某地铁列车转向架连接螺栓发生断裂事故，采用宏观观察、化学成分分析、拉伸试验、断口分析、金相分析等方法，对该连接螺栓的断裂原因进行了分析。结果表明：该连接螺栓的断裂性质为弯曲疲劳断裂，裂纹起源于螺纹根部。安装时未及时检查清理内螺纹孔的残留螺纹胶导致螺栓受力不均，以及螺纹根部存在轻微脱碳现象，再加上列车行驶时振动所产生的交变载荷，该地铁转向架连接螺栓于螺纹根部萌生裂纹，并以疲劳的方式扩展，最终发生断裂。

The connecting bolt of a metro bogie fractured. The fracture causes of the connecting bolt were analyzed by means of macroscopic observation, chemical composition analysis, tensile test, fracture analysis and metallographic analysis. The results show that the fracture property of the connecting bolt was bending fatigue fracture, and the crack originated from the root of the thread. During the installation, the residual thread glue in the internal thread hole was not checked and cleaned in time, which led to uneven force on the bolt. There was slight decarburization at the root of the thread, coupled with the alternating load caused by the vibration of the metro running. The connecting bolt of the metro bogie initiated cracks at the root of the thread, and then expanded in the form of fatigue, and finally fracture occurred.