中炭素鋼および二相ステンレス鋼における疲労き裂成長に及ぼす負の応力比の影響

抄録

Fatigue crack growth behaviors such as the crack growth rate and crack closure were studied using medium carbon steel (S45C) and two phase stainless steel (SUS329J1) under various combinations of the negative stress ratio, R and the maximum stress relative to the yield strength, σ_max/σ_y. The main results obtained are as follows;
(1) In the range of intermediate fatigue crack growth rate, the crack growth rate, da/dN increased with a decrease of R and an increase of σ_max/σ_y for both the carbon steel and the two phase stainless steel. In this case, it becomes clear quantitavely that the compresion stress in the load cycle relative to the yield strength, S_c/σ_y is a very important factor which dominates the crack opening level and consequently da/dN in the both materials.
(2) In the range of low fatigue crack growth rate, however, the compression stress in the cycle, S_c caused da/dN to decrease for the carbon steel but to increase slightly for the two phase stainless steel. This is closely related to the fact that the oxide deposite forms easily on the fracture surfaces of the carbon steel but does not form on the stainless steel.