The background of fatigue limit ratio of torsional fatigue to rotating bending fatigue in isotropic materials and materials with clear-banded structure

摘要

The fatigue limit ratios of torsional to rotating bending fatigue (τ{sub}w/σ{sub}w) have been determined in several steels. This ratio was found to be equal to about 0.55 in an annealed rolled carbon steel (C=0.2, 0.45%) and about 0.65 in an annealed cast carbon steel, in a diffusion annealed carbon steel (C=0.45%) and in a quenched & tempered carbon steel (C=0.45%). The difference between the two values is due to the microstructure. The annealed rolled carbon steels delivered as round bars have a clear-banded structure of ferrite and pearlite. The other three materials have no banded structure. Namely, the former materials are regarded as anisotropic, while the latter ones are considered as isotropic materials. The clear-banded structure in the axial direction greatly affects the torsional fatigue limit, but it does not affect the rotating bending fatigue limit. This is due to the fact that, in the carbon steel with a clear-banded structure, large local strain concentration occurs within the ferrite in torsional fatigue but not in rotating bending fatigue. Because of this fact, the torsional fatigue limit in a banded structure relatively decreases and, therefore, the value of fatigue ratioτ{sub}w/σ{sub}w becomes small.