抄録
Fatigue crack threshold values for 13Cr cast steel and 304 stainless steel were obtained in different environment, i. e., in room air, in hermetic sealed air which is quasi-inert environment and in water. Furthermore, the fatigue strength of 13Cr cast steel specimens with small shrinkage cavities was obtained in room air and in water.
The threshold stress intensity value for a small defect was lower than that for a large crack, especially at a negative stress ratio. The threshold cyclic-plastic-zone-size criterion, which was proposed by the authors, could evaluate the fatigue threshold values for different stress ratios, by regarding defects as cracks. In water corrosion environment, the threshold value for a small defect could not be established and it was markedly lower than that for a large crack.
The fatigue crack threshold values in room air were as high as twice of those in the quasi-inert environment. This was attributed to the wedging effect of metal oxide formed on crack surfaces. Water corrosion did not reduce the threshold value at a high stress ratio, while it reduced a lot of at a negative stress ratio. A model for oxide induced fatigue crack closure behavior was proposed, which explains the effects of the environment and stress ratio on the fatigue crack threshold values.
