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STD.API/PETRO 12-ENGL 1980 N 0732290 0579075 T B 1

FLORIDA ATLANTIC UNIVERSITY

I N F L U E N C E O F S E A WATER AND CATHODIC PROTECTION UPON F A T I G U E O F WELDED

S T E E L P L A T E S , A S A P P L I C A B L E T O OFFSHORE STRUCTURES

by W i l l i a m H. Hartt, T h o m a s E . Henke

and Philip E. Martin D e p a r t m e n t o f O c e a n E n g i n e e r i n g

College of Engineering Engineering and Indust rial

Experiment Station

I

COPYRIGHT 2003; American Petroleum Institute

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TR-OE-80-1

INFLUENCE OF SEA WATER AND CATHODIC PROTECTION UPON FATIGUE OF WELDED

STEEL PLATES, AS APPLICABLE TO OFFSHORE STRUCTURES

by William H . Hartt, Thomas E . Henke

and P h i l i p E. M a r t i n D e p a r t m e n t of Ocean E n g i n e e r i n g

F i n a l Report, F i r s t Two-Year R e s e a r c h E f f o r t , P r e p a r e d for

AMERICAN PETROLEUM INSTITUTE March 20, 1980



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S T D * A P I / P E T R O 12-ENGL 1 9 8 0 D 0732270 0 5 7 7 0 7 7 AS4 lg

TABLE OF CONTENTS

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . i i

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . i v

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

PROJECT OBJECTIVES A N D EXPERIMENTS . . . . . . . . . . . . . . . . . 3

EXPERIMENTAL P R O C E D U R E . . . . . . . . . . . . . . . . . . . . . . . 4

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . 7

Stress Anaiysis o f Specimens . . . . . . . . . . . . . . . . . 7

Strain D a t a . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Electrochemical and Corrosion Factors . . . . . . . . . . . . . 11 Fatigue Tests . . . . . . . . . . . . . . . . . . . . . . . . . 12

Influence of Weld Variables and Defects . . . . . . . . . . . . 14

Fai 1 ure Phenomenology . . . . . . . . . . . . . . . . . . . . . 17

Design Cri t e r i a . . . . . . . . . . . . . . . . . . . . . . . 18

Crack Growth Rate Approach t o Fatigue Analysis and Design . . . 20

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

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SUMMARY

Fatigue a t welded tubular jo in ts has been judged t o be c r i t i ca l

with regard t o integri ty of offshore structures i n deep water or rough

sea applications. Thus, numerous ocean structures are predicted t o

experience 107-109 s t ress cycles in the 138 N/mm2 (20 ksi) range and

below d u r i n g the design l i f e . However, most experimental data relevant

t o fatigue of welded tubular jo in ts extend only t o 106-107 cycles.

report presents resul ts o f a two year research e f fo r t , the purpose of

which was t o develop sea water fatigue data in the h i g h cycle regime

and thereby contri bute to improved design of marine structures.

T h i s

The experimental technique involved fatigue of modified taper canti-

lever beam, ABS D H 32 steel specimens, measuring 2.54 by 15.25 cm. (1 by

6 inches) in the cross section of the weld, by a reverse bend constant

deflection technique. All variables were selected so t o approximate as

closely as possible the conditions which ex is t a t welded jo in ts o f o f f -

shore structures b u t allowing u p t o lo8 cycles t o be developed i n one

year. These included a s t ress range of e i ther 69 or 138 N/mm2 (10 o r

20 k s i ) , frequency of e i ther 0.5 o r 3 Hz, natural sea water environment

w i t h temperature e i ther ambient or 4°C and w i t h specimens ei ther freely

corroding or cathodically polarized t o -0 .85~ . or -1.00~. versus a Cu-CUSOL,

electrode.

cracks was provided by in-situ visual examination and by s t ra in gage

measurements.

Information regarding development and progression of fatigue

Cycles-to-initiation and cycles-to-fail Ure data are discussed w i t h

regard to influence upon the fatigue process o f numerous factors, including

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STD.API/PETRO 1 2 - E N G L 1980 Sl 0 7 3 2 2 9 0 0 5 7 9 0 7 9 b27

st ress range, cyclic frequency, sea water, corrosion s t a t e , temperature

and weld defects.

priateness o f design procedures and c r i t e r i a for mitigation of fatigue

o f offshore structures.

Based upon th i s projections are made regarding appro -

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A C KNOW L EDG EM ENT S

Trie investigators express sincere grat i tude t o members o f the

APT Technical Advisory Committee on Corrosion Fatigue fo r t he i r con-

tributions t o the project.

by this Committee enhanced accomplished o f project objectives and pro-

vi’ded a highly professional environment for the project management.

Memb.ers of the Committee are

The ongoing evaluation and i n p u t provided

Dr. James E. Burke,

Mr. John E . Hansford,

Mr. Peter W. Marshall,

Mr. Maryin L. Peterson, Chairman,

Mr. Charles P. Royer,

Mr. Fred W. Schremp.

In addition the au thors are appreciative of assistance w i t h var ious

phases o f the project provided by Mr. James Gay and Mr. Thomas McNamara.

F u n d i n g of a portion of the experiments by the Sea Grant Office o f the

National Oceanographic and Atmospheric Administration i s also acknowledged.

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STD.API/PETRO 12-ENGL L w o m 0 7 3 2 2 9 0 0 5 7 7 0 ~ 2 8 5 m

INTRODUCTION

Fatigue has been judged t o be an important problem with regard t o in-

tegr i ty of fixed offshore structures of the "jacket" or "template" t ~ p e . l ' ~

The s i t u a t i o n i s particularly significant i n deep water or rough sea locations,

where i t i s anticipated t h a t these structures may experience lo7 - lo9 s t ress

cycles of relatively low amplitude over the design l i f e . 2

and fa i lure i s most l ikely t o occur a t the welded joints of tubular members,

since a t such s i t e s s t ress i s concentrated due t o both metallurgical and struc-

turai (geometri cai ) i rregulari t i e s .4

Fatigue cracking

A recent 1 i t e ra ture study5 conducted under API sponsorship has comprehen-

sively reviewed the s t a t e o f knowledge regarding fatigue of welded structural

steel in sea water, and a research plan fo r future studies was recommended.

Evaluations such as t h i s and others6 have placed emphasis upon the f ac t tha t

most existing data which i s applicable t o fatigue of welded s teel structures

i n sea water has addressed the low cycle ( re la t ively few cycles-to-failure)

regime.

obtained primarily from t e s t s performed a t stressing rates in excess of those

experienced by tubular jo in ts of offshore structures. Since i t i s generally

considered tha t damage due t o cyclic s t ress ing i n a corrosive environment i s

more accelerated the lower the frequency, such low s t ress range t e s t s could

resul t i n an incorrect and overly optimistic assessment of fatigue resistance.

Figure 1, which i s reproduced from Figure 12 of reference 5 , presents S-N

curves from various investigations , 7 - 1 0 t h u s summarizing the extent of existing

data for re la t ively large welded steel specimens i n sea water or a similar t e s t

The limited h i g h cycle fatigue data which i s available has been



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STD.API/PETRO 12-ENGL 2 7 8 0 I 0 7 3 2 2 7 0 0 5 7 9 0 8 2 111

electrolyte a t the time t h a t report was prepared.

AWS-X and AWS-X Modified design curves.ll

nates the "hot spot" value on the outside surface of the main member weld

toe for simple T, Y and K connections.

cycles-to-failure becomes more sensitive t o strain range beyond 2 x lo6 cycles,

the AWS-X Modified curve represents a more conservative stance where log

fatigue strength i s assumed t o decrease a t a constant r a t e with decreasing

log strain range, presumably as a consequence of severe notches and corrosion.

The degree of conservatism, i f indeed there i s conservatism, associated w i t h

either of these design curves i n the h i g h cycle range cannot a t this time

be projected based upon presently available test data which properly models

the stress-environment state of welded joints in offshore structures.

Included also are the

For these the strain axis desig-

While the AWS-X curve considers t h a t

The U.K. Offshore Steels Research Project has for the past several years

been comprehensively addressing the problem of sea water corrosion fatigue. l 2

This program i s investigating the numerous facets of this problem, including

not only development of S-N and crack growth rate data, b u t also welding tech-

niques and material properties, experimental and f ini te element evaluation of

the mechanics of welded connections and fracture mechanics and fracture tough-

ness testing. While the U . K . investigations have developed a d d i t i o n a l , rele-

v a n t S-N data in the lo6 - lo7 cycles-to-failure range, s t i l l no t e s t results

extending beyond th i s (cycles-to-failure > lo7) have been reported.

most of this da ta has been for freely cor roding specimens and l i t t l e new

information i s available regarding the cathodically protected case.

