Influence of Mean Stress on Fatigue-Crack Propagation in a Ferritic Weld Metal

Abstract

The separate contributions of the pulsating stress intensity, ∆K, and the maximum stress intensity, K _MAX, to the fatigue-crack propagation rate have been determined in a low-alloy ferritic weld metal. An effect of K _MAX on the rate was found only when ∆K exceeded some critical value; this coincided with a marked departure from plane-strain conditions and with a change in the mode of propagation from striation formation to void coalescence. It is suggested that the change in stress state is responsible both for the change in mode of propagation and for the K _MAX effect. In addition, high transient rates of propagation occurred when K _MAX was raised during the test; these are attributed to the needfor equilibrium to be re-established between the plastic zones characteristic of ∆K and of K _MAX. The advantages of performing tests at constant stress intensity rather than at constant applied load are pointed out.