Failure Analysis of Ductile Iron Crankshaft in Four-Cylinder Diesel Engine

Spheroidal graphite cast iron is widely used in automobile crankshafts due to features such as high strength, high toughness, good machinability, low cost, and ductility. The purpose of this paper is first to analyze the fatigue failure in fractured truck crankshafts between 400,000 and 1,350,000 km and then to predict the stress field under the influence of the mechanical loads caused by gas combustion. Several experimental studies including chemical composition, material strength, hardness, and microstructure are performed to evaluate the failure analysis. A nonlinear three-dimensional stress analysis model by the elastic-plastic finite element method is used to estimate the crankshaft stress field under cyclic bending combined with steady torsion. Failure analysis showed that the fracture surface consisted of three zones: fatigue crack initiation, fatigue crack propagation, and static fracture; therefore, the failure is in fatigue. Also, the microstructure results showed that the crankshaft nodularity is about 70%, which crankshaft structures are usually acceptable greater than 80% of nodularity. Besides, hardness gradient showed that the crankshaft crankpin had no hardened surface layer. Finally, numerical stress analysis indicated that the highest stress is in the crankpin-web fillet zone that it is in good agreement with those obtained in the experimental field measurements.