Effect of microstructure on low-cycle fatigue property of structural steels under large plastic deformation

Low-cycle fatigue behavior of the structural steels under large plastic strain was investigated using notched round specimens. Crack growth rates show good correlation with the value of effective J-integral range, Delta J(eff). Crack growth rates are strongly affected by microstructure of the steels. It was shown that steels with higher pearlite volume fraction have lower crack growth rate. Many voids were observed around crack tip in the steel with higher pearlite volume fraction, which shows that the void growth and coalescence mechanism is dominant in this type of steels and crack growth were suppressed by branching and curving of the crack. On the other hand, in the steel with lower pearlite volume fraction, voids were hardly formed near crack tip region and crack propagates by plastic blunting and re-shaping mechanism, keeping the crack shape strait. Fatigue failure life were affected by MnS inclusion content. Large amount of void nucleation and growth were observed in the inner region of the specimen of the steel with large MnS content during cyclic loading, and final failure was occurred with a very short crack extension. It can be said that final toughness is improved by reducing MnS inclusion content and pearlite volume fraction, which can prevent void nucleation and growth in the inner region during cyclic loading.