Prediction of Ductile Fracture in Steel Connections Using SMCS Criterion

Conventional fracture mechanics approaches have limited capabilities to accurately predict fracture under conditions of large scale yielding or in complex geometries where there are no significant pre-existing flaws. Such conditions are found, for example, in welded steel moment connections that meet stringent detailing and quality assurance requirements for seismic design that have been imposed in response to damage caused by the Northridge earthquake. The stress modified critical strain (SMCS) criterion provides an alternative approach for modeling ductile crack initiation by relating the fundamental process of void initiation, growth, and coalescence to macroscopic stresses and strains, obtained using detailed finite element models. The material-specific parameters of the SMCS model are calibrated for mild A572 Grade 50 steel using a series of notched tensile tests and fractographic data. Accuracy of the SMCS criterion is demonstrated for specimens with a range of triaxial constraint conditions, including both sharp-crack fracture specimens and blunt-notch specimens, the latter being examples of cases where conventional fracture mechanics approaches would not apply. The SMCS model is then applied in a practical demonstration to relate data from idealized pull-plate tests to the fracture resistance of beam–column moment connections.