Steel--Fatigue

Fatigue response of selected high strength steels in seawater was investigated
under conditions relevant to tension leg platform tendon applications. This involved both
freely corroding and cathodically protected, displacement controlled experiments upon
constant taper, bending specimens which were either notched or welded, ground and post
weld heat treated. High stress range freely corroding results indicated enhanced life
compared to structural steel, whereas at low stress range the opposite was true.
Corrections made to the high strength steel and structural steel data to correspond to
similar test conditions revealed no benefit of enhanced material strength on corrosion
fatigue life. Fatigue life for all cathodically protected specimens exceeded that for the
freely corroding ones, and with one exception testing was terminated prior to failure.
The single cathodically protected specimen failure involved a stress range approximately
one-third the value for other specimens which were run-outs. This suggests that an
environmental cracking process may operate precipitously at relatively low stress
amplitudes (alternately, high R values). Fatigue life for freely corroding notched
specimens was less than for grounded PWHT specimens, the difference increasing with
decreasing amplitude.

An experiment was designed to provide a simulated steel fatigue crack under cathodic protection in seawater for study. Calcareous deposits were found to form in the simulated crack at -0.800, -0.900, -1.000 and -1.100 volts (SCE) with cycle frequencies of both 0.5 and 1.0 Hertz. By X-ray and SEM analysis, calcium carbonate deposits were shown to grow in quantities sufficient to block and become compressed by this simulated crack. Increased fluid velocities from crack pumping were shown to negatively affect crystal nucleation and growth. Due to kinetic barriers to crystal growth, aragonite and brucite were determined to be the most likely deposit type; excluding calcite, dolomite and magnesite. The deposits formed at -1.000V were shown by current requirements and SEM to provide the best protection as a coating.

The fatigue response of selected high strength low
alloy steels, that are under consideration for future
offshore structural applications, was investigated. Freely
corroding and cathodically polarized experiments were
conducted under conditions relevant .to these applications.
Cathodic polarization experiments involved testing of A537
Direct Quenched (A537 DQ) steel at different potentials in
the -0.80 V to -1.10 V (SCE) range. The damage to freely
corroding high strength steel was more than on structural
steel for calculations based on the Wirsching wide band
spectrum. Polarization to -0.80 V (SCE) improved the
fatigue life of A537 DQ steel over the freely corroding
life. A definite conclusion with regard to the influence of
different levels of cathodic polarization on the fatigue
life of the steel investigated can not not be drawn based on
the results from this study due to data variability.

A model for calcareous deposit induced fatigue crack
closure has been previously reported based upon the
criterion that the deposit thickness within the crack
equals or exceeds one-half the minimum crack opening
displacement. In the present paper an expanded and refined
model is proposed by considering (1) compaction of
calcareous deposits during the closure period of the stress
cycle and (2) the relationship between R ratio and
threshold stress intensity range. Compression tests upon
calcareous films grown on steel surfaces have been
performed, and these reveal the change in deposit thickness
as a function of stress. The implications of these models
to calcareous deposit induced fatigue crack closure are
discussed in terms of mechanical and electrochemical
parameters.

In the present research butt welded ABS DH-32 steel
specimens were fatigue tested under freely corroding and
cathodic protection conditions by a time-series simulated
wide-band spectra of a North Sea wave climate. Previous
research pertaining to the probabilistic features of wave
dynamics, wave force models, Markov-Chain sea state
evolution and time-series simulation of wide-band spectra
are addressed as a background review. The resulting freely
corroding data are discussed on the basis of the SN plot
which developed from the previous investigations at FAU.
Calculation of the damage of failed specimens by employing
the "Equivalent Narrow-Band Approach" resulted in excellent
agreement with the linear damage summation assumption.

Reverse bend, uniform stress beam specimens of ABS DH-32
steel were subjected to fatigue cycles of randomly generated
amplitude in a sea water environment to determine the effect
of narrow band spectrum loading on cycles to failure. The
loading spectrum was the C/12/20 spectrum, developed at the
National Engineering Laboratory in Scotland to simulate the
severe conditions of the North Sea. Some misinformation
which has been published by NEL and other& regarding ~he
C/12/20 spectrum is addressed. A number ot means for
presenting random amplitude cycles to failure data in the
form of an SN plot are discussed. A means for taking into
account fatigue test cycling rate in failure predictions is
proposed, and the results of the freely corroding specimens
of this investigation conform to the proposed technique.
The random amplitude specimens exhibited reduced fatigue
lives compared with similar specimens of an earlier constant
amplitude investigation.

The fatigue crack growth behavior of welded ABS DH32 steel spec~mens
cathodically polarized to -1.00 volt (Cu-CuS04) and subjected to bending
fatigue of nominal stress + 69 MN/m^2 (+- 10 ksi) in seawater was
investigated. Small non-propagating surface cracks with lengths up to
4.93 mm formed in the specimens. There were no s~gns of failure after
36 x 10^6 cycles when the experiment was terminated. Comparison of
this test with similar freely corroding and -0.85 volt (Cu-CuS04)
tests indicated that -1.00 volt (Cu-CuS04) cathodic protection is conducive
to arrestment of small fatigue cracks. This suggests that
modest cathodic over-protection of steel in seawater may beneficial.

Constant deflection fatigue experiments performed on cathodically
protected, notched steel bars in natural sea
water produced stresses which caused short cracks to appear
soon after the beginning of testing . At stresses
above 22 ksi (152 MN/m^2), cracks grew continuously across
the bars. At stresses below 22 ksi (152 MN/m^2), and at
potentials of -1.20 volts , saturated calomel electrode SCE,
cracks did not grow, or arrested part way across the bar.
At potentials other than - 1.20 volts , SCE, stress levels
had to be less than 19 ksi (131 MN/m^2), before crack growth
could be arrested. Strain gage readings suggest calcareous
material may be forming on the fracture surfaces of cracks
in which growth was arrested. Examination of these surfaces shows a pattern of calcareous deposit and corrosion
product which suggests a potential gradient within the
crack electrolyte. A mathematical model is presented
which describes this potential gradient.

Fatigue tests were performed at low stresss intensity ranges
upon precracxed HY-80 steel specimens in air and sea water,
at stress ratios of 0.1 and 0.8 with and without cathodic
protection. Crack growth. was found to be higher at R = 0.8
than R = 0.1. Also, the relative positioning of fatigue
crack growth rate data for freely corroding and cathodically
protected specimens reversed order cs R changed from
0.1 to 0.8. Threshold stress intensity ranges were found
to be higher in sea water than in air. A comparison of
the deltaKth for HY-80 was made with. that of other steels.

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.