目前，各国规范普遍采用基于S-N曲线的名义应力法对钢桥细节进行疲劳评估，而这种方法往往依赖于构造细节的疲劳类别和荷载历史。当面对新型连接和节点或采用新型钢材的构造时，通常需要开展疲劳试验，但试验费用昂贵且结果往往缺乏普遍适用性。针对这一现状，本文基于断裂力学理论，开展了对钢桥疲劳问题的研究，主要的研究工作和成果如下：（1）在三种应力比条件下，完成了三种厚度、共计54个Q345qD钢紧凑拉伸试样及10.0mm厚、共计18个WNQ570钢紧凑拉伸试样的疲劳裂纹扩展速率试验。分别采用成组数据法和单试件数据法，提出了Q345qD和WNQ570母材及焊缝的基于Paris公式的疲劳裂纹扩展速率参数标准值，给出了开展疲劳评估时参数的选用建议，为相关工程的疲劳评估提供了重要的基础数据。（2）选取三种典型的焊接构造细节——不等厚对接焊缝、十字形传力角焊缝和十字形非传力角焊缝，完成了共计78个试件的疲劳试验，细致测量了试件的几何外观、初始裂纹位置和应变数据，并验证了现行规范中疲劳设计曲线的适用性。（3）实现了基于FRANC3D和ABAQUS交互平台的复合型疲劳裂纹扩展分析方法。利用已完成的焊接构造疲劳试验数据，充分验证了该方法的准确性和可靠性。在此基础上评估了三种焊接构造解析解的合理性，开展了焊接构造疲劳寿命的影响因素分析，并推荐了三种典型焊接构造的初始裂纹设置方式。（4）将复合型疲劳裂纹扩展分析方法推广至多尺度有限元模型，利用既有疲劳试验结果，验证了本方法在多尺度模型中的准确性和可靠性，并将本方法应用在一座典型铁路钢桁梁桥的疲劳评估之中。通过名义应力方法完成了该桥的疲劳构造细节危险程度排序，识别出其中最危险的两种构造细节，分别建立了每种危险细节含裂纹体的多尺度模型和单纯局部模型，预测了两种危险细节的剩余疲劳寿命。据此讨论了在开展宏观结构层面的疲劳裂纹扩展分析时采用多尺度模型的必要性，并考察了列车轴重增加对钢桥疲劳寿命的显著影响。本文形成了一种基于断裂力学手段、具有普遍适用性、经过可靠验证的疲劳评估方法，并将其推广应用至工程实际，可以有效地克服钢桥传统疲劳评估方法的不足，服务于我国钢桥建设的发展。
Fatigue assessment on structural details of steel bridges is usually implemented through the nominal stress approach according to the current design codes of different countries. The assessment is based on specific S-N curves and determined loading history for various structural details. This approach cannot be conducted for new types of connections, joints or structural details made by new types of steels. In such situation, fatigue tests usually will be employed, but the tests are extremely time and money consuming while the results generally cannot be entended to other cases. To cope with the situation, this paper focuses on solving fatigue problem of steel bridges by Fracture Mechanics. The major research work and achievements are as follows.(1) Fatigue crack growth rate tests under three different stress ratio loading cases have been conducted on 54 Q345qD specimens of three thicknesses and 18 WNQ570 specimens of 10.0mm thickness. The standard value of fatigue crack growth rate parameters has been proposed for Q345qD and WNQ570 base metal and butt welds, based on group data approach and single specimen data approach, respectively. Suggestions on how to choose parameters for fatigue assessment are presented in the meantime. This work can provide essential data support for the fatigue assessment of related engineering projects.(2) Fatigue tests on 78 specimens have completed involving three typical welded structural details, which are butt welds connecting plates with different thicknesses, cruciform joints with load-carrying fillet welds and cruciform joints with non-load-carrying fillet weld. The geometric profile, initial fatigue crack location and strain data for these specimens are carefully measured. The nominal S-N curves used in current design codes are proven to be appropriate.(3) The mix-mode fatigue crack propagation analysis approach is proposed based on the interactive platform of FRANC3D and ABAQUS. The approach has been verified to be accurate and reliable by the fatigue test results. On the basis of the validated approach, the reasonability of the analytical solutions for the three typical welded structural details has been investigated and the influence factor analysis on their fatigue lives is fulfilled. Recommendation on the choice of initial cracks for the three typical welded structural details is also provided.(4) The mix-mode fatigue crack propagation analysis approach has been employed in multi-scale finite element (FE) models furthermore. The accuracy and reliability are verified by the existing fatigue tests, while the validated approach is applied in the fatigue assessment of a typical railway steel truss bridge. The fatigue risk ranking of all possible structural details in this bridge is generated by conducting the nominal stress approach, during which the most dangerous two structural details are recognized. The multi-scale FE model and local FE model with pure solid elements are established for these two details respectively. Both the models contain three dimensional cracks. The residual fatigue life of the two details is predicted. Through comparing the differences of the results carried out by multi-scale model and local model, this paper conducts a discussion on the necessity of fatigue crack propagation analysis in multi-scale FE model for real structures. The remarkable influence of increasing train axle load is analyzed.This paper proposes a validated fatigue assessment approach with excellent generalizability based on Fracture Mechanics, which also has been applied in practical engineering projects. The approach can overcome the shortcomings of the traditional fatigue assessment approaches effectively and serve for the development of steel bridges in China.
