某汽车发动机凸轮轴正时带轮紧固螺栓在台架耐久试验中发生断裂。通过宏观检验、化学成分分析、硬度测试、金相检验以及断口分析等方法对螺栓的断裂原因进行了分析。结果表明：该紧固螺栓的断裂模式为疲劳断裂；螺栓未紧固到设计要求的预紧力，导致其在使用过程中发生松动，在交变应力作用下螺栓螺纹旋合部位萌生裂纹，最终导致螺栓发生疲劳断裂。

Clamping bolt for camshaft timing pulley of an engine fractured during the durability bench test. By means of macroscopic examination, chemical composition analysis, hardness test, metallographic examination and fracture analysis, the fracture causes of bolt were analyzed. The results show that the fracture mode of bolt was fatigue fracture. The bolt was loose during using because of insufficient pretightening force,and cracks initiated on the screwed part of bolt under the action of alternating stress, which finally led to bolt fatigue fracture.