Explanation of Fish-Eye Type Crack Behavior of Austempered Ductile Iron in Very High Cycle Fatigue

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This research has discussed a fish-eye fatigue mechanism of ADI at a range over 10^7 cycles, and focused on the following three items : a development of the high response tensile fatigue testing equipment, obtaining many fatigue data till 10^8 cycles, and observation of fracture surfaces and specimen surfaces which failed by fish-eye. The developed testing equipment achieved a performance of +/- 1% in a loading accuracy and 120-200Hz in responsiveness, which enabled us to obtain a test data of 10^8 cycles in a short period of 6-10 days. The S-N Curve leveled off at 10^6-10^7 cycles and at a long life area it dropped again. The fracture origins of short life below 10^6 cycles existed at surfaces (surface fracture) and those at a long life over 10^7 existed at interior (fish-eyefracture) . Confidence analysis by the competing risk model showed that characteristics of fatigue life distribution between surface and fish-eye fracture were completely different. From surface observation of the … More specimens fractured by fish-eye, surface cracks were found to stop their propagation over 10^7 cycles. From fracture surface observation, it clarified that the fish-eye cracks propagated even at the DELTAK region smaller than DELTAK_<th> of this material. Especially, fracture surfaces of fish-eye left traces of fine lammelar structures of austenite and ferrite which were not seen on surface fracture. We calculated fatigue life based on the usual law of crack propagation rate using DELTAK measured by an projection area of each fracture origin. Then the fatigue life of surface fracture is almost predictable but that of fish-eye fracture shows much smaller than the actual data. In conclusion, results clarified that transition from surface fracture to fish-eye fracture is caused by stoppage of surface crack propagation, fish-eye cracks propagate at quite a low stress, and the different mechanism is existed between surface fracture and fish-eye fracture. Especially, we found a possibility that fish-eye cracks are propagating much slowly than they are expected by usual law of crack propagation rate. Less