炭素鋼予ひずみ材の腐食疲労寿命とき裂進展

Abstract

The effects of tensile prestrain on corrosion fatigue life and crack growth rate in 0.16%C carbon steel were studied under completely reversed plane bending stress in salt water (3.0%NaCl). The conclusions obtained are summarized as follows.(1) The corrosion fatigue strength of the prestrained specimen in the region of high cyclic stress amplitude increased in comparison with that of the annealed specimen and it increased monotonously with increasing prestrain and yield strength of the specimen. On the other hand, the effect of tensile prestrain on corrosion fatigue life disappeared in the region of low cyclic stress amplitude.(2) It was observed by an optical microscope that 80% of cracks were initiated from corrosion pits on the specimen surface under low cyclic stress amplitude. They were independent of the degree of tensile prestrain. The number of cycles to crack initiation, Ni, was not affected by tensile prestrain under the corrosion fatigue of low stress amplitude. In the region of high cyclic stress amplitude, it was found that Ni and the number of cycles during crack propagation increased with increasing tensile prestrain.(3) Micro-cracks on the specimen surface subjected to corrosion fatigue of low cyclic stress amplitude were initiated in the early stage of fatigue life and after that the surface crack density did not change with stress cycling. The growth rate of each surface crack on the specimen depended on the stress intensity factor in the early stage of fatigue life. But in the late stage of life, the growth rate of surface crack per cycle was constant. This constant value varied for each crack and followed normal distribution because of interaction and connection of distributed cracks.