Also,

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PROJECT OBJECTIVES AND EXPERIMENTS

The generalized objective of the experiments described i n this report

was t o develop data applicable t o h i g h cycle fatigue of welded steel struc-

tures in sea water such tha t the appropriateness o f various design c r i t e r i a

m i g h t be better understood.

For the purpose of accomplishing this a research plan was developed which

involved fatigue of 2.54 cm. (1.00 i n . ) thick welded s teel specimens i n sea

water a t to ta l s t ress ranges o f 69 and 138 N/mm2 (10 and 20 k s i ) . Prescribed

water temperature was ei ther ambient or 4°C and corrosion s t a t e was w i t h spe-

cimens e i ther freely corroding or cathodically polarized t o - 0 . 8 5 ~ or -1.00~

(Cu-CuSO,).

correspond, respectively, t o no cathodic protection, adequate protection and

overprotection.

ra te o f 0.5 Hz, t h i s being i n the range experienced by deep water structures

undergoing dynamic amplification. A t t h i s r a t e of tes t ing l o 7 cycles can be

developed in approximately nine months (assuming no machine down time). All

of the 69 N/m2 t e s t s were a t 3 Hz.

w i t h respect t o stressing rate o f offshore s t ructures , i t was selected because

t h i s speed permits 108 cycles t o be developed i n approximately thirteen months.

Table I i s a detailed l i s t i ng of specific t e s t s and t e s t conditions which

were prescribed for this investigation.

These three corrosion conditions were selected because they

Most of the fatigue t e s t s a t 138 N/mm2 were a t a stressing

While t h i s represents an acceleration

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S T D - A P I / P E T R O L2-ENGL 1980 E 0 7 3 2 2 9 0 0579084 Tï4 9

EXPERIMENTAL PROCEDURE

A schematic overview of the t e s t system employed for the present study

i s presented as Figure 2 . From this the key components, including fatigue

machines, potentiostats, data acquis i t ion system and sea water flow system

and the way these interface, are apparent.

All specimens were prepared from a normalized, 1.52 m . x 3.05 m. x 2.54 cm.

(5 f t . x 10 f t . x 1 i n . ) ABS DH32 steel plate.

Columbus Laboratories and was from the same stock employed i n Phase I experi-

This was obtained from Battelle

m e n t ~ ~ ~ o f t h i s overall research e f for t .

of t h i s material have been previously reported5 b u t are summarized here i n

Table I I .

p r e v i ~ u s l y . ~

possible current, good practice i n the j o i n i n g o f tubular members for offshore

structures. T h u s , weld surface profiles merged smoothly w i t h adjoining parent

plate.

Physical and mechanical properties

The plate was flame cut and welded w i t h the weld detai l described

I t was intended that t h i s welding technique model as closely as

Specimens were sectioned from the welded plate and machined t o the geo-

metry i l lus t ra ted i n Figure 3.

beam, such tha t a constant s t ress would resu l t i n the region of the weld.

I t was intended t h a t the relat ively large cross section a t the weld, 15.24 cm.

T h u s , the specimen design was a modified tapered

by 2.54 cm. (6.00 i n . by 1.00 i n . ) , would f a c i l i t a t e retention of welding re-

sidual s t resses , despite sectioning and machining.

mounted on each specimen along the longitudinal axis and 14.6 cm. above the

weld center l ine.

n i n g of the test.

A single strain gage was

T h i s gage was employed t o s e t the fatigue load a t the begin-

After the experimental program was underway, i t was deter-

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mined t h a t useful information regarding specimen compliance changes could be

obtained by periodic monitoring of the output from this gage.

Specimens were sand-blasted t o remove mill scale prior t o mounting i n

the sea water bath. The environment chamber was of an a l l plexiglass construc-

t i o n and employed Devcon 30 (Flexane), a moldable elastomer, t o form the seals.

A 5.1 x 15.2 cm. platinum coated n i o b i u m s t r i p mounted on each o f the two side

faces of the bath and opposite the weld served as the counter or auxiliary elec-

trode for t e s t s involving cathodic protection. The distance between the

specimen face and the bath in te r ior was 0.60 t o 0.65 cm., and nominal water

velocity across the specimen was controlled a t 15 cm./sec. (0.5 f t . / sec . )

based upon this dimension. Figure 4 i s a photograph of a specimen and bath

subsequent t o the above preparation.

The fatigue t e s t s per se ut i l ized six Fatigue Dynamics Model LFE 500

reverse bend, constant deflection machines, each modified t o accommodate two

specimens. Thus, a maximum of twelve specimens could be tested a t any given

time. The specimen pair for each fatigue machine was mounted i n series w i t h

respect t o the sea water flow system. T h u s , the e lectrolyte exiting the bath

of the f i r s t specimen then entered the bath o f the second and subsequently was

discharged t o the drain.

w i t h the various components ident i f ied.

Figure 5 i s a photograph o f one of these t e s t units

Potentiostats were fabricated based upon a cathodic protection c i rcu i t

board obtained from Englehard Industries. Control potentials and freely cor-

roding potentials were measured relat ive t o commercial saturated calomel elec-

trodes, which were checked weekly for s t ab i l i t y . For cathodically protected

specimens current was determined from the voltage drop across a four ohm res i s -

tor i n ser ies w i t h the specimen and counter electrode.

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STD.API/PETRO 1 2 - E N G L 1980 m 0732290 05790Ab 8 6 7

An Esterline-Angus Model PD64 data acquisition system was employed t o

record potential of each specimen and current of cathodically protected speci-

mens a t six hour intervals.

alarms and a relay system such t h a t a par t icular fatigue machine and pxent io-

s ta t would be s h u t down in the event o f a potential excursion i n excess of

k 0.005 volts re la t ive t o the control value.

Further, the acquisition u n i t *JsTs equipped with

Environment for the fatigue tests was natural sea water, as i s available

a t the FAU Marine Materials and Corrosion Laboratory.

on the Atlantic Ocean in Boca Raton a t a s i t e f ree o f urban o r industrial run-

o f f .

and wellpoint, the l a t t e r being positioned approximately one meter below the

sand and several meters seaward o f mean low tide.

including measurement of sa l in i ty , conductivity, temperature, pH and dissolved

oxygen i s performed routinely a t the Laboratory, i n addition t o occasional

chemical analysis.

former parameters over the course of an annual cycle.

noted here are typical of semitropical Atlantic Ocean surface waters.

solved oxygen concentrations correspond closely to a i r saturation.

T h i s f a c i l i t y is located

Sea water i s delivered t o the Laboratory by an a l l p las t ic pump, pipe

Water characterization,

Figure 6 de ta i l s sea water hydrology i n terms of the f ive

The value and trends

Dis-

For conduct of the fatigue t e s t s in 4°C sea water (see Table I ) a re-

circulating loop w i t h a cooling u n i t was added t o the normal sea water flow

system.

new water a t ambient temperature was introduced continuously a t a ra te of

0.15 l i t e r s per minute.

tory thermometer positioned i n the cold loop.

i n the range of 10 t o 11 ppm were noted.

sea water also was air saturated.

T h i s loop had a to ta l water capacity of approximately 16 l i t e r s and

Temperature o f th i s water was monitored by a labora-

Dissolved oxygen concentrations

I t was concluded from this tha t this

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STD.API/PETRO 1 2 - E N G L 1980 m 0732270 0 5 7 7 0 8 7 7T3 iai

RESULTS AND DISCUSS ION

Stress Analysis o f Specimens. Figure 7 depicts the resul ts of an ELAS

f i n i t e element analysis of the s t ress prof i le for the present specimen geo-

metry. Such an analysis was considered necessary because o f the complicated

specimen shape and the necessity of knowing the actual nominal s t ress as ac-

curately as possible.

(strain) a t any particular point on the specimen, the nominal s t ress ( e l a s t i c )

i n the region of the constant s t ress taper could be calculated.

f i n i t e element analysis a blank (unwelded) specimen was machined t o the dimen-

sions i n Figure 3 , and three s t ra in gages were mounted across a l ine where

the weld was located on the actual specimens. One of these was centered on

the spec men and the other two were 5.0 cm. t o e i ther side. A fourth gage

was positioned 14.6 cm. above the normal position o f the weld center l ine.

This was above the topmost part of the sea water bath (see Figure 4 ) . All

actual specimens were instrumented with a s t ra in gage in t h i s l a t t e r position.

This plate was then deflected by an amount corresponding t o 34.5 N/mm2 i n the

tapered, constant s t r e s s region; and readings were taken from the four gages

and compared w i t h the f i n i t e element analysis. The difference between the

single, upper gage and the lower three was within one percent o f tha t pre-

dicted by the s t ress analysis.

