重型汽车发动机曲轴断裂分析

某重型汽车在正常行驶过程中，发动机曲轴突然发生断裂。对失效曲轴进行硬度测试、金相组织检查及断口宏微观观察等综合分析，结果表明：该曲轴断裂性质为弯曲-扭转疲劳断裂，其断口明显分为3个区域，即疲劳源区、扩展区和瞬断区；曲轴表面硬度比规定硬度值低，问时，材料表层和内部存在较多弥散分布的气孔及Al2O3、MnS等氧化物和硫化物夹杂，在弯矩和扭矩的共同作用下，疲劳裂纹从曲轴轴径油孔下方过渡圆角处等应力集中区域开始萌生，并沿与轴径约呈45°的方向扩展，最终导致曲轴断裂失效。

Failure Analysis of Crankshaft in a Heavy-duty Vehicle Engine

A crankshaft in the engine of a heavy-duty vehicle suddenly fractured during service. The failure cause was analyzed by microhardness testing, metallographic examination, and macro and micro observation on fracture surface. The results show that the failure mode of the crankshaft is fatigue fracture resulting from co-effect of bending and twisting, and the fracture surface could be obviously divided into three zones： fatigue source zone, propagation zone and final fracture zone. The surface hardness of the crankshaft was lower than the standard value, and there were many pores and inclusions of oxides and sulfides like Al2O3 and MnS in the crankshaft material. Therefore, under the co-effect of bending and twisting, fatigue cracks initiated from the stress concentration zones of the crankshaft, such as the transition round angle zone under the oil hole. Then the crack propagated along the direction 45°away from the crankshaft axis. As a result, the crankshaft fractured.