| 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. High levels of 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. High levels of 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.
For additional information on CC Technologies
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