Fatigue Limit Ratio of Torsional Fatigue to Rotating Bending Fatigue in Isotropic Materials and Materials with Clear-Banded Structure

摘要

In our experiments, the fatigue limit ratios of torsional and rotating bending fatigue ( τ~_w/σ_w) were about 0.55 for the annealed rolled round carbon steels (C=0.2, 0.45%) and about 0.65 for the annealed cast carbon steel, the diffusion annealed carbon steel (C=0.45%) and the quenched & tempered carbon steel (C=0.45%). The difference between the two values is due to the micro structure. The annealed rolled round carbon steels have a clear-banded structure of ferrite and pearlite. The other three have no banded structure. Namely, the former is regarded as an anisotropic material, and the latter is regarded as an isotropic material. The clear-banded structure in the axial direction affects the torsional fatigue limit greatly but does not affect the rotating bending fatigue limit. This is due to the fact that in the carbon steel having a clear-banded structure, the large local strain concentration is recognized within the ferrite in torsional fatigue but it is not recognized in rotating bending fatigue. Because of this fact, the torsional fatigue limit in a banded structure decreases relatively and therefore the value of fatigue ratio τ~_w/σ_w becomes small in the banded structure.