NEESR-II Large-scale testing and micromechanical simulation of ultra-low-cycle fatigue cracking in steel structures

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Cyclic inelastic deformations are the primary mode of seismic energy dissipation in steel structures. During earthquakes, beam-column moment connections undergo a phenomena called Ultra-Low Cycle Fatigue (ULCF), which is characterized by very few (10-20) large strain cycles. ULCF is quite distinct from low cycle fatigue, which has been more widely studied but does not address the conditions prevalent in seismic design. Relatively little attention has been given to characterizing the fundamental failure mechanisms associated with ULCF, due to the lack of suitable micro-scale models to simulate ULCF and the computational requirements necessary implement the models for studying large structural components. Existing research on ULCF of steel structures in earthquakes relies almost exclusively on semi-empirical methods, which cannot be transferred to varied structural configurations. Moreover, most of the existing empirical research is based on quasi-static testing, which does not account for earthquake loading rate effects. Such knowledge gaps represent serious issues for seismic hazard mitigation. The proposed research aims to (1) identify and quantify the underlying failure mechanisms of earthquake-induced ULCF, (2) develop and implement models to simulate ULCF in steel structures (3) conduct large scale subassembly tests at earthquake loading rates to verify and demonstrate the models (4) apply the ULCF models to develop practical guidelines and recommendations for earthquake resistant design. A recent study by the PI and his collaborator at Stanford succeeded in developing some of the first micromechanical models for predicting earthquake-induced ULCF crack initiation in steel structures. Based on these initial advances, the proposed study will integrate micromechanics concepts with advanced simulation techniques and parallel-computing to realistically simulate fundamental fatigue-fracture processes in steel structures. The first phase of the research will include integrated testing and analyses of welded components to calibrate the material properties in the micromechanical models. The second phase will use the fast hybrid testing facility at NEES Colorado to test full-scale welded steel connections results of which will be used to validate micromechanical simulations for predicting ULCF fractures. The third phase will use the micromechanical model-based simulation framework to address unresolved practical problems of interest to the design and construction industry, e.g. the initiation and propagation of ductile fractures in welded steel construction. Intellectual Merit of the Proposed Research: This research will develop powerful tools to model crack initiation and propagation at a very fundamental level in structural steel components under earthquake loading effects. The research will substantially advance the state of knowledge in fracture/fatigue mechanics and effectively demonstrate the power of micro-scale modeling for addressing important earthquake engineering problems. This will have both a positive effect on simulation practices in general and the migration towards a more extensive model-based simulation environment. Consistent design recommendations based on the simulations will address important detailing issues and mitigate earthquake hazard. The research will utilize the fast hybrid testing facility at NEES-Colorado, and the research team is committed to free sharing of data, simulation models, and other information through the NEESgrid and publication in refereed journals. Broader Impact of Proposed Research: This research will have a significant impact on the state of the art in earthquake fatigue mechanics, model based simulation, and design guidelines to protect against ULCF in earthquakes. Involving expertise from structural engineering, materials and computational science, this research will promote interdisciplinary technology transfer and collaboration between the between the two participating schools and the NEES site at CU-Boulder. An educational impact of this study will include the education of two doctoral students, one each at UC Davis and Stanford. Moreover, the PI and the co-PI are both responsible for teaching steel design classes at UC Davis and Stanford, which provide a natural educational opportunity for students to become engaged in the research through the NEES teleparticipation and database facilities. The project team is committed to involving under-represented groups in research and will collaborate with on campus engineering diversity programs to achieve these aims.