Transition between Fatigue and Ductile Fracture in Steel

Abstract

Cyclic tension and compression testing has revealed that cracking behavior in structural steel subjected to plastic strain reversals is transitional from a fatigue to a ductile mode with the elevation of strain amplitude. The transition can be identified by the plastic strain accumulation pattern, the fractographic appearance, the crack penetrating profile, and the convex feature of the Manson-Coffin's curve. Since the strain amplitude induced by a severe earthquake is large enough to cause a crack in a few or several cycles of plastic strain, the crack is not a fatigue crack, but a ductile crack. Thus, the crack onset under cyclic seismic overloading can be predicted from the crack initiation strain under monotonic tensile testing. Because the monotonic tensile test is easily carried out in practice, it can be used to qualify steels and welds for seismic applications.