某重型汽车的一桥摆臂在服役期间发生开裂。通过宏观检查、化学成分分析、微观分析、金相检验等方法，对摆臂的开裂原因进行了分析。结果表明：该开裂摆臂的开裂模式为疲劳开裂，摆臂在锻造过程中产生了折叠缺陷，折叠缺陷内伴生夹杂及微裂纹，在工作应力下折叠缺陷成为疲劳源，最终导致摆臂开裂。

The bridge swing arm of a heavy vehicle cracked in service. By means of macro examination, chemical composition analysis, micro analysis and metallographic examination, the cracking causes of the swing arm were analyzed. The results show that the cracking mode of the cracked swing arm was fatigue cracking. During the forging process, the swing arm had folding defects. The folding defects were accompanied by inclusions and microcracks. Under the working stress, the folding defects became the fatigue source, which eventually led to the swing arm cracking.