某汽车发动机连杆螺栓在发动机台架耐久试验中发生断裂。通过宏观检验、化学成分分析、扫描电镜分析、金相检验、能谱分析等方法，对螺栓的断裂原因进行了分析。结果表明：该连杆螺栓断裂模式为多源疲劳断裂；裂纹内部存在大量的磷和锌元素，说明在搓丝工序时螺栓已经产生了微小裂纹；在后期的磷化处理中，磷化液渗入微小裂纹中；台架耐久试验过程中裂纹逐步疲劳扩展并导致螺栓断裂。

A connecting rod bolt of an engine fractured during the durability bench test of the engine. The fracture reasons of the bolt were analyzed through macroscopic inspection, chemical composition analysis, scanning electron microscopy analysis, metallographic inspection, energy spectrum analysis and so on. The results show that:the fracture mode of the connecting rod bolt was multi-source fatigue fracture; there were plenty of elements of phosphorus and zinc inside cracks, which indicated that the cracks had been produced in thread rolling process; the phosphatizing liquid infiltrated into the micro cracks during the following phosphatizing treatment, and the fatigue cracks gradually expanded and led to the fracture of the bolt during the durability bench test.