某工程机械液压缸活塞杆仅使用200 h就发生了断裂。通过宏观分析、低倍检查、化学成分分析、金相检验和力学性能试验等方法，对活塞杆的断裂原因进行了分析。结果表明：大尺寸夹杂物的存在，使得活塞杆原材料基体不连续，在动态拉-压循环载荷的作用下，于夹杂物处产生微裂纹。加之螺纹加工精度不高，裂纹在使用过程中扩展，最终导致活塞杆发生疲劳断裂。

The piston rod in hydraulic cylinder of a construction machinery fractured when it was used for only 200 h. The fracture causes of the piston rod were analyzed by means of macroscopic analysis, macroscopical examination, chemical composition analysis, metallographic examination and mechanical property test. The results show that the matrix of the piston rod was discontinuous due to the existence of large size inclusions. Under the action of dynamic tension-compression cyclic load, microcracks occurred at inclusions. In addition, the machining accuracy of screw thread was not good, and the crack expanded in the process of use, which eventually led to fatigue fracture of the piston rod.