ねじり疲労強度に及ぼす引張圧縮疲労試験により導入した微小予き裂の影響

抄録

Reversed torsional fatigue tests were carried out on the 0.47%C steel specimens containing an initial small crack. The initial small semi-elliptical cracks ranging from 200μm to 1000μm were introduced by the preliminary tension-compression fatigue tests using specimens which contains a hole of 40μm diameter. The crack growth behaviour from a semi-elliptical surface crack was investigated. The effects of artificial small defects on torsional fatigue strength are also reviewed in relation with those of small cracks. The conclusions are summarized as follows:
(1) In rotating-bending and tension-compression fatigue, the effects of artificial small defects and small cracks are identical from the viewpoint of fatigue limit. However, in torsional fatigue these effects are not identical.
(2) Cracks leading to fracture were Mode I branched cracks initiated at crack tips. Initiation of Mode II cracks at the pre-crack tip was also observed in most cases in addition to Mode I branched cracks. However, these cracks stopped propagating as the Mode I branched cracks propagated.
(3) Torsional fatigue limit for a semi-elliptical small cracks is determined by the threshold condition for non-propagation of Mode I branched cracks.
(4) Torsional fatigue limit of specimens containing a small crack can be successfully predicted by the √area parameter model with a combination of σ_θmax criterion.