Effects of highly stressed volume on fatigue strength of austempered ductile irons

The relationship between fatigue strength and highly stressed volume was investigated with various combinations of specimen configurations and loading modes for austempered ductile irons (ADIs) in different heat treatments. Fatigue specimens were prepared in different notch geometries with stress concentration factors ranging from 1 to 2.64. High-cycle fatigue experiments were conducted in both rotating-bending and axial loading to determine the relevant stress-life curves. Experimental results indicate that the fatigue strength of ADI is higher when austempered at 360 degrees C in comparison to that austempered at 300 degrees 0C due to the greater toughness present. The fatigue limit in rotating-bending is higher than that in axial loading because of a smaller volume of high stress. ADI austempered at 300 degrees C is more susceptible to notch effects on fatigue strength due to its harder, more brittle matrix structure. The fatigue limit of ADI is generally decreased with an increase in the volume of material subjected to high stress. A prediction equation for the fatigue limits of ADIs at different heat-treated conditions is established in terms of highly stressed volume and toughness. (C) 1998 Elsevier Science Ltd.