某公司码头门机使用的DG20钢吊钩使用不到1 a（年）即于螺杆处发生径向断裂，采用化学成分分析、宏观分析、扫描电镜分析、金相检验等方法对吊钩螺杆断裂原因进行了分析。结果表明：吊钩螺杆断裂属于单向弯曲疲劳断裂，断裂起源于螺纹根部的微裂纹；现场操作不当使吊钩螺杆在螺纹根部产生严重的单向应力集中，促使该处萌生微裂纹并不断扩展，最终导致断裂失效。

The radial fracture of DG20 steel hook screw of a portal slewing crane happened after less than one year service on the wharf in a company. The fracture reasons of the hook screw were analyzed by means of chemical composition analysis, macro analysis, SEM analysis, metallographic examination, etc. The results show that the fracture of the hook screw was one-way bending fatigue fracture, and the fracture originated from the micro cracks at the root of the thread. Improper operation at the site resulted in serious unidirectional stress concentration at the thread root of the hook screw, which caused the crack initiation and continuous expansion and finally resulted in the fracture failure.