Fatigue analysis of crankshaft sections under bending with consideration of residual stresses

In this paper, the influence of the residual stresses induced by the fillet rolling process on the fatigue process of a ductile cast iron crankshaft section under bending is investigated. The stress concentration near the fillet of the crankshaft section under bending without consideration of residual stresses is first investigated by a two-dimensional elastic finite element analysis. The plastic zone development and the residual stress distribution near the crankshaft fillet induced by the fillet rolling process are then investigated by a two-dimensional elastic-plastic finite element analysis. In order to use the two-dimensional finite element analysis to shed light on the residual stress distribution due to the three-dimensional rolling process, the rolling depth after the unloading of the roller in the two-dimensional finite element analysis is determined by the fillet surface profiles measured by the shadowgraphs taken before and after the fillet rolling process. After the rolling process, a bending moment is then applied to the crankshaft section. With consideration of the stresses due to the rolling process and the bending moment, the fatigue failure near the fillet is investigated based on a linear elastic fracture mechanics approach. An effective stress intensity factor, which combines the stress intensity factors due to the bending moment and due to the residual stresses, is defined. An effective stress intensity factor range is then approximated and compared to an assumed threshold stress intensity factor range to determine if the crack can continue to propagate for a given crack length. The results indicate that the four-bubble failure criterion only determines the crack initiation life for small cracks initiated on the surfaces of fillets. The four-bubble failure criterion does not indicate whether a fatigue crack initiated on the fillet surface can propagate through or arrest in the compressive residual stress zone induced by the rolling process.