Phenomenological investigation of the influence of Cathodic protection on corrosion fatigue crack propagation behaviour, in a BS 4360 50D type structural steel and associated weldment microstructures, in a marine environment.

摘要

The influence of Cathodic Protection potential upon corrosionfatigue crack propagation rates in a medium7strength ferritic-pearliticstructural steel (ES 4360 grade 50D) and associated weldment microstructuresin simulated sea-water was studied and the results werepresented in bi-modal da/dN vs AK curves. Above transition propagationrate data was satisfactorily described by the Paris relationshipda/dN = C. AKm and a relationship of the formin = Aln C+D betweenParis exponent m and constant C was confirmed. In all microstructuresthe influence of cathodic protection on crack propagation rate wasfound to be dependent upon the level of cathodic potential appliedand crack depth. A. critical crack depth of approximately 5.0mm wasidentified.Optical crack monitoring combined with graphical data processing,was found to be particularly effective for the characterisationof corrosion fatigue crack propagation behaviour and resulted inminimal data scatter. The phenomena of transition which was evidentin the bi-modal da/dN vs LK curves was found to be associated withsecondary or branched-crack activity. Back-extrapolation of belowtransition data was used to estimate Arm values and showed that theweldment beat-affected zone microstructure was a major potentialsource of fatigue or corrosion fatigue fracture.The effect of a single cycle of simulated overload, to stormloadlevels, on both fatigue and corrosion fatigue crack propagationbehaviour in parent plate material was studied and the phenomenonof retardation was confirmed.