Very high cycle fatigue property of high-strength austempered ductile iron at conventional and ultrasonic frequency loading

•The very high cycle fatigue tests of austempered ductile iron were conducted.•The fatigue properties and fracture behaviors were investigated.•The effect of loading frequency and defect size on fatigue strength was analyzed. In this study, the very high cycle fatigue tests were conducted on a kind of high-strength austempered ductile iron (ADI) at 90Hz and 20kHz. The S–N curves decrease continuously and there is no fatigue limit in the very high cycle fatigue regime. The fatigue strength of the different frequencies essentially corresponds with each other and the specimens show similar fracture behaviors. Meanwhile, on the fracture surfaces of some specimens, the ‘granular-bright-facet’ (GBF) area is observed. The threshold stress intensity factor for internal crack growth is obtained and the fatigue limit is evaluated based on the defect size.