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While considerable progress has been made in simulating the overall seismic response of steel structures using nonlinear response history (dynamic) analysis, techniques to simulate fracture propagation under large scale inelastic cyclic loading are not as well developed. This is despite the fact that fracture is often a critical limit state that can precipitate structural failure and collapse. To address this, a new ductile damage-based cohesive zone model is presented. The proposed model is an extension of the established continuum-based local or micromechanical ductile fracture models for evaluating ultra-low cycle fatigue in structural steels. This model is implemented in the finite element program WARP3D, and evaluated against tests of notched bars that fail by ductile crack propagation. The preliminary results indicate that the model is an effective tool for predicting ductile fracture initiation and propagation in structural steels subjected to monotonic and cyclic large scale inelastic loading. Implications of this for characterizing the post-fracture response of structural steel components are discussed, along with limitations of the research. 

Since their first use in Japan about thirty years ago, Buckling Restrained Braces (BRBs) have been widely implemented in steel-framed buildings throughout the world. To date, most of the development and validation of BRB ductility has relied extensively on testing of full-scale braces under cyclic loading since no fracture evaluation method based on underlying micromechanics is currently available. Therefore, research is currently being undertaken to develop, validate and apply detailed finite element models to computationally simulate ductile fracture initiation and propagation in BRBs. As a part of this research, this paper presents an evaluation methodology of ductile fracture initiation using an Ultra-Low Cyclic Fatigue criterion, referred to as the Stress Weighted Damage Model (SWDM), along with detailed finite element analysis of BRBs.