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

循环非弹性变形是钢结构地震耗能的主要形式。在地震期间，梁柱弯矩连接经历超低周疲劳（ULCF）现象，其特征是很少（10-20）次大应变循环。ULCF与低周疲劳完全不同，低周疲劳已经得到了更广泛的研究，但并没有解决抗震设计中普遍存在的问题。由于缺乏合适的微观模型来模拟ULCF，以及研究大型结构部件所需的计算要求，对ULCF相关的基本破坏机制的描述相对较少。现有的钢结构在地震作用下ULCF的研究几乎完全依赖于半经验方法，无法转移到不同的结构构型。此外，现有的实证研究大多基于准静力试验，没有考虑地震荷载率的影响。这种知识差距是减轻地震灾害的严重问题。提出的研究旨在(1)识别和量化地震诱发ULCF的潜在破坏机制，(2)开发和实施模拟钢结构ULCF的模型，(3)在地震荷载率下进行大规模子装配试验，以验证和演示模型，(4)应用ULCF模型制定实用的抗震设计指南和建议。PI和他在斯坦福大学的合作者最近的一项研究成功地开发了一些用于预测钢结构中地震诱发的ULCF裂纹萌生的首批微力学模型。基于这些初步进展，提出的研究将把微观力学概念与先进的模拟技术和并行计算结合起来，真实地模拟钢结构的基本疲劳断裂过程。研究的第一阶段将包括焊接部件的综合测试和分析，以校准微观力学模型中的材料性能。第二阶段将使用NEES Colorado的快速混合测试设备测试全尺寸焊接钢连接，测试结果将用于验证预测ULCF裂缝的微观力学模拟。第三阶段将使用基于微力学模型的仿真框架来解决设计和建筑行业感兴趣的尚未解决的实际问题，例如焊接钢结构中韧性断裂的产生和扩展。建议研究的智力价值：本研究将开发强大的工具，在地震荷载作用下，在非常基本的水平上模拟钢结构部件的裂纹萌生和扩展。该研究将大大提高断裂/疲劳力学的知识水平，并有效地展示了微尺度建模在解决重要地震工程问题方面的力量。这将对模拟实践和向更广泛的基于模型的模拟环境的迁移产生积极的影响。基于模拟的一致设计建议将解决重要的细节问题并减轻地震危害。该研究将利用NEES-Colorado的快速混合测试设备，研究团队致力于通过NEESgrid免费共享数据、仿真模型和其他信息，并在评审期刊上发表。拟议研究的更广泛影响：本研究将对地震疲劳力学、基于模型的模拟和地震中防止ULCF的设计指南的最新进展产生重大影响。这项研究涉及结构工程、材料和计算科学方面的专业知识，将促进两所参与学校和科罗拉多大学博尔德分校NEES站点之间的跨学科技术转移和合作。这项研究的教育影响将包括两名博士生的教育，分别来自加州大学戴维斯分校和斯坦福大学。此外，项目负责人和联合项目负责人都负责在加州大学戴维斯分校和斯坦福大学教授钢铁设计课程，这为学生通过NEES远程参与和数据库设施参与研究提供了一个自然的教育机会。项目团队致力于让代表性不足的群体参与研究，并将与校园工程多样性项目合作，以实现这些目标。

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.