Failure Analysis of Commercial Vehicle Crankshaft: A Case Study

A failure analysis has been conducted on a diesel engine crankshaft used in a commercial vehicle, which is made from 42CrMo forging steel, and the crankshaft was after induction hardening process. The fracture occurred between the 4th journal and the 4th crankpin, fracture section indicates that fatigue is the dominant mechanism of failure for this crankshaft. In order to find the failure causes, the dimension of journal boss fillet, material chemical compositions, surface hardness and depth of hardness layer were evaluated. Chemical compositions of crankshaft material, surface hardness and the depth of hardness layer are within the specified range, however the dimension of journal boss fillet is less than specified, and the transition zone of hardness layer was found on the journal boss fillet. Heavy segregation and many non-metallic inclusions aggregate zones were found from metallographic, these would lead to the reduction of endurance bending strength. FEA analysis results suggest that the dimension of journal boss fillet have an important effect on the stress and stress amplitude of the crankshaft. So the disqualification machining and material property are the main causes for the crankshaft fatigue fracture.