Prediction of Propagation and Nonpropagation of Short Fatigue Cracks at Notches Under Mean Stresses

Abstract

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, Kopth, and the nonpropagating crack length, cnp, was approximated by Kopth=Kopth∞ [nemerical formula] c1≤cnp≤c2 Kopth=Kopth∞ c2≤cnp, where Kopth∞ was the value for long cracks, c2 was a linear function of Kopth∞, 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.