Failure analysis of ductile iron crankshaft in compact pickup truck diesel engine

Abstract

The crankshaft is one of the structural components of the engine that converts the linear motion of the piston into rotational motion. In this study, the failure analysis of spheroidal graphite cast iron (SGCI) crankshafts of compact pickup truck diesel engine was investigated that premature failure occurred in the web-crankpin fillet zone of all four crankpins. A series of experiments including chemical composition, mechanical properties, hardness, and tensile tests were performed. Optical microscope (OM) and scanning electron microscopes (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) were used to investigate the microstructure, defects, fracture surface, and failure cause. To estimate the crankshaft stress field under cyclic bending combined with steady torsion, a nonlinear three-dimensional stress analysis model was used. Fractography showed that the failure is a type of transient low cycle fatigue and includes three zones of crack initiation, fatigue crack propagation, and fast final fracture. Also, hardness and microstructure results revealed that there is no hardened surface layer on both the crankpin and the main journal (>600 HV0.1 proposed standard). In addition, the microstructural characteristics showed that the material nodularity used in the failed crankshafts is within the acceptable minimum range (>80% nodularity). Besides, numerical stress analysis showed that the highest stress is in the web-crankpin fillet zone, which is in good agreement with the experimental field measurements. Finally, recommendations have been provided to ensure long fatigue life and prevent premature failure.