钢桁架桥梁鸟嘴式焊接节点的疲劳性能与设计方法研究

As a new type of square hollow section (SHS) welded joint in steel truss bridges, bird-beak joint provides higher static strength and in-plane rigidity when compared to the conventional SHS joints. However, its fatigue behavior under cyclic loading is unknown. This project aims at systematically investigating the fatigue behavior and failure modes, as well as developing design methods for bird-beak SHS welded joints. By employing both experimental and numerical analyses, the influences of various non-dimensional geometric parameters of the connection on hop-spot stress will be determined, followed by developing stress concentrate factors (SCF) using mathematical fitting. In addition, high-cycle fatigue tests and crack propagation simulations will be conducted, and the fatigue fracture modes and damage mechanisms will be identified with respect to initial imperfection, crack propagation, and fracture morphology. An analytical model of crack propagating and hop-spot stress range versus number of cycles (S-N) curves will be established using Linear-Elastic Fracture Mechanics (LEFM). The ultimate fatigue failure criterion considering degradation of structural rigidity and load carrying capacity of the joint will be proposed based on non-linear FEA of the joint models with various levels of fatigue damage. Finally, the hot-spot stress based fatigue design method and the performance degradation based prediction approach of residual fatigue life will be developed, respectively for new construction and cracked in-service bird-beak SHS welded joints, and recommendations for engineering application will be proposed. The study will provide a scientific basis for the formulation and revision of respective code provisions, and will promote implementation of bird-beak welded joints in bridge structures.

鸟嘴式节点是钢桁架桥梁中一种新型的方钢管相贯焊接节点，它具有比传统方管节点更好的静力性能，但在交变荷载作用下的疲劳性能尚不明确。本项目围绕鸟嘴式节点的疲劳性能及其设计方法开展系统性研究。首先采用试验和数值方法对节点的热点应力特性开展研究，得到几何无量纲参数的定量影响规律，并建立应力集中系数（SCF）计算公式；随后开展节点的高周疲劳试验和裂纹扩展数值模拟，从初始缺陷、裂纹扩展、断口形态等方面揭示其疲劳破坏过程与机理；进一步基于断裂力学理论推演，建立节点的裂纹扩展速率模型和热点应力幅-疲劳寿命（S-N）曲线，并通过含裂纹节点的非线性有限元分析，提出基于刚度和承载力退化的疲劳失效判定准则。在此基础上，最终建立分别针对新建和在役鸟嘴式节点的疲劳设计方法和开裂后剩余疲劳寿命评估方法，并提出工程应用建议。研究成果将为制定、完善相关的设计规范条文提供科学依据，有利于此类节点在桥梁工程中的推广和应用。

本项目针对钢桁架桥梁中的鸟嘴式焊接方管X形和K形节点，系统性开展了疲劳性能及疲劳设计方法研究，具体研究内容和研究成果包括：.（1）制作了8个鸟嘴式X形节点试件和8个鸟嘴式K形节点试件，开展支管轴力、支管面内弯矩、支管面内复合受力工况下的应力集中试验研究，探明了此类节点的热点应力分布规律和敏感热点位置。同时基于有限元参数分析揭示了节点几何无量纲参数（β、γ、2τ）对应力集中系数（SCF）的影响规律，并提出了SCF计算公式。.（2）基于SCF实测值和目标热点应力幅，对16个节点试件进行高周疲劳加载，得到了节点的疲劳破坏模式、裂纹扩展过程、刚度退化及疲劳寿命等关键性能指标，结果表明：节点裂纹的初始萌生位置与最大SCF位置高度吻合；采用贯穿壁厚寿命N3作为此类新型节点的疲劳设计寿命是安全合理的。同时采用Franc 3D软件对试验节点的疲劳裂纹扩展过程进行了数值模拟，将疲劳破坏模式和裂纹扩展与试验结果对比，验证了边界元模型的准确性，并从断裂力学角度揭示了疲劳断裂机理。进一步对疲劳裂纹扩展全过程的节点刚度进行分析，建立了鸟嘴式节点的疲劳失效判定准则。.（3）将鸟嘴式节点试件的疲劳寿命实测值与国际焊接协会IIW疲劳设计规范中的Srhs-N曲线进行对比，结果表明：IIW设计曲线可用于鸟嘴式节点的疲劳寿命预测，在此基础上建立了鸟嘴式节点基于热点应力的疲劳设计方法。进一步基于裂纹扩展数据及疲劳失效判定准则，建立了鸟嘴式节点基于性能失效的开裂后疲劳寿命评估方法。.研究成果为鸟嘴式方管节点在动力荷载作用下的疲劳性能和疲劳设计方法提供了科学依据和重要参考，有助于推动此类新型方管节点在公路、铁路钢桁架桥梁工程中的应用。