桥梁、海洋平台等钢构件、节点可能承受循环往复的疲劳荷载和冲击荷载作用，若在低温条件下服役则其脆性断裂的风险会增加。目前各国的钢结构设计条文对钢结构低温下的疲劳尚没有明确规定。对防冲击脆断的设计主要体现在冲击功的规定上。为了深入探究钢结构在低温条件下的疲劳和动态断裂性能，本文完成了由普通建筑钢材Q345B焊接而成的两类构造细节共计60个试样的低温疲劳试验以及两类建筑钢材Q235B、Q345B及其焊缝共计60个试样的低温和常温示波冲击试验，并对低温动态断裂试验进行数值模拟，结合理想模型和工程算例探讨了动态断裂的应力强度因子放大效应。主要研究内容如下：（1）对两类焊缝受剪的疲劳细节，包括侧面角焊缝连接和正面角焊缝连接各30个试样进行了低温高周疲劳试验，得到了0℃、-20℃、-40℃环境温度中两类构造细节的S-N曲线，分析了两类构造细节破坏模式的差异，并与常温的试验结果和规范规定的S-N曲线进行对比。结果表明侧面角焊缝连接在低温下的抗疲劳性能略有提升，较为稳定；正面角焊缝连接在低温下疲劳性能主要受焊接过程影响，低温下可能出现疲劳寿命降低的情况。（2）对Q235B、Q345B及E4303、E5015焊缝四种材料在20℃、0℃、-20℃、-40℃、-60℃的温度条件下进行示波冲击试验，得到了四种材料在五个温度条件下的动态断裂韧性。结果表明，动态断裂韧性值随着温度降低并非单调下降，在低于-20℃后四种材料的动态断裂韧性都呈现了明显降低的趋势。根据Boltzmann函数拟合给出了四种材料的韧-脆转变温度。（3）使用ABAQUS显式动力学分析结合位移外推法计算Q235B钢材-60℃~0℃温度条件下的动态断裂韧性，使用ABAQUS隐式动力学分析给出动态断裂韧性随时间变化的JId-t曲线，使用两种计算方法得到的动态断裂韧性值在-60℃和-40℃拟合效果较好。（4）使用ABAQUS计算了带中心裂纹的二维板在理想阶跃荷载下的动态应力强度因子响应，分析了动态应力强度因子的放大系数与板件几何尺寸的关系。计算了两类实际工程中应用的带裂纹梁的动态响应，发现中心裂纹比边缘贯穿裂纹对动态荷载更加敏感，动载下两类裂纹的断裂安全系数显著下降。对交通信号灯杆受车辆撞击问题进行了模拟，给出了给定裂纹长度的容许撞击速度，并探讨了低温下容许撞击速度的变化规律。
Steel structures and joints such as bridges and offshore platforms may be subject to cyclic and impact loading. When exposed to low temperature, these structures are more vulnerable to brittle fracture. At present, there are no clear regulations regarding the fatigue problem at low temperature in the design codes of various countries. The regulation of anti-impact fracture is mainly reflected as the constraint of the Charpy energy. Therefore, in order to investigate the fatigue and dynamic fracture properties of steel structures under low temperature conditions, low fatigue tests were conducted on 60 samples of two structural details welded by Q345B. Meanwhile, ambient and low temperature instrumented impact tests on 60 samples made of Q235B and Q345B and their welds were conducted. The numerical simulation of low temperature dynamic fracture test was carried out. The effect of dynamic stress fracture factor amplification was discussed. The main research contents are as follows:(1) The low temperature and high-cycle fatigue tests were carried out on the side fillet welded joints and front fillet welded joints. The S-N curve of two structural details at 0℃, -20℃, -40℃ temperature were obtained and compared with test results at room temperature and the curve specified in the specification. The difference of failure modes of these two structural details were analyzed. The results show that the fatigue life of the side fillet welds is slightly improved and more stable at low temperature. However, the fatigue performance of front fillet welds at low temperature is mainly controlled by the welding process, and fatigue life may decrease at low temperature.(2) The instrumented impact tests were carried out on the Q235B, Q345B, E4303 and E5015 welds at 20℃, 0℃, -20℃, -40℃, -60℃. The dynamic fracture toughness of these materials were obtained at these five temperature. The results show that the dynamic fracture toughness does not decrease monotonously with temperature. The dynamic fracture toughness shows a significant decrease under -20℃ for all of the four materials. The ductile-brittle transition temperatures of four materials are given according to Boltzmann function fitting.(3) The dynamic fracture toughness of Q235B at low temperature was calculated by ABAQUS explicit kinetic analysis combined with displacement extrapolation. The JId-t curve was obtained by ABAQUS implicit kinetic analysis. The obtained dynamic fracture toughness agrees well with the experimental results at -60℃ and -40℃. (4) The dynamic stress intensity factor response of a two-dimensional plate with a central crack under an ideal step load was calculated by ABAQUS. The relation between the amplification factor of the dynamic stress intensity factor and the geometric dimensions of the plate was analyzed. The dynamic response of the two cracked beam applied in actual structures was also calculated. The results show that the central crack is more sensitive to the dynamic loads than the edge through crack. The case of vehicles hitting traffic signal poles was simulated by ABAQUS, giving the allowable impact velocity for a given crack length. The variation of allowable velocity at low temperature was also dicussed.
