Abstract
Rotating bending and torsional fatigue tests were carried out on plain specimens and small-hole specimens of rolled round carbon steel bars which were quenched and tempered. The present specimens are regarded as isotropic materials although the original materials have a banded structure. The fatigue limit ratio between rotating bending and torsional fatigue (τw/σw) in the specimens is different from that in an annealed rolled round carbon steel bar, and is approximately the same as that in isotropic cast carbon steel. We should use isotropic materials in discussing effects of combined stresses on fatigue. In the torsional fatigue tests, crack propagation on plain specimens was of the shear type in the initial stage and of the tension type in the later stage. Cracks of the shear type were generated in both the circumferential and axial directions. The crack growth laws in bending and torsion are expressed by dl/dN∝ σ anl or dl/dN∝τanl. The dependency of 1/Nf on the stress amplitude σa and τa in bending and torsional fatigue is approximately the same as the dependency of the crack growth rate on σa or τa, respectively.
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