EFFECTS OF HIGHLY STRESSED VOLUME ON FATIGUE-STRENGTH OF AUSTEMPERED DUCTILE IRONS

The relationship between fatigue strength and highly stressed volume w as investigated with various combinations of specimen configurations a nd loading modes for austempered ductile irons (ADIs) in different hea t treatments. Fatigue specimens were prepared in different notch geome tries with stress concentration factors ranging from 1 to 2.64. High-c ycle fatigue experiments were conducted in both rotating-bending and a xial loading to determine the relevant stress-life curves. Experimenta l results indicate that the fatigue strength of ADI is higher when aus tempered at 360 degrees C in comparison to that austempered at 300 deg rees 0C due to the greater toughness present. The fatigue limit in rot ating-bending is higher than that in axial loading because of a smalle r volume of high stress. ADI austempered at 300 degrees C is more susc eptible to notch effects on fatigue strength due to its harder, more b rittle matrix structure. The fatigue limit of ADI is generally decreas ed with an increase in the volume of material subjected to high stress . A prediction equation for the fatigue limits of ADIs at different he at-treated conditions is established in terms of highly stressed volum e and toughness. (C) 1998 Elsevier Science Ltd.