( s t r a in ) distribution of the present specimens had been adequately character-

ized and that the deflection t o yield a par t icular nominal s t ress i n the weld

region could be determined from the upper gage alone.

Based upon this analysis and by knowing the s t r e s s

To check the

On this basis i t was f e l t tha t the s t r e s s

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STD.API/PETRO 12-ENGL 1980 m 0 7 3 2 2 9 0 0 5 7 9 0 8 8 b3T

S t r a i n Data. From the periodic monitoring of o u t p u t from the s t r a in

gage on each specimen, compliance changes were noted w h i c h i n a t least some

instances were t h o u g h t t o re f lec t development and growth o f fatigue cracks.

Attention was focused upon two parameters, the to ta l s t ra in range and the

s t ra in asymmetry. The former i s defined as I (?in-?O) + (EO-EoUt) 1 , where

Ein i s the s t r a in reading w i t h the specimen deflected inward by the maximum

amount, tout i s the reading a t maximum outward deflection and EO i s the read-

i n g w i t h the specimen disconnected from the loading head (zero s t r a i n ) .

l a t t e r parameter, I I - I I , re f lec ts a t any given number of cycles

the extent t o which the s t ress cycle i s asymmetric. As a general rule varia-

tions i n these two parameters for individual specimens exhibited one of three

types of behavior. Figure 8 i s a plot of both s t ra in range and s t r a in asym-

metry versus cycles for Specimen 11, t h u s i l l u s t r a t ing what will be referred

t o as Type I behavior.

the average value for t h i s parameter d i d not change much from s t a r t t o f inish

o f the t e s t ( N = l o8 cycles).

than for s t ra in range; b u t here also the cycles average value has not changed

greatly, although i t does d i f f e r from the i n i t i a l value which was zero a t

N = O.

a t 69 N/m2 total s t ress range.

The

Although there were variations i n strain range w i t h N ,

Variations i n strain asymmetry were greater

This behavior was typical of cathodically protected specimens tested

Figure 9 i l l u s t r a t e s Type I I behavior, where a t some number o f cycles

(indicated by the discontinuous change i n slope of the dashed l i ne ) a d i s t inc t

transit ion i n s t r a in range and s t ra in asymmetry occurred. Type I I I behavior

i s indicated by Figure 10; and t h i s corresponds t o a s i tuat ion where only

strain range, b u t no t s t ra in asymmetry, underwent such a change.

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For specimens w h i c h exhibited Type I I or I I I behavior, a crack

apparent on the specimen edges and traversed approximately one-half

specimen thickness when s t ra in range had decreased t o approximately

cent of the i n i t i a l value. In the case of Type I I behavior, where

was

the

70 per-

ncreasing

s t ra in asymmetry accompanied the fal l -off i n strain range, the crack was always

located on t h a t side of the plate such tha t the absolute value o f the s t ra in

reading (ei ther I E - E * / or I E - E

so as t o open the crack.

behavior contained cracks on b o t h sides o f the plate.

I ) decreased when the plate was deflected in O out

On the other hand specimens which showed Type I I I

I n a l l instances the

fatigue cracks emanated from the weld toe and were oriented perpendicular t o

the plane of the specimen.

The above observations regarding Type I I and I I I behavior suggest tha t

the break in the strain range or s t ra in asymmetry versus cycles plot (Figures

9 and 1 0 ) corresponds t o occurrence o f a suff ic ient ly large fatigue crack that

macroscopic compliance changes resulted. Consequently, fatigue crack in i t i a -

tion, Ni, has been defined i n terms o f this number o f cycles. Experiments

have not yet been performed t o deduce what s ize crack i s required-to cause

the i n i t i a l decrease in strain range which i s apparent from Figures 9 and 10.

In the case o f specimens which were tested t o fa i lure a crack on the

specimen edge o f size less than one-half the plate thickness was seldom ob-

served.

men edge i t then extended t o the one-half plate thickness w i t h application i n

most instances o f fewer t h a n 2.5 x 105cyc1es. For f reely corroding specimens

T h i s suggests tha t once a crack grew along the weld toe t o the speci-

separation occurred a f t e r relatively few additional cycles.

fatigue cracks i n cathodically protected specimens generally arrested when

the crack reached the one-half thickness position.

On the other hand

In the case of cathodically

polarized specimens the cycles required for the crack to propagate across the

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remaining one-half plate thickness was often much greater, and during this

the strain reading with the specimen deflected i n the crack opening mode

often approached zero. Because o f this behavior, cycles-to-failure, N f ,

has been defined as the cycles required f o r the crack t o extend t o one-half

the specimen thickness.

I t has been projected t h a t load control fatigue tests may be representa-

tive of a non-redundant j o i n t of an offshore platform b u t t h a t such a tes t

technique yields unduly pessimistic results in the case of a joint in a redun-

dan t structure.14 I t may be reasoned w i t h regard t o the present E-N behavior

t h a t results of these experiments probably would n o t be significantly different

i f the fatigue tests had been o f the constant load type.

t h a t propagation would occur more rapidly in a constant load than in a constant

deflection tes t , b u t a quantitative assessment o f the difference between the

two requires that the crack size-compliance relationship be known, and this has

no t yet been determined. Differences between the two rates are expected t o be

small when the crack i s shallow and t o become more distinct as size increases.

I f , in the case o f Specimen 12 (Figure 9), however, average growth occurred

a t double the observed rate, as i t m i g h t in a constant load t e s t , then Nf-Ni

T h u s , i t i s expected

would equal 5 x lo5 cycles instead of 1.0 x lo6.

l i f e of the specimen by twenty-one percent, which i s a relatively modest reduc-

t i o n .

t o be only s ix percent.

should be the same for b o t h constant deflection and constant stress type loadings.

This would a l ter total fatigue

In the case of Specimen 13 (Figure l o ) , this difference is calculated

By this same rationale i t can be reasoned t h a t Ni

There presently exists no definitive explana t ion for the rather wide varia-

t i o n s i n strain asymmetry w i t h N , as were observed for Specimen 11 (Figure 8 ) .

Such scatter was more apparent for cathodically protected specimens t h a n f o r

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STD-APIíPETRO 22-ENGL 1980 S 0 7 3 2 2 9 0 0 5 7 9 0 9 2 1 2 4 I

freely corroding ones ( for example, compare Figures 9 and 10 w i t h Figure 8 )

and was greater for a control potential o f - 1 . 0 0 ~ (Cu-CuSO,) t h a n for -0 .85~.

Electrochemical and Corrosion Factors. Potential of freely corroding

specimens and potential and impressed current density of cathodically polarized

specimens were monitored d u r i n g each individual t e s t . I t was determined t h a t

in the freely corroding case potential was relat ively noble (posit ive) i n i t i a l l y

b u t then decreased t o a more active, steady-state value.

i s consistent w i t h the generally accepted perception tha t corrosion ra te of

s teel i n sea water i s under cathodic reaction control, a s governed by d i f f u -

Such an observation

sion of oxygen t h r o u g h corrosion product films. l 5

For most cathodically polarized specimens current density decreased with

exposure time, a typical example being as shown in Figure 11. Such behavior

i s generally a t t r ibuted t o accumulation of a calcareous deposit. In several

instances of specimens tested a t the higher s t ress range (138 N / m m 2 ) current

density decreased i n i t i a l l y b u t then increased w i t h subsequent exposure time.

Mean potential and current density values are l i s t ed in Table I I I along

w i t h other parameters which are t o be discussed l a t e r . The potentials are

i n the range of those reported i n the l i t e r a t u r e for s teel i n f lowing sea

water.l6 The current densit ies on the other hand exceed the value thought t o

be required t o cathodically protect s teel in sea water.17 This may have

resulted from the r e s t r i c t ive nature of the environmental chamber and a more

severe hydrodynamic flow s t a t e a t the metal surface than for the same nominal

velocity i n the open ocean. that calcareous films which formed on the present specimens were less developed

T h i s possibi l i ty i s supported by the observation

than ones which form i n the laboratory under quiescent conditions.

11



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~

STDSAPIIPETRO L2-ENGL 1980 a 0732290 U579092 Ob0

The observation tha t cathodic current density f o r specimens polarized

t o - 1 . 0 0 ~ ( C U - C U S O ~ ) was. less t h a n t h a t for specimens protected a t -0.85~

suggests t h a t a more res i s tan t deposit formed a t the more negative potent ia l .

