In order to examine fatigue life prediction methods at high temperatures where creep damage need not be taken into account, fatigue tests were carried out on plane bending specimens of alloy steels (SCM 435, 21/4Cr-1Mo) under superposed and combined superposed stress waves at room temperature and 500°C. The experimental data were compared with the fatigue lives predicted by using the cycle counting methods (range pair, range pair mean and zero-cross range pair mean methods), the modified Goodman's equation and the modified Miner's rule. The main results were as follows.
(1) The fatigue life prediction method which is being used for the data at room temperature is also applicable to predict the life at high temperatures. The range pair mean method is especially better than other cycle counting methods. The zero-cross range pair mean method gives the estimated lives on the safe side of the experimental lives.
(2) The scatter bands of N/Nes (experimental life/estimated life) becomes narrower when the following equation is used instead of the modified Goodman's equation for predicting the effect of mean stress on fatigue life.
σt; stress amplitude at zero mean stress (kg/mm2)
σB; tensile strength (kg/mm2)
σm; mean stress (kg/mm2)
σa; stress amplitude (kg/mm2)
k; modified coefficient of σB