Failure Analysis of a Four Cylinder Diesel Engine Crankshaft Made From Nodular Cast Iron

The premature breakage in some four cylinder diesel engine crankshafts was reported. All crankshafts were failed from the same region. Failures had occurred in the first crankpin, the nearest crankpin to the flywheel. Dynamic analysis and finite element modelling were carried out to determine the state of stress in the crankshaft. FEM results revealed that the first crankpin fillet is the most vulnerable point to fracture. Soderburg diagram of the studied crankshaft showed that the service operation point, which stands for mean and alternating stresses of the critical region (first crankpin fillet) was located in the safe region. Therefore, it can be concluded that fatigue fracture has not occurred in the crankshaft. SEM images of the fractured surface also showed cleavage fracture and put in evidence that the failure was brittle fracture. No sign of fatigue failure was observed. The fracture may have been caused by an overload. However, the results suggest re-evaluation of the design and manufacturing. The fillet rolling may play an important role in this matter. Optimization of the fillet rolling process by changing process parameters has been recommended to the manufacturer. This recommendation has been adopted by the manufacturer and no further fracture has been reported since.