–0.59 mass ppm). Previous studies indicate that the maximum upper bound of {(d a /d N ) H /(d a /d N ) U } is about 30 for small hydrogen contents. In this study, even for large hydrogen contents, the maximum upper bound remained at around 30. The upper bound value found in this study can be used as one parameter in the design of hydrogen-related devices. Further investigation is important to consider {(d a /d N ) H /(d a /d N ) U } at lower frequencies. (a ) Virgin materials (b) Torsional prestrained materials Figure 7 : Relations between fatigue crack growth rate and stress intensity factor range in S10C steel.  pre : Specific angle of twist. * The dotted line shows 30 times acceleration of fatigue crack growth rate for hydrogen-precharged specimen than the uncharged specimen.