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.
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