Abstract
The crack growth initiation behavior in delayed failure under superposed static or dynamic mode II stress on mode I static stress was investigated on the precracked specimens of Ni-Cr-Mo steel that was quenched and tempered at 673K. When the superposed mode II static stress intensity factor KII or the range of mode II dynamic stress intensity factor ΔKII on the static stress intensity factor KI was relatively small, the crack initiation time ti was longer than that when KII or ΔKII was zero. When KII or ΔKII was large, however, ti decreased with an increase in KII or ΔKII. The crack propagation direction under superposed KII or ΔKII on KI is dependent on fracture mechanisms. When hydrogen embrittlement governed the mechanism, the crack propagation angle θp once increased with an increase in ΔKII /KI and decreased with a further increase in ΔKII/KI. In the case of fatigue θp was small when ΔKII/KI was small and increased with an increase in ΔKII/KI. The crack propagation under superposed ΔKII on KI in water occurred by the hydrogen embrittlement type when ΔKII/KI was small, and changed to the fatigue type when ΔKII/KI became large.
