1. Fatigue Behavior of Welded Steel
Joints in Air and Seawater
G.H.G. Vaessen, Metaalinstituut-TNO
J. de Back, Delft tJ. of Technology
J.L. van Leeuwen, Delft U. of Technology
Summary
Constant-amplitude fatigue tests were carried out in air expensive and time consuming; therefore, it is possible
and artificial seawater on T -shaped welded steel joints. to perform only a very limited number of such tests. The
The main objective of the test program was to compare vast majority of corrosion fatigue tests has to be per-
the behavior of welded steel joints in air and seawater. In formed with laboratory-size specimens under conditions
addition, the effects of weld-profile/weld-finishing, simulating the conditions for offshore structures in the
stress relieving, and stress ratio were investigated. The North Sea.
influence of cathodic protection and overprotection on This paper describes the results of fatigue tests with
corrosion fatigue behavior also was studied. welded joints in air and artificial seawater. The tests
were carried out on nonload-carrying welded joints. The
Introduction parameters varied' in this investigation are environment
The recent use of offshore structures in deeper and (air and artificial seawater), stress ratio R (R=O.l and
rougher areas (e.g., the northern part of the North Sea), - 1), stress relief treatment [postweld heat treatment
where there are lower air and sea temperatures, high (PWHT)], weld profile, weld finishing [as-welded,
winds, and severe marine conditions, has increased the grinding, and tungsten inert gas (TIG) and plasma dress-
probability of fatigue failure. For safe design of offshore ings}, and cathodic protection and overprotection. The
structures in such areas, knowledge of the corrosion work forms part of a large research program on the cor-
fatigue behavior of steels and welded joints under rosion fatigue behavior of welded steel joints. Some
representative conditions is of vital importance. The vast preliminary tests results are given.
majority of fatigue data on which current fatigue design
rules are based have been derived from tests with Material
laboratory-size specimens in air. -5 The material used for the fabrication of the test
The corrosion effect normally is catered for, in the specimens was in accordance with Euronorm Fe 510 (BS
case of offshore structures, by extrapolation of the 4360 Grade D steel). The plate steel thicknesses were 40
design S-N curves beyond the fatigue limit. 6 Otherwise, and 70 mm. The chemical composition and mechanical
the same basic concepts as for "in-air" structures are ap- properties of the plates are described in Tables 1 and 2.
plied-"stress range philosophy" and Miner's rule. The The microstructure of the steel contained about 20 to
justification for this approach is a limited number of 25% pearlite (grain ~ize of 10.5 to 11 /tm according to
laboratory tests in simulated seawater, very limited ex- ASTM .specification). The impurity content of the steel
perience from laboratory test results that corrosion pro- was found to be low. The analysis and properties of the
tection is effective in delaying corrosion fatigue failures, steel are within specification values (Euronorm Fe
and service experience (which at present does not in- 510).7
.elude the North Sea).
To obtain fatigue data appropriate to steel offshore Experimental Work
structures, it is desirable to obtain more data about the Test Specimens (Design and Fabrication)
corrosion fatigue behavior of tubular joints under Fig. 1 illustrates the specimen configurations and weld
simulated North Sea conditions. However, these tests are proftles. Each specimen was fabricated such mat the
0149-2136l8210002-8621 $00.25
longitudinal direction of the specimen was aligned
Copyright 1979 Offshore Technology Conference parallel to the rolli!lg direction of the plate. Manual
440 JOURNAL OF PETROLEUM TECHNOLOGY

2. TABLE 1-CHEMICAL ANALYSIS OF PARENT STEEL
C Mn P S Si AI Cu Sn Cr Ni Nb
0.17 1.44 0.018 0.004 0.35 40 0.019 0.006 0.039 0.017 0.046
TABLE 2-MECHANICAL PROPERTIES OF PARENT STEEL
Elongation
R • (d p 5)· Zt Charpy V Energies·
(N/m~2)*. (%)** (%) at -3O D C (J) ••
40-mm steel plate 551 31 26 to 46 160
70-mm steel plate 530 30 28 to 62 155
-Mean value .
• "Transverse to rolling direction.
tThickness direction; range.
