3% NaCl水溶液中,各種鋼の下限界近傍疲労き裂伝ぱ特性とフラクトグラフ

抄録

Near-threshold fatigue crack growth properties and fractographic features in 3% NaCl aqueous solution were studied for high tensile steels SM50B and HT80, carbon steel S45C and stainless steels SUS304 and SUS403, under the condition where crack closure did not occur. The results obtained are summaried as follows:
(1) In air, fatigue crack growth properties were independent of the type of materials. The threshold values, ΔK, were equal to about 2.5MPa·m^1/2.
(2) Under the freely corroding condition in 3% NaCl solution, fatigue crack growth properties at a frequency of 30Hz coincided with those in air, when the stress intensity range, ΔK, was higher than 3MPa·m^1/2. However, the ΔK_th values were about 1MPa·m^1/2 for high tensile and carbon steels, and about 1.7MPa·m^1/2 for stainless steels.
(3) Decrease in frequency from 30Hz to 0.3Hz resulted in a sixfold acceleration of crack growth rate under the freely corroding condition.
(4) Under cothodic protection, fatigue crack growth properties including threshold were coincident with those in air at both frequencies of 0.3 and 30Hz.
(5) Under the freely corroding condition, the crystallographic fracture mode was observed near the threshold for stainless steels, while the fracture surfaces were dissolved for high tensile and carbon steels.
(6) Under cathodic protection, the fracture mode was transgranular at the region of ΔK<5MPa·m^1/2, while the intergranular cracking was formed at the region of ΔK≥5MPa·m^1/2.