Study on Fatigue Design Method of an Engine Component Produced by Flake Graphite Cast Iron for Different Loading Modes and Mean Stresses

In order to improve reliability of an automotive engine, a fatigue design method was studied based on the fatigue mechanisms in flake graphite cast iron. In the present study, fatigue test specimens were cut from an actual engine component. The material used showed nonlinear behavior in stress–strain relationship from lower stress level during tensile testing. Rotating bending fatigue testing and axial load fatigue testing were performed. Fatigue crack initiation and propagation behavior were observed during the fatigue tests by a replication technique. Fatigue cracking initiated from the tip of flake graphite in early stage of the total fatigue life when the applied stress was higher than the fatigue limit. On the other hand, fatigue cracking was not observed in the specimen tested with applied stress below the fatigue limit. The threshold stress intensity factor obtained with a small surface crack was lower than that obtained with a through-thickness crack. Fatigue limits under various loading conditions such as bending loading and axial loading were predicted well based on fracture mechanics using the area parameter. Threshold stress intensity factor for a small surface crack and the maximum graphite size assumed as an initial crack were used for the prediction. Axial load fatigue tests with different stress ratios were performed. The fatigue limit diagram obtained by the fatigue tests did not follow the modified Goodman relation. On the other hand, fatigue limits prediction based on fracture mechanics approach showed good agreement with fatigue limit obtained with fatigue tests in a wider range of mean stresses σ mean varying from − 150 to 50 MPa, in negative stress ratio. In case of σ mean higher than 50 MPa, in a positive stress ratio, a smaller effect of stress ratio in the fatigue limit diagram was observed. It was considered that this phenomenon could be explained by the lower effect on the crack closure in a small surface crack initiated from the tip of graphite flake and also influence of local plastic behavior of the matrix around the tip of graphite flake. It can be summarized that the fracture mechanics approach is an effective way to predict fatigue limits of an engine component produced by flake cast iron in different loading conditions and in wider range of mean stress conditions.