Ductile Crack as Trigger of Brittle Fracture in Steel

Abstract

Brittle fracture of structural steel members in buildings is generally triggered by a ductile crack initiated at a notched surface after undergoing a noticeable amount of plastic strain, as evidenced by structural damage in Kobe during the 1995 Hyogoken-Nanbu earthquake, as well as by large-scale fracture testing in laboratory. This study, based on fracture experiments and finite-element analyses, showed the conditions governing the initiation of such a ductile crack in conjunction with notch sharpness, material properties, and specimen size. Major findings are as follows. Ductile cracking is governed by three physical parameters, that is, averaged plastic strain in a notched section, peak stress triaxiality under the notch root, and uniform strain capacity pertinent to the material, in such a way that the strain at the onset of a ductile crack increases with the reduction in stress triaxiality and with the increase in uniform strain capacity. Their relations are established in an empirical formula. It is, furthermore, shown that size effect is not involved in the criterion for crack initiation.