Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 4 Fig. 4 shows the relation between hydrogen content ( C H ) and equivalent prestrain at the specimen surface (  eq,s ) along with the specific angle of twist (  pre ). Previous results [4] for S25C steel are also included in Fig. 4. Hydrogen content increased with the prestrain. Compared with that of S25C steel, C H of S10C steel is smaller at the same torsional prestrain due to the difference in the fraction of pearlite. As shown in Fig. 4, S10C steel has a higher ductility than S25C steel; thus, higher prestrain can be introduced into S10C steel. However, no clear difference in the maximum hydrogen contents in torsional fracture specimens between S10C and S25C steels exists. In S25C steel, for  pre ≥ 25.0 deg/mm, some cracks are initiated during the introduction of torsional prestrain. In contrast, for the prestrained S10C steel specimen, few cracks are initiated, even when  pre = 45.0 deg/mm. Therefore, a fatigue test with a hydrogen-precharged specimen containing a large hydrogen content ( C H = 36.7 mass ppm) could be conducted using torsional prestrained (  pre = 45.0 deg/mm) S10C steel. (a) S10C steel (b) S25C steel [4] Figure 3 : TDA hydrogen release profiles during continuous heating of hydrogen-precharged S10C and S25C specimens.  pre : Specific angle of twist. Figure 4 : Relationship between hydrogen content and  eq,s . Effects of a large amount of hydrogen on the fatigue property of carbon steel Fig. 5 shows the relations between the number of cycles to failure ( N f ) and equivalent strain at the specimen surface (  eq,s ). The uncharged virgin specimen fractured after 3.67 × 10 4 cycles, whereas the fatigue lives of both S10C and S25C steels increased with the torsional prestrain due to work hardening. However, in specimens precharged with hydrogen, fatigue life decreased with increasing prestrain. Fig. 6 shows the relations between crack length (2 a ) and number of cycles ( N ) for S10C and S25C steels. With respect to crack initiation, there is no obvious difference between the uncharged and