Quantitative Estimation of the Fatigue Crack Propagation under Varying Load Conditions

Abstract

In the previous report authors clarified the existence and the cause of the two main behaviours, the accelerated crack propagation behaviour and the stagnated one under varying load conditions. In this paper, a quantitative estimation of the crack propagation under varying load condition is attempted considering the accelerated crack propagation behaviour. As this behaviour depends upon the crack opening phenomenon, quantification is attempted by the following method. As the area which sustains the compressive stress under lower stress condition in a high-low type varying load fatigue test (the cracked surface does not sustain the compressive stress) is smaller than one under constant stress amplitude fatigue test (the cracked surface sustains the compressive stress), the section modulus for the former is smaller than that for the latter. Now, these section moduluses are calculated by considering the crack depth, then the actual nominal stress under low stress amplitude in the high-low type varying load fatigue test is obtained quantitatively by calculating the ratio of the net nominal stresses at the crack tips which are estimated by using the above section moduluses. The crack propagation curves for the above actual nominal stresses are interpolated among the curves for the constant stress amplitudes, then these become the estimated crack propagation curves. Reasonable estimation is possible for the crack propagation under varying load fatigue test of carbon steel specimens.