Such a reversal in the normal cathodic current density-potential t r e m for

steel polarized i n sea water has recently been reported.18

Fatigue Tests. Table I I I l i s t s N i and N f values for the various fatigue

t e s t s (Table I ) which have been performed. A p o i n t which has been considered

in evaluating th i s data i s t h a t d i f ferent specimens encountered different en-

vironment conditions, depending upon the t e s t du ra t ion and time period. This

i s apparent from comparison o f the t e s t dates (Table I I I ) and environmental

data which was presented ea r l i e r as Figure 6. However, i n sp i te of the rather

wide range i n the ambient values (see temperature d a t a , f o r example) no inf lu-

ence of these var ia t ions upon the fatigue t e s t s resu l t s , as reflected by N i and

N f values, was noted. Also included i n Table I I I i s the type of E-N behavior

(Type I , I I or I I I , as discussed above) t h a t was exhibited by each specimen.

The above Ni and Nf data are displayed graphically as log s t ra in range-

log cycles p l o t s i n Figures 12 and 13.

freely cor roding specimens and the l a t t e r (Figure 13) t o cathodically polar-

ized ones.

i s apparent for the 3 Hz, ambient temperature t e s t s . Such good agreement for

the different specimens must be viewed w i t h i n the perspective tha t re la t ive ly

few t e s t s were performed fo r any specif ic se t o f conditions, and so probabi-

l i s t i c aspects o f fatigue have no t been fu l ly addressed by these experiments.

On the other hand, these data will subsequently be compared w i t h resul ts from

past experiments by other investigators; and the observed agreement is such

t o promote confidence i n the trends despite the small number o f tes ts .

The former (Figure 12) pertains t o

W i t h regard t o the freely corroding data re la t ively l i t t l e s ca t t e r

12



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~ ~ ~~ ~

STD.API/PETRO 1 2 - E N G L 1980 %I 0732290 0579093 T T 7

The relatively close agreement between N f values for Specimens 2 and 15

(ambient temperature, freely corroding, 3 Hz) and Specimens 12 and 13 (4"C,

freely corroding, 0.5 Hz) suggests e i ther t h a t frequency and temperature varia-

tions i n the range considered have l i t t l e or no effect or tha t the e f fec t o f

each i s offsett ing.

Comparison of Figures 12 and 13 reveals an enhancement in fatigue res i s -

tance, as reflected by both Ni and N f , as a consequence of cathodic polarization.

Such a finding i s consistent w i t h the resul ts of numerous previous investiga-

tions for notched,20,21 and ~ e l d e d ~ , ~ * , ~ ~ s tee l . Results of the

four t e s t s a t 138 N/mm (20 k s i ) tota s t ress range (6.67 x total s t ra in

range) suggest that fatigue l i f e for his level of cathodic polarization

(-0.85v, Cu-CuSO,) i s s l igh t ly greater a t 3 Hz than a t 0.5 Hz. This finding

i s consistent with the general perception of the influence o f frequency upon

corrosion fatigue l i f e , which projects damage t o occur a t a higher ra te the

lower the frequency.

sions upon such a small number of t e s t resul ts .

Of course, caution must be exercized when basing conclu-

Fatigue testing of Specimens 22 t h r o u g h 25 (Test Numbers 9 and 10) i s

s t i l l i n progress, and so f inal data from these i s n o t yet available for com-

parison.

case 8.1 x lo6 cycles have expired w i t h o u t indication of crack in i t ia t ion

suggests for the case of cathodically protected specimens, f i r s t , tha t the

colder water i s no more damaging than the warm (compare Specimens 22 and 23

w i t h 20 and 21) and, second, tha t polarization t o -l.OOv, Cu-CuSO,, i s not

detrimental compared t o - 0 . 8 5 ~ (compare Specimens 24 and 25 w i t h 20 and 21) .

T h i s projection may contrast w i t h the resu l t s of Jaske e t a1,13 who found a

forty percent reduction in fatigue l i f e for a single specimen fatigued in

However, the fact tha t i n one case 7.5 x lo6 cycles and i n the other

13



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD.API/PETRO 1 2 - E N G L 1980 R 0732290 û57909Y 733 M

sea water a t 4°C compared t o one a t 20OC.

t o -0.85v, Cu-CuSOb. However, i n the former case the frequency was 1.0 Hz

and i n the l a t t e r 7.5 Hz; and so t h i s difference in fatigue l i f e may re f lec t

an influence of frequency.

Both specimens were polarized

Cathodically protected specimens fatigued a t 69 N/mm2 (10 ks i ) total

t o t a l s t r a in range) survived lo8 cycles w i t h o u t s t ress range ( 3 . 3 3 x

in i t ia t ion i n a l l b u t one instance.

in i t ia t ion and fa i lure were on ly about one-third greater than for comparable

freely corroding Specimens. Metallographic examination o f sections o f Speci-

men 17 i n the vicinity of the fatigue crack revealed undercutting a t the

weld toe.

sections depth of the undercut was determined t o be 0.30 t o 0.35 mm. This

exceeds the AWS code for transverse welds of tubular s t ructures23 (maximum

undercut depth 0.25 mm), and defects such as t h i s can be detected i f the

welding inspector exercises proper quali ty control.

Influence o f Weld Variables and Defects.

In th i s case (Specimen 17) cycles-to-

Figure 14 i s a photograph of t h i s defect. From t h i s and similar

Numerous material and pro-

cedural factors have been judged t o influence fatigue strength of welded

steel specimens. These include: i ) composition and properties of the

parent material, 2 ) specimen w i d t h and thickness, 3) type and method of

weld preparation, 4) welding process and type o f electrode, 5 ) welding

position, 6 ) weld defects, 7 ) weld shape, 8) post-weld machining and

9 ) post-weld heat treatment.

made i t d i f f i cu l t his tor ical ly t o correlate resul ts from different investi-

Such an extensive l i s t of variables has

gations. However, f o r a single study factors 6 ) and

source of specimen-to-specimen variations, since the

by the investigator. T h u s , i n experiments where the

7 ) are the most l ikely

others can be controlled

we1 d reinforcement has

14



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~ ~ ~~ ~~

STD.API/PETRO 1 2 - E N G L 1780 0732290 0579095 ô7T R

been removed by machining, fatigue performance comparable w i t h t h a t o f

the parent plate has been realized, provided weld defects (lack of fusion,

1 ack of penetration, voids , i ncl usions, undercuts and cracks ) were no t

extreme. 2 4

been found t o be instrumental in i n i t i a t i n g fatigue fa i lure .

In such situations (reinforcement removed) weld defects have

For si tuations where the reinforcing metal i s present the s t ress

concentrating influence o f the geometric discontinuity a t the weld toe

has been judged t o control fatigue strength in many instances. Height

and angle of the weld reinforcement and weld toe radius are parameters

which determine the magnitude of t h i s s t r e s s concentration, and experi-

ments have been performed demonstrating the significance of

Additionally, analytical treatments have been developed whereby the stress

concentration factor a t a weld toe can be quantified in terms of each o f

these three factors. 27

I t was n o t intended t h a t the present specimens be comprehensively

(quantitatively) analyzed with regard t o weld defects and weld shape.

Indeed, the fact that experiments have continued u p t o the present time

has precluded extensive post-test evaluation.

t i o n of welds from the present program has indicated good specimen-to-

specimen uniformity w i t h regard t o reinforcement profile.

w i t h the fact t h a t each specimen included approximately 60 cm. of weld

toe, t h u s p r o v i d i n g a spectrum of local weld heights, reinforcement angles

and weld toe r ad i i , probably contributed t o the relat ively good agreement

between Ni and Nf values among specimens o f the same t e s t group.

17 which contained an undercut, as discussed ea r l i e r , was an exception t o

this.

However, qual i ta t ive inspec-

This coupled

Specimen

15



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

Previous researchers have observed a greater number of fatigue

cracks in steel specimens fatigued in an aqueous electrolyte t h a n for

ones tested i n air.19,2*-30 T h i s has been interpreted t o mean t h a t in

the former case (corrosion fatigue) the crack initiation process i s less

sensi tive t o local (microscopic) factors associated with material struc-

ture or properties t h a n in the l a t t e r (fatigue in a i r ) .

cycles associated w i t h crack propagat ion, N p , can be taken as the dif-

ference between Nf and Ni, then Table IV points o u t t h a t for the present

specimens where such da ta i s available the cycles associated with crack

growth was relatively constant for tests a t the same stress range, irres-

pective of corrosion state. Thus, differences in fatigue l i f e fo r cathodi-

cally polarized specimens as opposed t o freely corroding ones were probably

due t o a n influence of potential upon crack i n i t i a t i o n or upon growth when

the crack was relatively small. The fact t h a t in Specimen 17 a relatively

small b u t sharp undercut s i g n i f i c a n t l y compromised fatigue l i f e i s indica-

tive of a dependence of crack initiation o r init ial growth upon weld toe

geometry fo r -0.85~ tests.

suggested t h a t more cracks occurred i n these specimens than for cathodically

protected ones.

concentrations influence fatigue l i f e t o a lesser extent in the freely

corroding, a s opposed t o the cathodically protected, case.

re la t ive ly l i t t l e scatter i n Nf was apparent for freely corroding tests

a t stress range 69 N/m2 may be due t o a lack of dependence of initiation

o r early growth or b o t h upon reinforcement geometry. Such a conclusion

is projected t o apply primarily where strain range i s small, since here

the environmental component of corrosion fatigue i s most significant.

