Low-cycle fatigue of austempered ductile irons at various strain ratios

The effects of mean stress on the low-cycle fatigue (LCF) properties of austempered ductile irons (ADIs) were investigated. Uniaxial LCF tests were conducted under strain-control with three strain ratios, R=−1, 0, and 0.5, on a couple of different grades of ADIs. These ADIs were respectively produced at two austempering temperatures, 300 and 360°C, from the same base ductile iron. Results indicated that LCF lives in both austempered conditions were decreased with an increase in strain ratio due to mean stress effects. As ADI austempered at 300°C is more susceptible to the mean stress effects, a greater reduction in LCF strength at higher strain ratios of R=0 and 0.5 was observed in comparison to ADI austempered at 360°C. All the LCF data obtained under various combinations of heat treatments and strain ratios were well correlated by a modified SWT approach for lifetime prediction.