Studies on the Evaluation of Corrosion Fatigue Crack Initiation Life in Notched Mild Steel Plate

Abstract

In the ballast tank of oil tanker and offshore structures, corrosion damage caused by sea water and fatigue damage occurred by wave-induced forces usually go on occurring simultaneously. So the fatigue cracks often initiate on the members of such structures, even if stress lower than the fatigue limit in air acts on those in sea water.In this study, fatigue crack initiation life Nc of a notched steel plate subjected to pulsating tensile load in air and in sea water was analyzed theoretically by applying Neuber's rule with stress-strain function D proposed by Topper et al. as a parameter of damage. Experiments were carried out with notched plate specimen, of which the elastic stress concentration factor Kt was varied from 1. 0 to 6. 0, under pulsating tensile loading fatigue test in order to verify the theoretical analysis. And completely reversed strain controlled fatigue test was conducted with smooth round bar specimen on the consideration of reversed stress state induced on notch root under pulsating tensile loading. Results of pulsating tensile loading fatigue test of notched specimen were a good agreement with theoretical values evaluated with stress-strain function from completely reversed strain controlled fatigue test results. The corrosion effect factor Kc of notched specimen was expressed as an exponential function of corrosion time Tc corresponding to Nc irrespectively of Kt value, while the Kc of smooth specimen is less than that of notched specimen because of the difference on corrosion action.