炭素鋼の引張圧縮疲労強度に及ぼす応力繰返しの間の引張大荷重の影響

抄録

Fatigue tests were carried out on smooth specimens of 0.33% carbon steel by applying one high tensile load or periodic high tensile loads during stress cycling, and the results were discussed on the basis of variations of plastic strain range at cyclic stress and high tensile load, and surface observations during stress cycling.
The main conclusions obtained are summarized as follows:
(1) In the fatigue tests with one high tensile load during stress cycling, the high tensile load corresponding either to the lower yield point (σ_H=36.5kg/mm²) or to 9.1% strain (σ_H=57.0kg/mm²) of the virgin specimen does not influence the fatigue life.
(2) In the fatigue tests with periodic high tensile loads (σ_H=36.5kg/mm²) during stress cycling, the fatigue lives are almost the same as those of the virgin specimen in the range of overstress levels, but the fatigue failure occurs by understress levels and the S-N curve bends slightly to the side of shorter life.
(3) In the fatigue tests with periodic high tensile loads (σ_H=57.0kg/mm²) during stress cycling, the fatigue lives are almost the same as those of the preloaded specimen in the range of overstress levels and the S-N curve bends abruptly to the side of shorter life in the range of understress levels. The fatigue lives are shorter than those in the case of σ_H=36.5kg/mm².
(4) In the two fatigue tests with one high tensile load and periodic high tensile loads during stress cycling, the plastic strain range at cyclic stress after high tensile loads tends to reduce in comparison with the virgin specimen. This is due to work hardening by high tensile loads.
(5) As mentioned above, the plastic strain range is smaller than that of the virgin specimen, but the correlation between plastic strain range and fatigue life does not exist. This results from that the fatigue failure of specimens subjected to high tensile loads occurs by local causes such as the stress concentration due to non-uniformity of strain distribution and the opening of slip bands or micro-cracks by high tensile loads.
(6) When the fatigue tests with periodic high tensile loads are regarded as the two steps repeated multiple loads fatigue tests, the values of cumulative cycle ratio are very small. Especially, it is interesting and important that, in the case of σ_H=36.5kg/mm², the fatigue failure occurs by the combination of stresses that do not cause failure individually.