I f the number of

Examination of freely cor roding fracture faces

This i’s consistent w i t h the projection above t h a t stress

The fact t h a t

16



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

The generalized observation tha t data for high s t ra in range (low cycle)

fatigue tes ts upon uncracked specimens in a i r and sea water are essent ia l ly

the same suggests t h a t weld defects and weld shape are important i n this

si tuation, irrespective of environment.

Failure Phenomenology. Based upon the above observations certain

projections can be made regarding the fatigue fa i lure process i n the present

specimens. These are summarized by Figure 15, which i l l u s t r a t e s the various

sequential steps that are thought t o be important.

polarized case fatigue crack in i t i a t ion is thought t o have occurred a t tha t

region of the weld toe corresponding t o the most severe s t a t e of s t r e s s , as

determined by height and angle of the weld reinforcement, by weld toe rad ius

and perhaps by weld defects.24 For the freely corroding case factors asso-

ciated w i t h a synergystic interaction between electrolyte and plast ical ly

deforming metal may be important,lg and fatigue crack in i t ia t ion i s projected

t o have occurred a t the local s i t e or s i t e s where this process was most pro-

nounced.

resul t of the in i t ia t ion process was a surface crack for which the c/a

ra t io (a i s the crack depth and c the half-length along the surface) was

relatively large. Johnson e t al 3 1 have determined for nonwelded plate

specimens in bending t h a t surface cracks tend t o develop such tha t c/a

For the case of welded material, however, it i s not unreal is t ic tha t the

residual s t ress profile and a lso the s t r e s s concentrating influence of

the reinforcement would render t h i s r a t io even more extreme. The resu l t

i n this l a t t e r case i s a shallow crack which spreads la te ra l ly along the

weld toe.

be influenced by local nature of the reinforcement i n the crack t i p v ic in i ty ,

Thus , i n the cathodically

In e i ther case (freely corroding o r cathodically polarized) the

4.

A t any given time i n t h i s propagation process growth rate should

17



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

b u t these variations should average o u t such t h a t overall the number of

cycles involved i n crack extension i s re la t ively invariant for specimens

of identical tes t conditions and overall geometry. That t h i s was probably

the case was mentioned above in conjunction w

As discussed e a r l i e r i t i s t h o u g h t t ha t

gated t o the specimen edge i t then grew r e l a t

position. I n this configuration the s t a t e of

duced t h a t the crack often arrested.

t h the d a t a in Table IV.

once a fatigue crack propa-

vely fas t t o the half-thickness

s t ress was suff ic ient ly re-

Design Criteria. Fatigue design c r i te r ia have been established by

the AWS in terms of a ser ies o f S-N design curves, each pertaining t o a

particular type of weld member and s t a t e o f 1oad ing . l l The AWS-X and

ANS-X Modified curves have been considered t o represent appropriate design

c r i t e r i a for hot spot s t ress i n practical welded hardware, as discussed

briefly in conjunction w i t h Figure 1. Ini t ia t ion and f a i lu re data from

the present study was referenced t o these curves i n Figures 12 and 13.

More recently, the API-X and API-X' curves have been developed and

are t o be included as part o f API-RP Z A Y 1 1 t h Ed., 1980. These l a t t e r

design curves are s l igh t ly more conservative t h a n the AWS counterparts

and, as such, accommodate some recent tubular j o i n t data which l i e t o

the unsafe side of the AWS c r i t e r i a .

are discussed in terms o f the AUS-X and AWS-X Modified curves, since these

represent the industry practice a t t h i s time.

The data from the present research

I t can be projected that cathodic polarization o f a welded s teel plate

invariably prolongs to ta l fatigue l i f e compared t o the freely corroding

s i tuat ion, as mentioned ear l ie r . Apparently, t h i s statement applies even

when cathodic polarization i s as negative as - 1 . 1 7 ~ ~ Cu-CuSO,, as indicated

18



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~

STD.API/PETRO 12-ENGL 1980 W 0732290 05'79099 4 1 5 D

by the results of Solli.22 Such a finding does contrast w i t h results of

crack growth rate determinations, however, as will be discussed subsequently.

I n view o f the above, resuits from freely corroding fatigue tests

should represent the worst case or poorest fatigue performance.

discussed i n the preceding section, fa t igue d a t a from tests of freely cor-

roding specimens a t low stress range are expected t o be relatively indepen-

dent of weld reinforcement profile; t h a t i s , of the weld toe stress concen-

tration fac tor . I n view o f this possibility Figure 16 i s a p l o t o f log

strain range-log cycles-to-failure which compares the present data w i t h

recent results from the UKOSRP e f f0r t .~~ ,32-34 All of th i s d a t a i s for

the freely corroding situation and b o t h b u t t and various f i l l e t welds are

represented.

i s comprised of the Booth and de Back e t a132 specimens, w h i c h were of a

relatively severe weld profile, whereas in the case of the longer l i f e

d a t a (references 22, 34 and the present study) the reinforcement merged

more smoothly w i t h the parent material.

Also, a s

While the d a t a does conform t o a single band, the lower range

The fac t t h a t some o f the f i l l e t weld d a t a i n Figure 16 fa l ls below

the ANS-X curve CNf

worst case (freely co r rod ing) s i t u a t i o n . A l s o significant i s the f a c t t h a t

four d a t a po in ts from the present tes ts a t t o t a l strain range 3.33 x

l i e below the ANS-X curve. T h i s suggests t h a t such a des ign analysis;

t h a t i s , one based upon the freely co r rod ing situation and where large

numbers of low amplitude stress excursions are involved, should reference

a design curve more conservative t h a n AAS-X.

l o6 cycles) renders this criterion suspect under the

On the other hand this la t te r

AWS-X Modified

gure 16 t o even

da ta (present study) does fall t o the safe side of

curve, as does an extrapolation o f the d a t a trend

the

n F

19



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD.API/PETRO 12-ENGL Lq80 0732290 0579100 Tb7

greater cycles. Consequently, the design criterion represented by this

curve would seem t o be appropriate for freely corroding steel with well

contoured b u t t and f i l l e t welds.

Figure 17 presents results of b o t h the present s tudy and t h a t of

S o l l i 2 2 for b u t t welded specimens fatigued a t a constant cathodic poten-

t ia l of -0.85v, Cu-CuSOb.

another and w i t h one exception are positioned t o the safe side o f the

AWS-X curve.

the AWS-X curve by 0.13 x T h i s l a t te r specimen con-

tained an undercut, as discussed earlier in conjunction with Figures 13

and 14.

The two sets of data merge smoothly w i t h one

The exception i s Specimen 17 ( N f = 27 x l o 6 ) which i s below

strain units.

T h u s , while the AWS-X curve i s appropriate for welds polarized t o

-0.85~ and which conform t o AWS cr i ter ia ,23 i n the presence of sharp under-

cuts, such as the one in Figure 14, a more conservative criterion i s required,

a t least for non-redundant situations.

The da ta for Specimen 17 is consistent w i t h the projection above

regarding appropriateness of the AWS-X Modified curve.

the undercut in Specimen 17 had been of sufficiently large size, fatigue

l i f e might have been reduced t o the freely cor roding value ( N 2 18-20 x

lo6 cycles) o r even t o the value corresponding t o the AWS-X Modified Curve

(9.9 x l o6 cycles). However, what defect size would be necessary t o cause

t h i s is presently no t known.

design are included i n the following section.

