Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 5 hydrogen-precharged specimens in spite of the large amount of hydrogen introduced by torsional prestrain. For the virgin specimen, hydrogen does not affect crack growth or initiation. However, for all torsional prestrained specimens, the presence of hydrogen accelerated crack growth; thus, the N f values of torsional prestrained specimens precharged with hydrogen were clearly decreased. Figure 5 : Relations between number of cycles to failure and  eq,s . (a) S10C steel (b) S25C steel Figure 6 : Relations between crack length and number of cycles.  pre : Specific angle of twist. Effects of a large amount of hydrogen on the fatigue crack growth acceleration rate of carbon steel Fig. 7 shows the relation between fatigue crack growth rate (d a /d N ) and stress intensity factor range (  K ) for S10C steel. For the virgin material, no obvious effect of hydrogen on the fatigue crack growth rate was observed. For torsional prestrained materials, the acceleration ratios {(d a /d N ) H /(d a /d N ) U } were estimated, where (d a /d N ) H represents the value of d a /d N for the hydrogen-precharged specimen, and (d a /d N ) U is the d a /d N for the uncharged specimen. For instance, when  pre = 15.0 deg/mm, {(d a /d N ) H /(d a /d N ) U } = 4 at  K = 15 MPa √ m, indicating that the fatigue crack growth rate of the hydrogen-precharged specimen is 4 times higher than that of the uncharged specimen. For  pre = 30.0 deg/mm, {(d a /d N ) H /(d a /d N ) U } = 13 at  K = 15 MPa √ m. These results show that the fatigue crack growth rate of hydrogen- precharged materials increases with the hydrogen content ( C H ). For  pre = 45.0 deg/mm the results were drown by dot lines at positions apart far from other prestrained specimens. {(d a /d N ) H /(d a /d N ) U } for  pre = 45.0 deg/mm was about 16 at  K = 15 MPa √ m, which is almost the same as that obtained for  pre = 30.0 deg/mm. Fig. 8 shows the relation