Repeated two-step tests which composed of a large number of cycles of understress with or without tensile mean stress and the intermittent overstress were carried out on smooth specimens of a 0.38% carbon steel. The effects of frequency ratio of overstress to understress in one block on the cyclic plastic strain and the fatigue damage by the stress below fatigue limit were investigated. The main results obtained are as follows;
(1) The frequency ratio of overstress to understress affected the cyclic plastic strain, and the smaller ratio caused the larger plastic strain range-pair corresponding to the high level stress and the smaller plastic strain range-pair to the low level stress.
(2) The effect of frequency ratio of overstress to understress was found not only in the relationship between the plastic strain range-pair and the fatigue life but also in that between the stress and the fatigue life. It was concluded that the effect of frequency ratio on fatigue life could not be explained only by considering the variation of the plastic strain range-pair due to stress interaction.
(3) Although the fatigue damage amount per one cycle of low level stress did not change largely when the number of low level stress cycles, n2, was less than about 102 cycles, the larger n2 decreased the damage amount. And the fatigue damage during one block of low level stress cycling became saturated at a certain number of cycles of n2 and the damage could not accumulate any more beyond it.
(4) In the multi-step loadings the saturation of fatigue damage during understressing was sometimes disturbed and the fatigue life prediction based on the results of repeated two-step tests where the saturation of damage existed might be dangerous.