Of course, i f

Additional comments pertaining t o fatigue

Crack Growth Rate Approach t o Fatique Analysis and Design.

applications of fracture mechanics t o fatigue studies have indicated a

unique relationship between crack growth rate, da/dN, and the range of

Recent

20



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD.API/PETRO 12-ENGL 1980 9 0732290 0 5 7 9 3 0 2 9T3 D

the s t ress intensity parameter, A K , assuming other variables such as

material microstructure, mean s t ress intensi ty range, environment, tem-

perature and others are maintained constant. 3 5 For intermediate values

of A K t h i s relation has been determined t o be o f a power law form,

da/dN = C ( A K ) ~ , (1) where C and m are constants. For fatigue i n the low AK regime, however,

where s t ress intensity range approaches the threshold value, A K ~ ~ , below

which n o crack extension can be detected,36 Equation 1 must be modified

t o the f0 rm3~

da/dN = C ( A K - AKt,)". ( 2 )

T h u s , the number o f cycles involved with crack propagation can be calcu-

lated by integration of Equation 2 between appropriate l imits for crack

s ize , - a . Usually, the lower l imit corresponds t o the s ize of some i n i t i a l

crack-like defect, whereas the upper limit i s tha t of a c r i t i c a l crack nec-

essary t o cause b r i t t l e ( f a s t ) fracture.

appropriate for si tuations where f a i lu re o f the component i n question has

catastrophic consequences ; or , i n other words , i n appl i cations where a

most conservative design or analysis i s warranted.

Such an analysis i s particularly

In si tuations involving unwelded hardware the specification of s ize

of an i n i t i a l crack may be d i f f i cu l t . An improper choice for the crack

s ize can introduce a large e r ror i n the l i f e calculation, since crack

growth ra te i s most strongly dependent upon s t r e s s intensity range in

the low AK regime. I t has been suggested, however, tha t the fracture

mechanics approach t o fatigue i s par t icular ly appropriate for welded com-

p o n e n t ~ , ~ * such as tubular joints o f offshore s t ructures , since defects

i n the s ize range 0.1 t o 0.5 mm are invariably present from the welding

21



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD.API/PETRO 1 2 - E N G L 1980 111 073.2290 0577102 83T

process39 and only a small number of cycles may be associated w i t h i n i t i a -

t i o n . T h u s , the designation of an i n i t i a l flaw size has been projected t o

be more straightforward here.

The s t ress intensity range for a semi-elliptical surface flaw, as

might occur a t the toe of a weld (see Figure 1 4 ) , has been analyzed and

determined t o be o f the form40

where @ o i s the complete e l l i p t i c in tegra l , Ms i s a f ree surface correction,

M, a f i n i t e thickness correction, M, a p las t ic i ty correction and M, a s t ress

magnification due t o the weld reinforcement.

variations in M, and M, with c/a and plate thickness, respectively, and upon

dependence of Mk upon reinforcement geometry, as discussed ea r l i e r .

cedures outlined above have been applied t o welded specimens and good agree-

m e n t between fatigue l i f e , as calculated from Equations 1 and 3, and experi-

mentally determined S-N curves has been realized. 38

Attention has been focused upon

The pro-

A problem associated w i t h S-N curve prediction based upon fracture me-

chanics analysis becomes apparent when the influence of environment and C a t h -

odic protection upon ordering of the crack growth ra te curves i s considered.

For example, Scot t and S i lves te r ,37 ,41 have performed comprehensive fatigue

crack growth determinations upon BS 4360:

environment conditions comparable t o those experienced by offshore struc-

tures.

Grade 50D steel under s t ress and

These researchers concluded tha t for R < - 0.1 and for AK - < 18 MN*m'3/2

crack growth ra te i n sea water was equal t o or less t h a n in a i r .

result i s i n qual i t a t ive agreement w i t h data of Vosi k o v ~ k y , ~ ~ who fatigued

X-65 l ine pipe s teel in a 3.5% NaCl-distilled water solution. If one con-

This

siders though that the major po r t ion of l i f e of a welded member fatigued

22



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

i n the h i g h cycle regime occurs w i t h the crack small, such tha t s t r e s s

intensity range i s low (< 18 M N . m - 3 / 2 ) , then the fracture mechanics approach

predicts l i f e i n sea water t o be greater than or equal t o t h a t i n a i r .

i s i n the low s t ress range of the S-N curve, however, where corrosion reac-

tions have been found t o have the most compromising influence upon fatigue

l i f e . T h i s par t icular point i s apparent from Figure 1.

I t

Scott and Silvester37 further observed that i n the range 20 < A K <

40

polarization (3 0 . 0 7 ~ ) reduced crack growth re la t ive t o the freely corroding

value by a factor of 0.3 t o 0 .4 , b u t the ra te increased w i t h further polari-

zation and for C$ < - 1 . 0 0 ~ (Cu-CuS04) cracks advanced a t a ra te in excess

of t h a t w i t h no polarization. For A K < 20 MN-m'3/2 cracks propagated a t

about the same rate i n both freely cor roding and cathodically protected

specimens. I t i s , however, d i f f i c u l t t o project trends for t h i s l a t t e r

experimental condi tion , f i r s t , because data i s 1 imi ted and, second, because

crack growth ra te i s highly sensi t ive t o variations i n stress intensi ty

range.

( R < 0.1 and frequency = 0.1 Hz) small amounts of cathodic

In the case of welded members i t can be reasoned that crack growth

rates a t h i g h mean s t ress intensi ty should be considered.

R = 0.7 and 0.85 f a t i g u e crack propagation i n sea water under freely corrod-

ing conditions i s apparently greater than in a i r down t o a t l eas t lo'*

meters per cycle.41 However, i n the low A K range growth rates are not

changed significantly by cathodic polarization. A t intermediate s t ress

intensity ranges (AK 2 10-20 MN-m'3 /2 ) growth ra tes a t h i g h mean s t r e s s

intensi ty and w i t h cathodic protection are greater than freely corroding

ones.

For the cases o f

T h i s resu l t contrasts w i t h the ordering of S-N curves w i t h potential ,

23



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD-APIIPETRO 32-ENGL 3980 0732290 0579304 b02 m

as determined by the present study (Figure 13) and othersY22 where cathodic

polarization in sea water invariably enhances fatigue 1 i f e .

A possible explanation for the apparent lack of correspondence be-

tween the S-N and da/dN - A K approaches t o fatigue l i f e p red ic t ion a t low

stresses i s tha t the in i t i a t ion stage cannot be neglected, as assumed by

the fracture mechanics rationale.

zat ion apparently has a marked beneficial influence upon crack nucleation

or, a l ternately, upon i n i t i a l development o f cracks from weld defects. On

the other hand i t may be tha t the interact ive influence o f sea water and

cathodic polarization upon fatigue crack growth i s not yet adequately

characterized i n the low A K ( threshold) regime.

t h a t growth character is t ics of cracks of the size of the surface flaws pro-

jected in Figure 14 (several millimeters or l e s s ) m i g h t be different from

larger cracks, a s projected by Bardal,43,44 should be considered. A t any

rate i t would appear tha t caution should be exercised in S-N curve prediction

based upon crack growth r a t e data u n t i l the apparent contradictions discussed

above can be rationalized.

A potentially useful interface of fracture mechanics w i t h existing

If this i s the case, then cathodic polari-

The added possibi l i ty

design c r i t e r i a can be proposed, based upon a modification o f a representa-

t i o n developed by Sprowls e t a l .45 A generalized, sequential i l l u s t r a t ion

of this is projected by Figure 18. On the l e f t hand side the procedure

involves (1) characterization of the j o i n t per se and o f the weld according

t o i t s geometrical severity and the resultant hot spot s t r e s s (Figure 18a),

( 2 ) development of a fatigue design curve for this par t icular type of weld,

based upon experimental data o r theoretical analysis or both (Figure 18b)

and ( 3 ) selection of a design s t ra in range, based upon projected design l i f e

24



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

(Figure 18b).

AWS procedure.

I n essence, this technique i s identical t o the present

Consider now a strain range - crack size p l o t (Figure 18c), where

the design strain range from ( 3 ) above appears as a horizontal line.

From da/dN - AK da ta (Figure 18d), where material , fatigue and environment

variahles have been appropriately selected f o r the application in question,

the threshold value f o r fatigue crack growth i s determined.

mental relationship between A K , stress range (a l t e rna te ly , strain range)

and crack size (Equation l ) , the second curve in Figure 18c i s developed.

I f s tate o f the member or j o i n t i s such t h a t it fa l l s within the lower

le f t region or "Safe Zone" then neither fatigue crack initiation nor

growth of any existing cracks should occur within the design l i f e .

crack size designated as "significant" indicates the minimum size flaw

which should propagate due t o cyclic stressing o f magnitude equal t o the

design strain range.

t h a n the size o f crack-like defects which are present from welding, then

crack growth should n o t occur.

