鋼材の低サイクル疲労に関する研究 (第1報)

低サイクル疲労における断面収縮型破壊

抄録

A Method for prediction of the life of necked-out type fatigue fracture under low cyclic loading was proposed. Practically, in case of the necked-out type fatigue fracture, it may be assumed that fatigue life has been used up when necking began. In such a case, when accumulated permanent strain, ε_m, amounts to critical value, ε_us, the fatigue failure occurs, and the critical value is nearly equal to the general elongation of material, ε_u0. Then a relation between strain and number of cycles is expressed approximately as follows;
Δε_m·N_us=ε_us-ε₀=ε_u0-ε₀
where Δε_m is a cyclic creep rate at the stationary stage which occupied major part of whole fatigue life, N_us is number of cycles to the fatigue failure and ε₀ is a permanent strain at the first cycle. The cyclic creep rate Δε_m is correlated to nominal maximum stress, σ_max, and stress ratio, R, and the following relation is assumed.
(σ_B-σ_max) /σ_Y= (∑ A_i·Rⁱ) ·Δε_m^β
Constants, A_i, and exponent, β, in the above equation are able to be obtained from statical test results. In order to prove the validity of these equations, mild steel specimens were tested under constant repeated axial loads with several stress ratios, and the test results were compared with the values calculated by the equations, and fairly good agreements between them were obtained.