Abstract:
The influence of Cathodic Protection potential upon corrosion
fatigue crack propagation rates in a medium7strength ferritic-pearlitic
structural steel (ES 4360 grade 50D) and associated weldment microstructures
in simulated sea-water was studied and the results were
presented in bi-modal da/dN vs AK curves. Above transition propagation
rate data was satisfactorily described by the Paris relationship
da/dN = C. AKm and a relationship of the formin = Aln C+D between
Paris exponent m and constant C was confirmed. In all microstructures
the influence of cathodic protection on crack propagation rate was
found to be dependent upon the level of cathodic potential applied
and crack depth. A. critical crack depth of approximately 5.0mm was
identified.
Optical crack monitoring combined with graphical data processing,
was found to be particularly effective for the characterisation
of corrosion fatigue crack propagation behaviour and resulted in
minimal data scatter. The phenomena of transition which was evident
in the bi-modal da/dN vs LK curves was found to be associated with
secondary or branched-crack activity. Back-extrapolation of below
transition data was used to estimate Arm values and showed that the
weldment beat-affected zone microstructure was a major potential
source of fatigue or corrosion fatigue fracture.
The effect of a single cycle of simulated overload, to stormload
levels, on both fatigue and corrosion fatigue crack propagation
behaviour in parent plate material was studied and the phenomenon
of retardation was confirmed.
