某挖掘机连接下车架与回转支承的36支高强螺栓在使用过程中全部发生断裂。采用宏观检验、化学成分分析、力学性能测试、金相检验等方法，结合螺栓装配、挖机工况等因素，对该批螺栓断裂的原因进行了调查分析。结果表明：螺栓断裂的性质为疲劳断裂；断裂的原因是螺栓装配不良，导致螺栓在使用中轻微松动，在挖掘机工作过程中的剪切力作用下，螺栓松动处产生疲劳裂纹，最终导致螺栓断裂。

The fracture occurred to all the 36 high strength bolts connecting the nether frame with the slewing bearing of the digging machine during use. By means of macro examination, chemical composition analysis, mechanical property testing, metallographic examination and so on, the fracture reasons of high strength bolts were investigated and analyzed combined with the factor of bolt assembly and working condition of the digging machine. The results show that: the fracture mechanism of the bolts was fatigue fracture; the fracture reason was that the assembly of the bolts was unqualified and it made the bolts slightly loose during use; under the action of the shear force of the digging machine, the fatigue cracks generated in the loose position of the bolts during working process, and finally the bolts fractured.