長寿命域ならびに乾湿繰返しにおける腐食疲労強度

概要

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Corrosion fatigue behaviors have been studied on a low carbon steel exposed to continuous or alternate immersion in 1% NaCl under low stresses in the long life range up to 10^9 cycles corresponding to life time up to 10^4 hours. S-N curves of the long life corrosion fatigue are composed of two curves having different gradients, those belonging to the short and long lives being corresponded to the corrosion fatigue stage and to the corrosive crack-extension stage, respectively. In the former stage mechanical repetitive stresses extend fatigue cracks directly, whilst in the latter corrosive dissolution at crack tips brings crack extension. Fractured surface morphology reveals also the change corresponding to the S-N curves, the inter-and transgranular mixed modes in the corrosion fatigue stage being replaced by the corrosive dissolution modes in the corrosive crack-extension stage. The life cycles under the alternate immersion are longer than those under the continuous one when compared at the same net immersing duration. This is due to the delayed growth of corrosion pits and the increased wedge action of corrosion products in the corrosion fatigue stage and to the delayed extension of crack tips by corrosive dissolution in the corrosive crack-extension stage.
1976-09-15