ROLE OF SMALL CRACKS AND UNDERSTRESS IN PREDICTION OF FATIGUE LIFE UNDER TWO-STEP STRESS LOADING.

The effect of understress in two-step stress 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 sigma //w defined by the failure of a specimen containing a small crack, and the other is the critical stress sigma //w//t which is the threshold value for fatigue crack initiation from the small crack. The equations to predict sigma //w and sigma //w//t were derived here as the function of crack size. If the value of sigma //L is lower than sigma //w defined on the basis of crack length l, the repetition of sigma //L does not contribute fatigue damage. However, if the crack propagates due to the repetition of overstress sigma //H and the value of sigma //w//t decreases below sigma //L , sigma //L 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.