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 : せん断平均応力および引張りまたは圧縮平均応力の影響

抄録

Torsional fatigue tests under shear mean stress (τ_m) and tensile or compressive mean stress (σ_m) were carried out on the 0.47% C steel (JIS S45C) specimens containing an initial small crack. An initial small semi-elliptical crack was introduced by the preliminary tension-compression fatigue tests using specimens which contain a hole of 40 μ_m diameter. The initial branching angle of cracks that emanated from the initial crack tip was approximately ±70.5°to the initial crack plane, where the local tangential stress σ_o has the maximum value. The branched cracks eventually propagated perpendicularly to the plane where the range of normal stress has the maximum value, i.e. ±45°to the specimen axis. The torsional fatigue limit under mean stress is determined by the threshold condition for non-propagation of Mode I branched cracks emanating from the initial crack tip. The prediction method for torsional fatigue limit of specimens containing an initial small crack is proposed by the extended application of the √area parameter model with a combination of the maximum tangential stress (σ_omax) criterion.