Retardation Effects of Fatigue Crack Propagation in the Optimum Ferrite-Bainite Steel

Abstract

It is clarified that fatigue crack propagation rate of the optimum ferrite-bainite (F/B) steel shows superior resistance in comparison with conventional ferrite-pearlite (F/P) steel. In this study, fatigue crack propagation properties of both F/P steel and the F/B steel are investigated in consideration of opening/closure of fatigue crack tip. Fatigue crack propagation tests have been carried out under various loading conditions ( constant amplitude cyclic loading, block amplitude cyclic loading and constant amplitude cyclic loading and a few static large spike loads ) for acceleration, retardation and non-propagation of crack growth rate. From the results of this study, fatigue crack propagation phenomena are shown in hysteresis loop for fatigue crack tip, and fatigue crack growth retardation and non-propagation bring long tail of hysteresis loop for fatigue crack tip. And effects of retardation in the F/B steel are larger than that in F/P steel.