INTERACTIVE NATURE OF CATHODIC POLARIZATION AND FATIGUE

This report, authored by Dr. Carl E. Jaske of CC Technologies, describes the results of a fatigue study of cathodically polarized steel in seawater. The study was divided into the four tasks: (1) comprehensive literature search, (2) fatigue of base metal in seawater, (3) fatigue of welded joints in seawater, and (4) development of a fatigue model. The experimental work of Tasks 2 and 3 was performed on specimens made from 19-mm thick ASTM A710, Grade A, Class 3 steel plate.

The comprehensive literature search extended a previous review of publications on corrosion-fatigue of steels in marine environments (Jaske et al., Corrosion Fatigue of Metals in Marine Environments, Springer-Verlag, New York, 1981). Results from the literature review were used to finalize the plans for the fatigue testing and to help develop the fatigue model. A total of 296 documents were obtained and reviewed. The reference citation and notes relevant to the current study were recorded in a computerized database and used to prepare the detailed bibliography in Appendix A. It uses the key words and types of steels listed in Appendices B and C, respectively.

Task 2 consisted of fatigue testing axially loaded smooth (unnotched) and notched base-metal specimens to develop stress versus number-of-cycles-to-failure (S-N) curves and crack-growth testing of base-metal compact-tension (CT) specimens to develop cyclic crack-growth rate (da/dN) data. The smooth specimens were tested in both air and synthetic seawater at room temperature (RT). The notched specimens were tested only in synthetic seawater at RT. For testing in seawater, the loading was sinusoidal at a stress ratio (R) of 0.1 and a frequency of 1 Hz, and the specimens were cathodically polarized to either -0.90 or -1.13 V vs. Ag/AgCl (adequate or over cathodic protection).

For adequate cathodic protection, the fatigue crack initiation resistance was slightly better than that in air, while the fatigue crack growth rate was about the same as that in air. Over cathodic protection degraded the fatigue crack initiation resistance slightly, but did not reduce it below that in air. The fatigue resistance of notched specimens with stress concentration factors of 2.0, 3.5, and 5.0 decreased as the stress concentration factor increased. Over cathodic protection reduced fatigue crack growth rate by producing calcareous scale deposits within the crack that reduced the effective range of stress intensity factor. The results of this study were in good agreement with published data from other corrosion-fatigue studies of ASTM A710 steel in seawater.

Task 3 consisted of fatigue testing of butt-welded and fillet-welded joints under cyclic three-point bending to develop crack-initiation and crack-growth data. The seawater environment, loading, and cathodic polarization conditions were the same as those used in testing the base-metal specimens. Specimens with as-welded or ground weld toes had similar fatigue strengths, while specimens with undercut weld toes had very low fatigue strengths. The butt welds had higher fatigue resistance than the fillet welds. The level of cathodic polarization did not have a significant effect on the fatigue strengths of the welded joints.

The DC electric potential drop method was used to detect crack initiation and measure crack growth in the welded-joint specimens. The fatigue crack growth behavior of welded joints was well characterized using the standard fracture-mechanics approach. The crack growth data obtained from tests of welded joints agreed with those from tests of standard fracture mechanics specimens.

The fatigue crack initiation resistance of notched specimens was well correlated with that of unnotched specimens by using Peterson’s (1974) fatigue strength reduction factor to calculate local stress values at the notches. The fatigue strength reduction factor was determined using the stress concentration factor, a material parameter, and the notch root radius. This same fatigue strength reduction factor also gave good predictions of the fatigue crack initiation resistance of both butt-welded and fillet-welded joints.

Total fatigue life of welded joints can be predicted as the sum of crack-initiation life and crack-growth life. Crack-initiation life is obtained from a S-N curve using the local stress at the weld toe. Local stress is computed using the fatigue strength reduction factor. Crack-growth life is computed by integrating the crack-growth rate relationship. This approach can be applied to the fatigue design of welded joints in marine structures.

Look for Report SSC-412 at http://www.shipstructure.org/ or NTIS Report #PB2000-108444 at http://www.ntis.gov/.

For additional information see The Effect of Cathodic Polarization on Fatigue Behavior.

Top of Page