TABLE 3-SPECIFICATION OF THE TlG AND PLASMA DRESSING METHODS
TIG Dressing Plasma Dressing
(vertical position, (vertical position,
downhill direction) downhill direction)
thoriated tungsten nozzle (¢ = 3.2 mm)
electrode (¢=3.2 mm) plasma gas (95% argon
and 5% hydrogen)
cup diameter = 11 mm flow (plasma gas)=0.75 dm 3 /min
shielding gas argon shielding gas argon = 10 dm 3/min
heat input = 13.1 kJ/cm heat input = 21.5 and
(first and second run) 18.1 kJ/cm
current = 21 0 A current = 130A
voltage = 12.5 V voltage = 28 to 30 V
320
t
I
t 920 tJ
i-
! I I"
I II 11
218
520
t t
1. 1520 IJ
I II Fig. 2-lIIustration of weld finishing by means of toe burr
grinding.
~~
lSwldtd illProwed profile
Fig. 1-Details of fatigue test specimens and schematic il-
lustration of fatigue tests.
FEBRUARY 1982 441

3. metal arc welding (vertical, uphill), using covered basic quency of 2 to 5 Hz; the tests in artificial aerated
electrodes (4)=3t,4 mm for the root runs and 4>=4 mm seawater were performed at a loading frequency of 0.2
for the following runs) according to AWS Code E 7016, Hz.
was employed to fabricate the specimens and a For defining the applied stress, strain gauges were
preheat/interpass temperature of 100 to 150°C was used. bonded to the test specimens at a distance of about 25
Three specimens were welded simultaneously without mm from each weld toe. In addition, for obtaining infor-
restraint. The effect of a steep and a smooth weld profile mation about crack initiation, strain gauges were bonded
on fatigue behavior has been investigated for the 40-mm to some test specimens in the immediate vicinity (about 2
specimens. mm) of the weld toe at both sides of the gusset. Failure
Stress relieving of the test specimens was performed was defined to have occurred in a test whenever the
by heating the specimens to 580±20°C at a mean rate of crack reached half the specimen thickness.
2oo°C/hr (above 300°C), holding 2.5 hrs, and furnace At this stage, the fatigue crack growth rate was so fast
cooling at a rate of l00°C/hr to 300°C. followed by air that the· number of cycles remaining to complete separa-
cooling. tion would have been very small compared with the
number of cycles already applied.
Test Procedure After test completion, some specimens were examined
The specimens were tested in four-point bending under for a detailed failure analysis. The corrosion fatigue ex-
sinusoidal constant amplitude loading. Seven loading periments were carried out by pumping artificial
machines under servovalve control in an MTS ™ or seawater (according to ASTM Sr-cification D 1141-52)
Schenck ™ (250-kN machine) closed-loop testing from a reservoir (about 500 dm ) through a transparent
system were used. The layouts of these machines were box (about 50 dm 3 ) surrounding the central part of the
similar. The force exerted by the actuator was transferred specimens. The seawater flow rate was of the order of 1
to the specimen so that it was loaded in four-point bend- dm 3 /min.
ing (Fig. 1). A fresh mix of seawater was substituted periodically
The 40-mm test specimens were tested at a stress ratio (about every 3 months). The temperature (20± 10c) and
R;;=O.I; the 70-mm test specimens were tested at R=O.l the pH (8 to 8.3) of the seawater were controlled and
and - 1. The tests in air were performed at a loading fre- monitored continuously. The chemical composition
.n
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Fig. 3-lnfluence of stress ratio (as welded). Fig. 5-lnfluence of stress relieving (air).
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442 JOURNAL OF PETROLEUM TECHNOLOGY

4. (salinity, bicarbonate ion concentration, chlorinity, etc.) Figs. 3 and 4 show, for the 70-mm T-shaped
.was controlled periodically. The seawater tests were per- specimens as welded (Fig. 3) and stress relieved (Fig .
fonned at free corrosion potential. 4), the effect of seawater on fatigue lifetime. The test
specimens have been tested at two different stress ratios
Fatigue Strength Improvement Techniques (viz., R=O.1 and -1).
