1989 年 38 巻 435 号 p. 1383-1389
Threshold strength of delayed failure, KISCC, under cyclic mixed mode I and II loading conditions was investigated for a Ni-Cr-Mo steel (SNCM439), that was quenched and tempered at 673K. When the mode II static stress intensity factor KII or the range of mode II dynamic stress intensity factor ΔKII was superposed on the static stress intensity factor KI, the threshold strength KISCC decreased with an increase in KII or ΔKII. The amount of decrease of KISCC due to the superposition of ΔKII was larger than that of KII. When the ratio of stress intensity factor KR was large, the crack propagation direction under superposed KII or ΔKII on KI at near the KISCC was normal to the maximum tensile stress. Therefore, when the ratio of stress intensity factor KR is large, the threshold condition on the delayed failure crack growth initiation can be described by the maximum tensile stress criterion, which is given by σθmax√2πr=constant.