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。在地震过程中，梁柱弯矩连接会经历一种称为超低周疲劳(ULCF)的现象，这种现象的特点是极少数(10-20)大应变循环。超低周疲劳与低周疲劳有很大的不同，低周疲劳得到了更广泛的研究，但没有解决抗震设计中普遍存在的情况。由于缺乏合适的微观模型来模拟ULCF，以及实现研究大型结构部件的模型所需的计算要求，对与ULCF相关的基本失效机制的表征相对较少。现有的钢结构在地震作用下的ULCF研究几乎完全依赖于半经验方法，这种方法不能转移到不同的结构构型上。此外，现有的经验研究大多基于拟静力试验，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。本研究的目的是：(1)识别和量化超低剪力纤维在地震作用下的破坏机理；(2)建立和实施模拟钢结构超低剪力纤维的模型；(3)在地震荷载作用下进行大型组件试验，以验证和论证模型；(4)应用超低剪力纤维模型，制定实用的抗震设计准则和建议。斯坦福大学的PI和他的合作者最近的一项研究成功地开发了一些第一个用于预测地震引起的钢结构超低碳纤维裂纹萌生的微观力学模型。在这些初步进展的基础上，拟议的研究将结合细观力学概念、先进的模拟技术和并行计算来逼真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，以校准微观机械模型中的材料特性。第二阶段将使用位于科罗拉多州NEES的FAST混合测试设施来测试全尺寸焊接钢连接，其结果将用于验证预测ULCF断裂的微观模拟。第三阶段将使用基于微观机械模型的模拟框架来解决设计和建筑业感兴趣的悬而未决的实际问题，例如焊接钢结构中延性断裂的起始和扩展。拟议研究的智力价值：这项研究将开发强大的工具来模拟地震荷载作用下结构钢构件在非常基本的水平上的裂纹萌生和扩展。这项研究将极大地提高断裂/疲劳力学的知识水平，并有效地展示微观模拟在解决重要地震工程问题方面的力量。这将对总体模拟实践和向更广泛的基于模型的模拟环境过渡产生积极影响。基于模拟的一致设计建议将解决重要的细节问题，并减轻地震风险。这项研究将利用NEES-Colorado的快速混合测试设施，研究团队致力于通过NEES网格免费共享数据、模拟模型和其他信息，并在被引用的期刊上发表文章。拟议研究的更广泛影响：这项研究将对地震疲劳力学、基于模型的模拟和地震中防止ULCF的设计指南的最新水平产生重大影响。这项研究涉及结构工程、材料和计算科学的专业知识，将促进两所参与学校和位于CU-Boulder的NEES网站之间的跨学科技术转让和合作。这项研究的教育影响将包括两名博士生的教育，加州大学戴维斯分校和斯坦福大学各一名。此外，PI和共同PI都负责在加州大学戴维斯分校和斯坦福大学教授钢设计课程，这为学生提供了一个自然的教育机会，通过NEES远程参与和数据库设施参与到研究中来。该项目团队致力于让代表不足的群体参与研究，并将与校园工程多样性计划合作，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.