某45钢传动电机轴在使用中发生早期断裂，通过宏观与微观分析、化学成分分析、硬度测试和金相检验等方法对该轴的断裂原因进行了分析。结果表明：传动轴的断裂性质为旋转弯曲疲劳断裂，疲劳源位于轴肩根部。主要原因是根部倒圆半径偏小引起应力集中和调质处理不合格导致抗疲劳强度偏低，最终传动轴在交变载荷下发生早期疲劳断裂。

A 45 steel drive motor shaft fractured in the early stage in use. The causes of fracture on shaft were analyzed by means of macro and micro analysis, chemical composition analysis, hardness test and metallographic examination.The results show that the fracture property of the drive shaft is rotational bending fatigue fracture, and the fatigue source is located at the root of shaft shoulder. The main reason was that the stress concentration caused by the small radius of root rounding and the fatigue strength was low due to unqualified quenching and tempering treatment, which led to the early fatigue fracture of drive shaft under alternating load finally.