低サイクル片振り引張疲れにおける鋼材の塑性ひずみ挙動

抄録

Low cycle fatigue tests with plain specimens and circular-holed plate specimens were carried out under constant load pulsating tension. The materials used are mild steel (SM 41) and quenched and tempered high tensile strength steel (HT 60). The development of plastic strain around a circular hole was experimentally analysed by the Moiré method at successive stages of fatigue, and the following characteristics were clarified concerning the plastic behavior in the specimens under low cycle fatigue.
In the case of SM 41, the distribution of plastic strain is relatively even for both plain and circular-holed specimens through all the stages of fatigue. In contrast to SM 41, the tendency of localization of strain to narrow regions is remarkable in the case of HT 60.
It is seen that the generalized plastic strain which occurred in the specimens only fractured in ductile manner under pulsating tension shows nearly a constant value, irrespective of the number of cycles at fracture.
It is also found that the generalized plastic strain near the fracture initiation point shows the behavior of cyclic creep which generally consists of three stages. The second stage, the stationary stage, of it usually shows the minimum creep rate and occupied the major part of the whole fatigue life. The following relation is found to exist in this stage, irrespective of materials, size of specimens, existence of circular hole, and frequency of load cycle,
(Δε) · N_f=ε₀/2
where Δε is the minimum creep rate,
N_f is the number of cycles at fracture,
ε₀ is the generalized plastic strain at fracture in simple tension.