CRITERION FOR DUCTILE CRACK INITIATION IN STRUCTURAL STEELS UNDER TRIAXIAL STRESS STATE

Abstract

Brittle fracture of structural steel for building use, in general, is triggered by a ductile crack initiated at notched surface after undergoing a noticeable amount of plastic strain. This study showed the conditions governing the initiation of such a ductile crack in conjunction with the notch sharpness, material properties, and specimen size. Monotonic tensile test of round bar specimens having a circumferential notch at the center was performed to observe the behavior of crack initiation, and then finite element analysis was done to investigate the triaxial states of stress and strain in the notched section. Major findings are as follows : The crack initiation strain tends to increase with the reduction in stress triaxiality underneath the notched surface and with the increase in uniform elongation capacity pertinent to the material, and their mathematical relation is proposed in an experimental formula. Furthermore, it is clarified that size effect is not involved in the crack initiation.