抄録
For preventing unstable brittle fracture of welded structure under cyclic loading, it is an important problem to clarify the transition behaviors from fatigue crack growth to unstable cleavage type fracture. Studies have been carried out of effect of specimen geometry and loading conditions on fatigue fracture toughness of low toughness material under cyclic loading. In the present experiment were used three point-bending notched specimens made from HT 80 which was subjected to heat treatment in order to take the brittle material. Fatigue crack of low toughness notched material under cyclic loading changes to unstable cleavage fracture at a certain critical stress intensity factor Kcf which is lower than Kc-value observed in static fracture toughness testing. When the maximum value of cyclic load is not relatively larger, brittle step can be observed during fatigue crack growth due to local embrittlement which is caused by pre-straining in the vicinity of fatigue crack tip. Although the Kci-value at which the first brittle step occurs is affected by initial loading conditions, Kcf at which unstable cleavage fracture occurs is independent of specimen geometry and load conditions.
