Abstract
Characteristics and mechanism of fatigue crack growth in mild steel have been investigated taking account of crack tip deformations: crack tip opening displacement δt and the size of highly deformed zone ahead of crack tip Rx0.2 (the size of the zone with accumulated strain above 0.2). δt was measured directly at midsection of the specimen with a profile projector. Rx0.2 was obtained from strain distribution ahead of crack tip determined by the use of the recrystallizalion phenomenon. It is revealed that crack growth rates ranging from 0.02 μm/cycle to 200 μm/cycle are expressed well by a second power function of both δt and Rx0.2. Abrupt increases in μt and Rx0.2 occured at a growth rate of about 1 μm/cycle. At this growth rate, fracture appearance changed from striation to dimple. These transitions are due to the transition of stress state. It is also shown that fatigue fracture strain is constant independently of crack growth rate and is equal to the ductile fracture strain in monotonic loading. The constant fracture strain is the criterion for fatigue failure of ductile steels.
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