Abstract
It was already reported that the fatigue fracture surface for TMCP steels with large SImax. value has a lot of micro-separations which distribute with each length of 0.010.02 mm and with their direction parallel to their plate surface. These micro-separations can be supposed to generate between the boundary of ferrite and pearite structures in the vicinity of a crack tip as a crack propagates.In this report, we investigate the effect of existence of micro-separations on the fatigue crack propagation life and the change of crack contour line by using model test specimens with edge notches parallel to loading axis in which the fatigue crack goes through the passage between their notches in semi-elliptical quasi-surface crack. Moreover we investigate the density and the length distribution of ferrite-pearite structures which are supposed to be the generation parts of micro-separations and then propose an estimation method of the fatigue crack propagation life for steels in which micro-separations are generated as a fatigue crack grows in accordance with above investigated structures distribution.This estimation method shows the good agreement with experimental change of aspect ratio for surface crack and experimental crack propagation life for thickness direction.
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