Steel--Fracture

In order to investigate the cracking tendency of
cathodically polarized SEACURE superferritic stainless
steel, a series of potentiostatically controlled tests were
conducted in flowing natural seawater. U-bend and tensile
specimens were used to determine the threshold potential and
limiting fracture stress.
The microstructure was examined using a metallurgical
microscope and a chemical analysis was conducted across the
grain boundary. These results were then correlated to
hydrogen assisted cracking, by performing SEM and EDAX
analyses on fractured U-bend and tensile samples. Finally a
model was developed which explained the predominantly
intergranular cracking in the static stress condition and
also accommodated the transgranular failure during dynamic
loading.

A summary of some pertinent previous research by
other authors is presented separately from the actual
laboratory report. The report presents the results of air
and corrosion fatigue tests which are. interpreted in terms
of time to failure, crack growth history, and fracture
mechanics techniques. In addition, the results of cathodic
protection of corrosion fatigue tests are interpreted in
terms of simple-design criteria, as well as relative to
Critical Corrosion Rate Theory. One of the more significant
results presented is the greater longevity of corrosion
fatigue, as compared to air fatigue, notched specimens
stressed within 10-15 KSI above the air endurance limit.
Also, the potential required to cathodically protect such
notched specimens is shown to be 50 mV lower than for
comparable smooth specimens. Both phenomena are attributed
in part to the lowering of crack pH to acidic levels in
notched specimens.

Near-threshold fatigue crack growth rate data were generated for six high strength steels at stress ratios of 0.5 and 0.8 under different environmental conditions, which included air, freely corroding in seawater, and cathodically polarized in seawater to -0.80v, -0.95v and -1.10v (vs SCE). The influence of stress ratio, loading frequency, magnitude of cathodic polarization and initial DeltaK on fatigue crack growth behavior for these high strength steels was experimentally characterized. The results disclosed general trends of fatigue crack growth under conditions relevant to offshore structure applications and circumscribed variables that are primarily influential with regard to service performance. The rate controlling processes and cracking mechanisms of near-threshold fatigue crack growth, particularly under cathodic polarization in seawater, were also studied in conjunction with crack mouth opening displacement measurements, fractographic observation and crack profile examination. The data and observations suggest that calcareous deposit induced closure was the rate-controlling factor for near-threshold fatigue crack growth under cathodic polarization in seawater although the cracking involved a brittle failure mechanism as a result of hydrogen embrittlement. With different FCGR determining procedures which included natural K-decreasing, enhanced K-decreasing, natural K-increasing and constant DeltaK, the FCGR under cathodic polarization in seawater was found to be time-dependent and, as such, was not a unique function of DeltaK but depended upon K-history or experimental procedures. These observations were consistent with results of a theoretical analysis which involved kinetics modeling of the interaction between fatigue crack growth and calcareous deposit thickening. The model also predicted the existence of an upper limit transition DeltaK, DeltaK Upp/T, between the Paris and threshold regions and it was found that the predicted DeltaK Upp/T was in agreement with the experimental one. The basis for applying this critical parameter (DeltaK Upp/T) to offshore structure designs was addressed in conjunction with experimental results.