From the funda-

The

If this "significant crack sire" (SCS) i s greater

F i g u r e 19 develops this approach more quantitatively, where strain

range versus crack size curves are included for three values of threshold

stress intensity range. These curves were developed from the fundamental

relationship A K = a(m) ' /2 , and so the crack dimension referred t o here

i s depth.

responding t o lo7, l o 8 and lo9 cycles.

here bound thresholds w h i c h have been reported in the literature. 37y46947

The curves in Figure 19 ind ica te t h a t the "significant crack size" cor-

responding t o A K t . , = 2 MN-m'3/2 i s o f the same order as inherent weld

Also shown are the ANS-X and ANS-X Modified strain ranges cor-

The various AKth values considered

25



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

STD.API/PETRO 12-ENGL l7ôO W 0732270 17579LOb 405

defects39 (0.1 - 0.5 mm) i f the des ign l i f e i s l o 7 cycles (SCS = 0.2 mrn

fo r design t o AWS-X and 0.28 mm for AWS-X

for this same threshold and a design l i f e

corresponding t o these two des ign curves

increased to 0.62 and 2.35 mm, respective

Modified). On the other hand

of lo9 cycles, note t h a t the SCS

ANS-X and AWS-X Modified) i s

y. T h i s approach considers then

that from a practical standpoint, as crack s ize becomes large a threshold

stress i s approached below which neither crack in i t i a t ion or growth of

existing cracks occur. I f th i s i s the case, then design curves should

become horizontal i n this region.

Figure 20 i s a refinement o f Figure 19, where the expression for

s t ress intensi ty range has been modified t o include the concentrating

effect of the reinforcement.

i s presented, th i s value being typical of what has been reported for fatigue

of structural s teel a t h i g h mean s t r e s s intensity.46947 The curves i n

Figure 20 are based upon the analysis of Gurney4* for transverse b u t t welds

with the reinforcement in the form of a c i rcu lar a rc and employing Equa-

t i o n 3 w i t h c/a >> l. The reinforcement angle i s i n one case 20" and i n

the other case 45'.

i s to shif t the crack length curve t o the l e f t , thereby reducing the safe

zone area.

Data for a s ingle threshold ( 3 M N . I I I ' ~ / ~ )

The consequence of an increasingly severe geometry

Significance o f the present fatigue data w i t h regard t o this rationale

can be recognized by recall ing t h a t the lowest s t r e s s range employed i n the

present experiments (69 N/m2 or 10 ksi) corresponds t o a s t r a in range of

3 .33 x T h u s , i f one assumes t h a t threshold stress intensi ty range

for the present material was 3 MN.1n-31~ and tha t the s t r e s s intensity

calculations o f Gurney48 for a 20" b u t t weld can be a p p l i e d t o the present

26



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~ ~ ~ ~~ ~-

~ ~

STD.API/PETRO 12-ENGL 1780 0732290 0577307 3Ll

specimens, then the SCS corresponding t o the above s t ra in range (3.33 x lo"+)

i s 0.25 mm. For t h i s same type of weld and w i t h design t o the AWS-X curve

a t l o 8 cycles the SCS i s 0.33 mm. These numbers are consistent w i t h the

t e s t resul ts from Specimen 17, which contained an undercut o f depth 0.30 - 0.35 mm and which fai led a t 27 x lo6 cycles.

th i s s ize were detected f o r Specimens 6 , 7 , 10, 11 16 and tha t these speci-

mens sustained l o 8 cycles without apparent fatigue damage further supports

t h i s rationale.

l o m 4 , freely corroding) d i d f a i l in less than lo8 cycles ( N f = 17.7-20.2 x

lo6 cycles) indicates e i ther tha t A K t h for this t e s t condition i s lower

t h a n when cathodic polarization i s involved or tha t corrosion assisted fatigue

crack i n i t i a t i o n was important.

The fac t t h a t no defects of

The fac t t ha t Specimens 4 , 5 , 8 and 9 ( s t ra in range 3.33 x

Interestingly, the AWS Structural Welding Code specifies a maximum under-

Considering t h i s t o be the SCS, t h e n cut depth of 0.25 mm (0.01 i i ches).23

Figure 20 indicates tha t the corresponding s t r a in range o f a 20" transverse

b u t t weld i s 3.3 x 10'4.

above the AWS-X s t ra in range a t l o8 cycles. However, i n applying data such

as t h a t i n Figure 20 t o fatigue of actual welded systems, i t must be recog-

nized t h a t development of fatigue cracks from the weld toe may occur a t

localized s i t e s where the reinforcement geometry i s particularly severe.

This suggests t h a t a factor o f safety should be incorporated into the de-

signation of s ignif icant f law size. This could be accomplished e i ther by

a reduction i n design strain range, by a reduction i n the assumed AKth o r

by an increase i n weld defect or crack s ize tha t i s assumed t o be present.

In this regard, note that a l l specimens in the present t e s t program fai led

to the safe side of the AWS-X Modified curve.

Th s value i s approximately 0.3 x s t r a in units

A t l oa cycles this l a t t e r

27



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

- ~-

STD.API/PETRO 12-ENGL 1 7 8 0 M 0732270 0579308 258 D

criterion corresponds t o significant crack sizes of 1 . 3 and 0.8 nnn for

the 20" and 45" b u t t welds, respectively, in Figure 20. Thus , i t may be

t ha t the Modified curve, as i t presently exists, reflects an appropriate

rationale for SCS designation and for adequate fatigue design in the h i g h

cycle range.

28



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

CONCLUSIONS

1. For present experimental conditions cycles-to-crack in i t ia t ion

and cycles-to-failure of welded steel in sea water were increased

a s a consequence of cathodic polarization, the difference being

greater for s t ress range 69 N/mm2 (10 ks i ) than for 138 N / m m 2 (20 ks i ) .

No distinction w i t h regard t o fatigue resistance was apparent between

specimens polarized t o - 0 . 8 5 ~ . and those a t - 1 . 0 0 ~ . (Cu-CuS04).

For the present specimens fatigued i n sea water under freely corroding

conditions a t s t ress range 138 N/mm2 (20 ks i ) fatigue l i f e was the

same a t 3 Hz, ambient temperature as i t was f o r 0.5 Hz, 4°C.

For the present specimens fatigued a t s t r e s s range 138 N/mm (20 k s i )

fatigue l i f e was s l igh t ly greater a t a frequency of 3 Hz t h a n for

0.5 Hz.

A t relatively low s t ress range fatigue l i f e of freely corroding

specimens was not influenced significantly by weld reinforcement

geometry. W i t h cathodic polarization t o e i ther -0.85~. or - 1 . 0 0 ~ .

( C U - C U S O ~ ) we1 d reinforcement prof i 1 e and presence of undercutting

influenced fatigue performance.

Data from the present test specimens support appropriateness of the

AWS-X Modified curve as a design cr i ter ion for mitigation of fatigue

fa i lure under f reely corroding conditions in the h i g h cycle range.

Data from the present specimens indicate tha t the AWS-X curve i s an

appropriate design cr i ter ion f o r mitigation of h i g h cycle fatigue

2 .

3.

4.

5.

6.

29



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~ ~~

STD-API/PETRO 32-ENGL 3780 M 0732290 0577330 70b

fa i lure a t a potential of e i ther -0.85~. or -1 .00~. (Cu-CUSOL, )

b u t i t may not be appropriate fo r the freely corroding condition.

For a single specimen tested a t stress range 69 N/mm2 (10 ksi)

and -0 .85~. (Cu-CuSO,) and w i t h an undercut of depth 0.3 mm f a i lu re

occurred sooner than predicted by the AWS-X design curve. However,

the AWS-X Modified curve i s s t i l l appropriate for fatigue design

w i t h undercutting of this size.

7. Influence of potential (cathodic) upon e i ther fatigue crack i n i t i a -

t i o n or growth of small cracks or crack-like defects s ignif icant ly

influenced fatigue l i f e of the present specimens tested a t s t r e s s

range 69 N/mm2 (10 ks i ) . Consequently, S-N curves for these t e s t

conditions cannot be predicted based upon a f racture mechanics

analysis.

30



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

1.

2.

3.

4.

5.

6.

7.

a.

9.

10.

li.

12.

~

STD.API/PETRO L2-ENGL L9BO O732290 05791l1 8 4 2 U

BIBLIOGRAPHY

J . G. Hicks, "Material and Structural Problems in Offshore Installations" , proceedings Conference on Welding i n Offshore Constructions, 1974, p. 1.

H. Wintermack, "Materials and Welding in Offshore Constructions", 1975 Portevin Lecture, International Ins t i tu te of Welding.

P. W. Marshall , "Problems i n Long-Life Fatigue Assessment for Fixed Off- shore Structures", preprint 2638 , ASCE National Water Resources and Ocean Engineering Conference, San Diego, April, 1976.

