Fracture Mechanics- and Image Processing-Aided Assessment of Fatigue and Fracture Toughness of Austempered Ductile Cast Iron

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To examine quantitatively the effects of spheroidal graphite and matrix on strength of ductile cast iron, low-cycle fatigue tests were mainly conducted. Three kinds of ductile cast irons, ferritic, pearlitic and bainitic ductile irons with same chemical composition were prepared by heat treatment. First, graphite morphology such as nodule count, nodularity, area fraction etc. of each material was measured by an image processing analyzer. Then, a laser interometric measuring instrument for measuring closure behavior of a spheroidal graphite-initiated small crack with a higher accuracy was newly developed.Through the fatigue tests, it was found that the matrix structure primarily controls the fatigue strength and that the spheroidal graphite plays an important role during the fatigue process from microcrack initiation to final failure. In cases of bainitic and pearlitic materials, a microshrinkage formed during fabrication in the matrix was found to be responsible for the crack initiation site. Finally, the results of macroscopic crack propagation tests revealed that the matrix structure hardly influences the macroscopic crack propagation rate.