Fatigue improvement techniques (grinding and TIG and The influence of stress relieving on fatigue lifetime is
plasma dressings) were applied to some T -shaped shown in Figs. 5 and 6. The effect of stress relieving has
40-mm test specimens. been investigated in air and seawater and at two different
Toe burr grinding is achieved by grinding the weld toe stress ratios (R=O.l and -1).
with the burr tool illustrated in Fig. 2. This technique ef- The effect of weld profile and weld finishing (grinding
fectively removes the undercut and defects that occur at and TIG and plasma dressings) on fatigue endurance is
the weld toe region. The average depth of grinding at the shown in Figs. 7 and 8.
weld toe was approximately 0.6 mm. The manual TIG- In Fig. 9, the test results of cathodic overprotection are
dressing/plasma-dressing procedure involved remelting given.
of the weld toe by one· single weld run, followed The fatigue data obtained in air and seawater have
[because of an unacceptable hardness in the heat-affected .been compared with relevant fatigue design curves for
zone (HAZ) after the first weld run] by a second weld the joint geometry under consideration (Class F). JO The
run with nearly the same heat input. 8,9 The second weld mean design curve and the most commonly used design
run was 2.5 to 3 mm from the toe of the first run in the curve (the mean-minus-two-standard-deviations curve)
direction of the original groove weld. Table 3 gives some have been used (Fig. 10). In comparing the data ob-
details of the applied dressing techniques. tained, remember that the design curves are intended to
be applied primarily to axially loaded joints and that the
Results present results have been obtained under four-point
Because the stress range generally is considered the bending. Results obtained under bending are expected to
primary variable detennining the fatigue life of welded correspond to longer lives than those obtained under ax-
joints, the results of this investigation have been ex- ial loading.
pressed in tenns of the applied stress range against the During the fatigue experiments, the cracks always
number of cycles to failure (on logarithmic axes). The began near the toe of the weld, generally at both sides of
test results are shown in Figs. 3 through 9. the gusset. Usually, after some time, crack development
t
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dressing).
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250 ~l1 ....t""""1d
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Fig. 8-lnfluence of weld finishing (TIG dressing and im- Fig. 10-Comparison of results with fatigue design curves.
proved profile).
FEBRUARY 1982 443

5. 0,7
A 40 - 1 - 5 - L - Ro
iE5TSPEC1MEN QqO-l<~-l-RO N(jH1N~l STI'IESSAANGE "-144 tl/M"!;?
~o.
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1.0
STR~INS AT CR~CKEO SIDE mm _
Fig. 11-Strain gauge records during fatigue lifetime. Fig. 12-Striation spacing as a function of crack depth as
measured by transmission electron microscopy
(TEM).
became predominant at one side. lifetime between the welded steel joints tested in air and
The strain gauge measurements showed that crack in- in seawater of SoC under freely corroding conditions.
itiation usually takes place in the early stage of a fatigue This leads to the tentative conclusion that the deleterious
experiment (about 10 to 2S% of the total fatigue life). A effect of seawater on the fatigue behavior of welded steel
characteristic strain gauge record is shown in Fig. 11. joints is strongly dependent on seawater temperature.
A detailed fractographic analysis of one of the This conclusion also needs further verification.
specimens tested in air showed that the cracks originated The fatigue life obtained for the welded steel joints
from an undercut in the weld metal and that crack prop- tested in air corresponds well with the Class F (the rele-
agation took place afterward in a coarse-grained area of vant joint classification) mean S-N curve of the design
the HAZ. A diagram showing the measured striation curve. The fatigue life of the joints tested in seawater
spacing as a function of the actual crack depth is shown corresponds well with the Class F mean-minus-two-
in Fig. 12. Striations smaller than about 0.04 to 0.06 J.tm standard-deviations S-N curve of the design curve (Fig.
are usually nearly invisible. For that reason, the mean 10).
striation spacing in the vicinity of the crack origin may
be overestimated. Influence of Steel Plate Thickness
The S-N curves obtained for the 40- and 70-mm test
Discussion specimens in both air and seawater reveal no significant
Effect of Seawater effect of plate steel thickness on fatigue strength,
The S-N curves (based on stress range) obtained in air although such an effect has been predicted for similar
and in seawater of 20°C under freely corroding condi- joints on the basis of theoretical fracture mechanics
tions for the welded steel joints are different. The fatigue analyses. Booth 3,4 found a plate thickness effect in 2S-
lifetime has been found to be at least a factor of two to and 38-mm transverse joints and explained this effect on
three times shorter in seawater (Figs. 3 and 4) than in air the basis of the interdependence among stress intensity
when the same stress range is applied. The seawater ef- factors, stress concentration factor, and plate steel
fect on the fatigue behavior of the welded steel joints thickness.
