Effects of Stress History and Corrosive Environment on Fatigue Crack Propagation

Abstract

The effects of stress history and corrosive environment on fatigue crack propagation of a mild steel are discussed under the constant and the variable stress amplitude, and the corrosion fatigue strength under service load is made clear as follows ; The crack initiation of corrosion fatigue is very early but the crack rate in saline and in water becomes smaller than that in air from a certain value of crack depth. This may be due to the wedge action of metal oxide in cracks. In hydrochloric acid the decrease of the crack rate is not observed because of the dissolution of metal oxide. Under high-low two-step stress history, both in air and in saline the crack propagation is delayed for a considerable period after the stress change and the subsequent crack rate is nearly equal to the expected one for the lower stress. Under continuously decreasing stress history as well, both in air and in saline the crack rate becomes smaller than the expected one based on the linear cumulative damage hypothesis. This is due to the deceleration effect corresponding to the delay in crack propagation under high-low stresses. On the other hand, under low-high two-step stress history, the crack rate in air after the stress change is the same as expected, while the rate in saline is greater for a while after the stress change due to the balance of the metal oxide produced on the crack wall and the opening of the crack, and then gradually decreases to the expected one. The acceleration effect of the crack propagation, however, tends to diminish at a smaller increasing rate of the stress amplitude under continuously increasing stress history. ∫(dn/N) under the various stress patterns may be estimated by considering the acceleration effect and deceleration effect. For example, ∫(dn/N)≅l is concluded in saline under two-stress repeated block history and under stress varying up and down in a short period because of the mutual offset of the acceleration effect and the deceleration one. Since the life of smooth specimens in corrosive environments is wholly occupied by crack propagation, the same argument for notched specimens mentioned above may be applicable to the corrosion fatigue of smooth specimens under various stress histories.