Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 8 Figure 10 : Relation between fatigue crack growth rate and number of cycles ( f = 30 Hz).  pre : Specific angle of twist. Figure 11 : Relation between residual hydrogen content in the specimen and time after the end of hydrogen charge for the torsional prestrained S10C and S25C steel.  pre : Specific angle of twist. Effects of a large amount of hydrogen on the fatigue limit of carbon steel Fig. 10 shows the relation between d a /d N and N , while Fig. 11 shows the relation between the residual hydrogen content (diffusible hydrogen content) in the specimen and time, following the first hydrogen charging of the torsional prestrained (  pre = 45.0 deg/mm) S10C steel and (  pre = 20.0 deg/mm) S25C steel. The results shown in Fig. 11 were obtained by measuring the hydrogen content of the hydrogen measurement specimen (Fig. 1(b)) used for TDA at 30  C and heated to 520  C after 48 h. The hydrogen content values during the fatigue test shown in Fig. 10 were calculated using the results given in Fig. 11. The fatigue test of the torsional prestrained (  pre = 20.0 deg/mm) uncharged S25C specimen was first conducted at  a = 135 MPa. Under this condition, a non-propagating crack was observed at the edge of the artificial small hole, and the fatigue crack growth rates were d a /d N = 2.0 × 10 −11 m/cycle at N = 8–9 × 10 6 cycles and d a /d N = 4.1 × 10 −11 m/cycle at N = 9–10 × 10 6 cycles. Hence, d a /d N = 2.0 × 10 −11 to 4.1 × 10 −11 m/cycle was defined as the threshold d a /d N value between non-propagation and propagation in this paper. After hydrogen charging, the observed d a /d N was greater than 10 −10 m/cycle, indicating crack propagation. Following this propagation, d a /d N then decreased to 3.8 × 10 −11 m/cycle, while the estimated hydrogen content decreased to 7.1  4.2 mass ppm. Then, the same specimen was hydrogen d a /d N defined as the boundary between non- propagating and propagating.