Triaxiality and Fracture of Steel Moment Connections

Abstract

The connections of welded steel moment frames undergo a complex multiaxial state of stress that leads to high levels of stress triaxiality. As triaxiality increases, the propensity for fracture increases. Classic engineering models of fracture and modern microscale models of fracture mechanisms explicitly consider the role of triaxiality. Nonetheless, triaxiality is generally not directly considered by structural engineers. In this paper, triaxiality is defined as the ratio of the maximum principal stress to the von Mises stress. Triaxiality and maximum principal stress demands are investigated for tests on fractured notched round bars, small-scale tension specimens, and a full-scale moment connection. Based on analysis of the tests, it is proposed that, for fractures driven by triaxiality demands, the maximum principal stress at fracture is a function of the level of triaxiality. Calculation of the triaxiality demands requires 3D nonlinear analysis and depends on the loading, connection geometry, and postyield stress-strain relationships of all parent and weld metals. Examination of a welded steel moment connection indicates particularly high triaxiality demands. The triaxiality demands indicate that fracture of these connections may be governed by triaxiality even when high toughness parent and weld metals are used.