Ductile crack initiation behavior of structural steel under cyclic loading

Ductile crack initiation behavior of structural steel under cyclic loading was investigated using notched round bar specimens with comparing to monoto nic loading. FEM analysis was carried out to determine the effects of equiv alent plastic strain and stress triaxiallity on ductile crack initiation. U nder cyclic loading, ductile crack initiated at notch tip for all specimens differing in root radius, even for the specimen with root radius of 1.0 mm which crack initiated in the center of the specimen under monotonic loadin g. In the case of the specimen with large root radius under monotonic loadi ng, it is considered that void growth and coalescence is promoted by a larg e stress triaxiallity and it leads to ductile crack initiation in the cente r of the specimen. On the contrary, under cyclic loading or under monotonic loading with small root radius, deformation in the center of the specimen is small, and crack initiated at the notch tip by accumulation of equivalen t plastic strain under cyclic loading or concentration of strain at notch t ip. It can be stated that, effect of stress triaxiallity on ductile crack i nitiation at notch rip is small, and crack initiates at notch tip when equi valent plastic strain exceeds a certain level of strain.