seems to be more pronounced for the stress-relieved The steel plate thickness effect also has been observed
specimens than for the as-welded specimens. Stress· in other investigations-however, usually with relatively
relieving has a favorable effect on air fatigue endurance thin plates. The preliminary conclusion of the present
(Figs. 3 and 4). This effect is larger at R= -1 than at work that increasing the thickness above 40 mm has no
R=O.I, which can be explained on the basis of crack significant effect on fatigue behavior in air and in
closure effects. In seawater, however, the favorable ef- seawater needs further justification.
feet of stress relieving is much smaller (Fig. 6); even at
R =0.1, the effect is negligible because of the larger ef- Influence of Stress Ratio
fect of the seawater on the fatigue endurance of the The results of the fatigue tests with the 70-mm transverse
stress-relieved specimens. joints, tested in bending at stress ratios R=O.1 and -1,
Booth,3,4 with similar test specimens and test pro- indicate some effect of the applied stress ratio on the
cedure, has found no significant difference in fatigue fatigue strength.
444 JOURNAL OF PETROLEUM TECHNOLOGY

6. It can be' observed that, under bending loading, an in- seawater on fatigue endurance. In seawater, fatigue en-
crease in stress ratio from R= -1 to R=O.l tends to durance was found to be at least two or three times'
result in a small decrease in the fatigue strength. It ap- shorter than in air. .
pears that the influence of the stress ratio is about the 2. The plate steel thickness does not influence
same for both air and seawater. The results clearly in- significantly the fatigue strength of 40- and 70-mm weld-
dicate that the influence of stress ratio is more pro- ed steel joints in air and seawater.
nounced for the stress-relieved welded joints than for the 3. The stress ratio has a small influence on the fatigue
as-welded joints (even more so in seawater than in air). strength (based on stress range) of welded steel joints
This may be explained on basis of the fact that, in the lat- loaded in bending in air and seawater. An increase in
ter joints, high tensile residual stresses exist. Under the stress ratio from R= -1 to 0.1 results in some decrease
applied loading (R=O.1 and -1), the stresses near the in fatigue strength. The effect was found not to depend
weld in the as-welded specimens remain largely tensile on the environment (air or seawater) and to be more pro-
even under compressive loading (R= -1). Under these nounced for the stress-relieved specimens than for the as-
circumstances, the stress range is assumed to be'the ma- welded specimens.
jor variable determining fatigue and no large effect of 4. Grinding and plasma dressing of the weld toe in-
stress ratio is expected when the results are expressed in crease the fatigue life in air as well as in seawater. TIG
terms of the applied stress range. dressing of the weld toe gives only a slight improvement,
especially at long lives. A less steep toe angle (45 vs.
70°) has only a slight beneficial effect.
Influence of Fatigue Improvement Techniques
5. The test results in air are in good general agreement
Fig. 7 shows that grinding and plasma dressing of the with existing fatigue data and seem to be described safe-
weld toe significantly increase the fatigue strength of the ly by the fatigue design rules.
,welded steel joints in both air and seawater. The effect 6. Cathodic protection was found to be most effective
seems to be more pronounced in air than in seawater. at lower stress ranges and gives an improvement of a fac-
Obviously, as with the stress-relieved vs. as-welded tor of four in fatigue endurance. Cathodic overprotection
specimens, the detrimental effect of the seawater on has an unfavorable effect on fatigue life compared with
fatigue endurance becomes more predominant after cathodic protection. However, compared with the basic
fatigue improvement techniques are applied. corrosion fatigue results, no disastrous effect need be
In air, TIG dressing seems to be favorable only in feared.
short lives (Fig. 8). It is assumed that TIG dressing in
thick plates (40 mm) causes high welding stresses, which
influences the fatigue life unfavorably, especially at a
high number of cycles. The improvement of TIG dress-
Acknowledgments
ing of welded specimens tested in seawater seems to be This work forms part of a European Offshore Steels
small. Changing the weld angle from 70 to 45° seems to Research project sponsored by the European Coal and
increase the fatigue life in air as well as in seawater only Steel Community. We are indebted to SMOZ (Founda-
at a high number of cycles. The improvement is not very tion Materials Research in the Sea) for coordinating the
significant. Dutch part of the ECSC work. We also are indebted to
our colleagues J.J.W. Nibbering at Delft U., J.L.
