抄録
The crack growth initiation behavior in delayed failure under superposed cyclic mode II stress of various stress wave forms was investigated on the precracked specimens of Ni-Cr-Mo steel quenched and tempered at 673K. When the range of mode II cyclic stress intensity factor ΔKII was superposed on the static stress intensity factor KI, the crack growth initiation time tg decreased with an increase in ΔKII, the degree of which increased with an increase in applied stress increasing period at a constant frequency; i.e. tg decreased most under cyclic load with positive saw tooth wave, followed by those with negative saw tooth wave and trapezoid. The intergranular fracture ratio ψ of crack growth initiation surface was affected little by stress wave forms of ΔKII. The reason why the crack growth initiation time tg decreased when the stress wave form of mode II had a long stress increasing period seems to be that the corrosive reaction is activated during the stress increasing period, since the protective film in the vicinity of crack tip is destroyed mechanically by cyclic stress.
