1988 年 37 巻 417 号 p. 670-675
The influences of bending and torsional loading modes on cyclic SCC crack initiation behavior were investigated in a high-strength steel, SCM435, sensitive to hydrogen embrittlement type SCC. Under bending loads, an intergranular cyclic SCC crack by tensile stress is initiated at the bottom of a corrosion pit, and the strength of cyclic SCC is smaller than that of fatigue in laboratory air. Under torsional loads in laboratory air and in a 3.5% NaCl solution at higher stress levels of τmax≥780MPa, specimens are fractured by transgranular cracking normal to the longitudinal direction, with no influence of environment on strength. At τmax<780MPa, however, cyclic SCC strength is smaller than fatigue one. At 500MPa≤τmax<780MPa, a logitudinal crack by shear stress, which is considered to be caused by anodic dissolution, is initiated at the bottom of a corrosion pit, and then an intergranular cyclic SCC crack by hydrogen embrittlement at an angle of 45 degrees against the longitudinal direction (45-degree crack), which is dominated by a principal stress, is initiated at the tip of the longitudinal crack. At τmax<500MPa, however, an intergranular 45-degree crack by hydrogen embrittlement is immediately initiated at the bottom of a corrosion pit. In the case of bending and torsional loads at τmax<500MPa, the mechanical condition for crack initiation is determined by the KIFSCC value, which is obtained by assuming corrosion pits as sharp cracks. In a similar way, a longitudinal crack is initiated when a KIII value exceeds the KIIIFSCC; a 45-degree crack is initiated when a Mode I stress intensity factor of a longitudinal crack exceeds the KIFSCC value.