Influence of Cathodic Protection Overbeeke at Eindhoven U., and W. Dortland and H.
The results of the tests on cathodically protected Wildschut at TNO for their active participation in this
( -900-mV) and overprotected (- 11 OO-mV) 70-mm investigation.
specimens (stress relieved) are shown in Fig. 9. The
results indicate that cathodic protection is effective only
at low(er) stress range values. This is in agreement with
Booth's findings. 3,4 Cathodic overprotection reveals an References
unfavorable effect on fatigue life compared with the 1. "Regulations for the Structural Design of Fixed Structures on the
results of cathodically protected specimens. However, Norwegian Continental Shelf," Norwegian Oil Directorate (April
1977). . ,
the end endurance of the overprotected specimens is still 2. Oumey, T.R. and Maddox, S.J.: "A Re-analysis of Fatigue Data
somewhat higher than that of the basic test specimens in fOT Welded Joints in Steel," Report El44172, Welding Inst., Cam-
seawater. bridge, England (1972).
PWHT could have influenced the effect of cathodic 3. Booth, O.S.: "Constant Amplitude Fatigue Tests on Welded Steel
Joints Perfonned in Air," European Offshore Steds Research,
protection beneficially; however, test results of Cambridge, England (Nov. 1978).
cathodically protected as-welded specimens, reported by 4. Booth, O.S.: "Constant Amplitude Fatigue Tests on Welded Steel
Booth,3,4 show a beneficial effect on fatigue life (at Joints Perfonned in Seawater," European Offshore Steels
lower stress ranges). Research, Cambridge, England (Nov. 1978).
5. Solli, 0.: "Corrosion Fatigue of Welded Joints in Structural Steel
and the Effect of Cathodic Protection," European Offshore Steels
Research, Cambridge. England (Nov. 1978).
Conclusions 6. Berge, S.: "Constant Amplitude Fatigue Strength of Welds in
Constant amplitude fatigue tests were conducted in air Seawater Drip," European Offshore Steels Research, Cambridge,
and seawater on transverse nonload-carrying welded England (Nov. 1978).
7. Wildschut, H., Dortland, W., de Bach, J., and van Leeuwen,
steel joints. The following conclusions are drawn. J.L.: "Fatigue Behavior of Welded 'Joints in Air and Seawater,"
1. TheS-N curves (based on stress range) obtained for European Offshore Steels Research, Cambridge, England (Nov.
air and seawater clearly reveal a deleterious effect of the 1978).
FEBRUARY 1982 445

7. 8. Haagensen, P.J.: "Effect of TIG Dressing on Fatigue Perfor- SI Metric Conversion Factors
mance and Hardness of Steel Weldments," paper STP 648,
ASTM, Philadelphia (1978). Btu x 1.055 056 E+OO kJ
9. Millington, D.: "TIG Dressing for the Improvement of Fatigue OF (OF-32)/L8 °C
Properties in Welded High Strength Steels," Contract Report in. x 1.54* E+01 mm
CZlS/2ZI7I, Welding Inst., Cambridge, England (July 1971).
to. Gurney, T.S.; "Fatigue Design Rules for Welded Steel Joints," L x 1.0* E+OO = dIn 3
Research Bull., Welding Ins~. (May 1976) 17. . lbf x 4.448 222 E+OO N
micron X 1.0* E+OO I+m
Original manuscript received in Society of Petroleum Engineers office Feb. 15, 1979. psi X 6.894 757 E-03 = N/mm 2 (MPa)
Paper accepted for publication Jan. 17,1980. Revised manuscript received Nov. 10,
1981. Paper (SPE 8621, OTC 3421) first presented altha 11th OffshOre Technology
Conference held in Houston April30-'May 3, 1979. *Conversion factor is exact. JPT
446 JOURNAL OF PETROLEUM TECHNOLOGY