The effect of understress in two-step stress fatigue tests on fatigue life was investigated with emphasis on the behavior of small cracks.
In fatigue under variable amplitude loading, two critical stresses play important roles in fatigue damage prediction. One is the fatigue limit σw defined by the failure of a specimen containing a small crack, and the other is the critical stress σwi which is the threshold value for fatigue crack initiation from the small crack. It should be noted that both σw and σwi are the function of crack size and accordingly the degree of contribution of understress σL to fatigue damage varies at every moment with the variation of crack size, i.e. crack propagation. The equations to predict σw and σwi were derived here as the function of crack size.
If the value of σL is lower than σwi defined on the basis of crack length l, the repetition of σL does not contribute fatigue damage. However, if the crack propagates due to the repetition of overstress σH and the value of σwi decreases below σL, σL starts to contribute to fatigue damage. This fact explains the reason for a wide scatter of the values of cumulative fatigue damage D of modified Miner's rule obtained in previous studies. A method is also discussed to predict the lower limit of D.
