在长期往复交通荷载作用下，钢结构桥梁的危险焊接细节极易发生疲劳开裂破坏，而在冬季寒冷和严寒地区服役的钢结构桥梁还面临着低温冷脆破坏的威胁。但是，目前各国规范尚未有关于钢桥低温疲劳问题的规定，相关研究也十分匮乏。因此，为了进一步明确钢桥焊接细节在低温环境下的疲劳性能，以大量的疲劳试验研究为基础，结合数值模拟和理论分析，从钢材和焊缝、焊接细节到构件层面，系统研究了低温对其疲劳性能的影响规律和机理。主要研究工作和结论如下:（1）在室温~-60oC范围，以Q345qD和Q370qE钢材为对象，展开了共计39个光滑圆棒试样的应变控制疲劳试验、72个夏比V型缺口试样的冲击试验、91个标准紧凑拉伸试样的疲劳裂纹扩展速率试验和裂纹扩展门槛值测试，揭示了低温对钢材疲劳性能的影响规律，积累了宝贵的桥梁钢材低温疲劳数据。结果表明低温增强了两种钢材抵抗裂纹萌生和裂纹扩展的疲劳性能，降低了对接焊缝的抵抗疲劳裂纹扩展性能。（2）选取典型焊接构造——等厚板对接焊接接头和十字形非传力角焊缝接头，在室温和-60oC条件下，完成了共计43根有效试样的高周恒幅疲劳试验，并进行了基于局部应变法的疲劳裂纹萌生寿命预测和基于三维裂纹扩展分析的疲劳裂纹扩展寿命预测分析，揭示了低温对焊接细节疲劳性能的影响规律和机理。结果表明焊接细节的低温疲劳寿命不会低于室温，其疲劳性能主要受焊接过程和低温环境等因素共同作用，但是在低温环境下其疲劳裂纹萌生可能更容易发生。（3）基于三维裂纹扩展分析，进一步验证了含敏感细节受弯构件的低温疲劳性能，结合典型焊接细节的疲劳分析结果表明，由于现行低合金桥梁钢材一般都具有较高的断裂韧性，低温下临界裂纹尺寸的降低对焊接细节疲劳性能的影响几乎可以忽略。（4）结合疲劳试验和数值分析结果，提出了适用于室温~-60oC环境下钢桥疲劳评估的Paris参数特征参考值，从钢材选择、焊接细节的低温疲劳性能及其疲劳性能的改善三个角度，为服役于低温寒冷和严寒地区钢桥焊接细节的抗疲劳设计提供了建议。
The welded details in steel bridges are prone to experience fatigue damage induced by the cyclic action of long-term traffic loading. Additionally, the low-temperature brittle fracture also threatens the safety of such infrastructures residing in the cold and severe cold regions. However, there are no specific codes stipulating the requirements with respect to the low-temperature fatigue problems of steel bridges so far, and relevant investigation is still inadequate. Thus, in order to get a better understanding of fatigue properties of welded components in steel bridges under the low temperature ambient, a large number of fatigue tests, together with the numerical simulation and theoretical analysis, helped to systematically study the rule and mechanism of the low temperature influencing the fatigue properties in terms of the level of materials, welded details and components. The primary research work and main conclusions lead to the four following sections. (1) Within the range of the room temperature to -60 oC, this study carried out a series of strain-controlled fatigue tests using 39 smooth coupons, finished numerous Charpy impact tests with 72 V-notch specimens, and performed a number of fatigue crack propagation rate tests and stress intensity factor threshold tests utilizing 91 standard compact tension specimens, as all those specimens were made from the bridge steel Q345qD and Q370qE. The test programs explored the effect of low temperature on the fatigue properties of bridge steels, and collected the valuable low-temperature fatigue data of bridge steels. Results indicate that the low temperature enhances the resistance to fatigue crack initiation and propagation in bridge steels, but impairs the resistance to fatigue crack propagation in butt welds. (2) At the room temperature and -60 oC, this dissertation conducted the high-cycle constant-amplitufe fatigue tests of the butt-welded joints and the non-load-carrying fillet-welded cruciform joints, and the valid data points were 43 in total. Then, using the local strain method predicted the fatigue crack initiation life of those two kinds of joints, and applying the three-dimension crack propagation analysis based on linear-elastic fracture mechanics assessed the fatigue crack propagation life of the above joints. The combinated results of fatigue tests and numerical analyses revealed the variation law and mechanism of the low temperature influencing the fatigue properties of welded details. Results demonstrate that the fatigue life of the welded details at low temperatures would not be inferior to that at the room temperature, and both of the low-temperature ambient and the welding process will pose a coupled effect on fatigue properties of the welded details. But the fatigue crack initiation of the welded joints is more likely to occur at a low temperature ambient.(3) On the basis of the three-dimension crack propagation analysis, this study further validated the low-temperature fatigue properties of the flexural members with critical details from NCHRP 102 and 147. Results together with the above investigation exhibit that the reduced critical crack length has a marginal impact on the low-temperature fatigue crack propagation life of welded details, because the current low-alloy bridge steels generally possess relatively higher fracture toughness.(4) Finally, this dissertation provided the characteristic values of Paris-law parameters applicable to the fatigue damage assessment of welded details in steel bridges at the room temperature to -60 oC, and proposed some suggestions for the fatigue design of welded details of steel bridges depolyed in cold and sever cold areas, covering the material selection, low-temperature performance of welded details and their improvement approaches.
