Very High Cycle Fatigue Performance of Ductile Cast Iron With Different Microstructures

The very high cycle fatigue performance of four ductile cast irons, a solid solution strengthened ferritic, ferritic-pearlitic, and two austempered ductile cast irons, was investigated by ultrasonic fatigue testing under fully reversed loading conditions. The step-stress fatigue testing method incorporating re-use of runouts was employed, with a $3\times 10^8$ cycles to failure runout criteria. The guarantee of failed specimens enables extensive fractography and characterization of fatigue-initiating defects, facilitating extreme value analysis of defect distributions. Furthermore, the fatigue and defect data were used for evaluation of stress–cycle–defect relationships as well as fatigue strength distributions. The obtained relationships between applied stress and fatigue-initiating defect size were used to adjust the data with respect to a given defect size, to compare and evaluate fatigue-initiating defect size equivalent fatigue strength distributions, emphasizing the role of the different matrix microstructures.