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 arbitrary stress ratio. The resistance curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plate of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, Kop th, and the nonpropagating crack length, cnp, was approximated by Kop th= Kop th∞[(cnp-c1)/(c2-c1)]l/2, (c1lcnplc2), Kop th= Kop th∞, (c2<cnp), where Kop th∞ was the value for long cracks, c2 was proportional to Kop th∞, 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.