PREDICTION OF PROPAGATION AND NONPROPAGATION OF SHORT FATIGUE CRACKS AT NOTCHES UNDER MEAN STRESSES

A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, K(op th), and the nonpropagating crack length, C(np), was approximated by [GRAPHICS] where K(op th-infinity) was the value for long cracks, c2 was a linear function of K(op th-infinity), and c1 was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the nonpropagating crack length. The fatigue limit for crack initiation of a Stage II crack was almost constant and equal to the value predicted for the case of R = -1.