T. R . Gurney, Fatigue of tdelded Structures , Cambridge University Press , 1968, pp. 19-30.

C . E. Jaske, J . E. S la ter , D. Broek, B. N. Leis, W . E. Anderson, J . C. Turn and T. Omar, "Corrosion Fatigue of Welded Carbon Steel for Appli- ca t ion t o Offshore Structures" , interpretative Report submit ted t o API by Batte1 l e Columbus Lab. , February 1, 1977.

E. C. Rodabaugh, "Review of Data Relevant t o the Design of Tubular Joints f o r Use in Fixed Offshore Platforms", Final Report submitted t o Welding Research Council by Battelle Columbus Lab. , July, 1978, Chapt. 3, pp. 43, 44.

J. W. Kochen, J , P. Tralmer and P. id. Marshall, "Fatigue o f Structural Steel for Offshore Platforms", paper no. 2604 presented a t Offshore Technology Conference, Houston, May, 1976.

K. J . March, T. Martin and J . McGregor, "The Effect of Random Loading and Corrosive Environment on the Fatigue Strength o f Fillet-Welded Lap Joints" , National Engineering Laboratory ( N E L ) Report No. 587, Feb., 1975.

F. E. Havens and D . M. Bench, "Fatigue Strength of Quenched and Tempered Carbon Steel Plates and Welded Joints i n Sea Water", paper no, 1046 presented a t Offshore Technology Conference, Houston, May, 1969.

J . C. Walter, E. Olbjorn, O. Alfstad and G . Eide, "Safety Against Corro- sion Fatigue Offshore", Det Norske Veritas, Pub. No. 94, April, 1976.

"Section 10. Desi-gn o f New Tubular Structures" , Structural Welding Code, ANS D1.1-79., American Welding Society, Inc. , 1978, pp. 145-173.

Vol. 1 and 2 Preprints, Select Seminar, European Offshore Steels Research, U.K. Department of Energy Offshore Steels Research Project, Nov. 27-29, 1978 , Cambridge , U. K.

31



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~~

~~

STD-APIIPETRO 12-ENGL 1780 0732270 0577112 787

13. C . E . Jaske, D . Broek, J . E. S l a t e r , D. A. Utah and C . J . Plar t in , "Corrosion Fa t igue o f Ca thod ica l ly P r o t e c t e d , Welded Carbon S tee l i n Cold Sea Water", Final Report submi t ted t o API by B a t t e l l e Col umbus Lab., Feb. 11 , 1977.

14. E . C. Rodabaugh, "Review of Data Relevant t o the Design o f Tubula:. Joints f o r Use i n f i x e d Offshore P la t fo rms" , Final Report submi t ted t. Welding Research Council by B a t t e l l e Columbus Lab., J u l y 1978, Chapt. i , pp .6 , 7.

15 . W. K. Boyd and F . W . F i n k , Corrosion o f Metals i n Marine E n v i r o w e n t s , MCIC Report 78-37 , B a t t e l l e Columbus L a b o r a t o r i e s , March, 1978, - . 4 and 13.

16. F. L. Laque, Proc. American S o c i e t y for Tes t ing i 'Sater ia ls , vo l . 51, 1951, p. 495.

17. "Recommended Practice: Control o f Corrosion on S t e e l , Fixed Offshore P l a t - forms Assoc ia ted w i t h Petroleum Product ion" , NACE Standard RP-01-76, A p r i l , 1976, p. 8.

18. W.

19. D.

20. w.

21. w.

22. o.

H . H a r t t and S. L. Wolfson, " P r o p e r t i e s o f Calcareous Deposi ts upon Cathodic S t e e l Su r faces i n Sea Water", paper no. 152 t o be p re sen ted a t CORROSION 80, Chicago, March 3-7, 1980. Manuscript submi t ted t o Corro- s i o n J o u r n a l .

J . Duquette and H. H . Uhl ig , "Effect o f Dissolved Oxygen and NaC1 on Corrosion Fa t igue of 0.18% Carbon S t e e l " , Trans. ASM, v o l . 61, 1968, pp. 449-456.

C. Hooper and W . H. H a r t t , "The Influence o f Cathodic P o l a r i z a t i o n upon Fa t igue o f Notched S t r u c t u r a l S t e e l i n Sea Water", Corrosion Journa l , vo l . 34, 1978, PP* 320-325.

H . H a r t t and W . C . Hooper, "Endurance L i m i t o f Notched, 1018 S t e e l i n Sea Water - Specimen S i z e and Frequency Effects", paper no. 214 presen- t e d a t CORROSION 78, March 6-10, 1978, Houston. Corrosion J o u r n a l .

To be publ i shed i n

S o l l i , "Corrosion Fa t igue o f Welded J o i n t i n S t r u c t u r a l S t e e l s and the Effect of Cathodic Protection", paper no. 70 presented a t Select Seminar European Offshore S t e e l s Research , Nov. 27-29, 1978, Cambri dge , U. K.

23. "Sec t ion 3. Workmanship", S t r u c t u r a l Welding Code, ANS D1.1-79, American Welding S o c i e t y , Inc. , 1978, p . 41.

24. R. P. Newman and T. R. Gurney, "Fa t igue T e s t s on P l a i n P l a t e Specimens and Transverse B u t t Welds", British Welding J o u r n a l , vo l . 6 , 1959,pp. 569-594.

25. W . W . Sanders , A. T. Derecho and W . ti. Munse, "Effect o f External Geometry on Fa t igue Behavior o f Welded J o i n t s " , Welding Research Suppl ., v o l . 44 ( 2 ) , 1965, pp. 495-555.

26. F. V. Lawrence, "Es t imat ion o f Fa t igue Crack Propagat ion Life i n B u t t Welds", Welding Research Supplement, 1975, pp. 212s-220s.

32



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~~ ~~ ~~

STD.API/PETRO 12-ENGL 1780 D 0732270 0577113 bL5 m

27. R.

28. B.

29. U.

30. M.

31. R.

32. J .

33. G.

34. s.

35. P .

36. R.

37. P.

J . Mattos arid F . V. Lawrence, "Es t imat ion of the Fat igue Crack I n i t i a - t i o n L i f e i n Welds Ilsin9 Low Cycle Fa t igue Concepts", FCP Report No. 19, Col lege o f Engineer ing, Un ive r s i ty o f I l l i n o i s , Oct. , 1975.

Westco t t , i n Corrosion Handbook, H. H. U h l i g , e d i t o r , J . Wiley and Sons, Inc . , i4ew York, 1948, p. 578.

R. Evans, The Corrosion and Oxidat ion o f Metals , Arnold, Ltd. , London, 1960, p . 709.

S. Baxa, Y . A. Chang and L. H . Burch, "Effects o f Sodium Chloride and Shot Peening on Corrosion Fa t igue of AIS1 6150 S t e e l " , Fleta1 Transac t ions A , vo l . 9A, 1978, P P * 1141-1146.

Johnson, I . B re the r ton , B. Tomkins, P.M. S c o t t and D. R . V. Silvester, "The Effects o f Sea Water Corrosion on Fa t igue Crack Propagat ion i n S t r u c t u r a l Steel ,'I Paper- no. 15 presented a t S e l e c t Seminar European Offshore S t e e l s Research, Nov. 27-29,1978, Cambridge, U . Y,.

de Back, W . Dort land and H . Wildschut , "Fa t igue Behavior o f Welded J o i n t s i n Air and Sea Water", Paper no. 9 presented a t S e l e c t Seminar European Offshore Steel Research, Nov. 27-29, 1978, Cambridge, U. K.

S. Booth, "Constant Ampl i tu e Fa t igue Tests Performed on Welded S t e e l J o i n t s i n Sea Water", Paper no. 9 presented a t S e l e c t Seminar European Offshore S t e e l Research, Nov. 27-29, 1978, Cambridge, U . K .

Berge, "Constant Amplitude Fa t igue S t r eng th o f Welds i n Sea Water Drip", Paper no. 12 presented a t S e l e c t Seminar European Offshore S tee l Research, Nov. 27-29, 1978, Cambridge, U. K.

C. P a r i s , Fa t igue - An I n t e r d i s c i p l i n a r y Approach, Proc. Tenth Sagamore Conference, Syracuse Un ive r s i ty Press, Syracuse , New York, 1964, p . 107.

J . Bucci, P . C. P a r i s , R. W . Her tzberg , R . A. Schmidt and A . F. Anderson, ASTM Spec. Tech. Pub. 513, 1972, pp. 125-140.

M. S c o t t and D. R. V. S i l v e s t e r , "The In f luence o f Mean Tensile S t r e s s on Corrosion Fa t igue Crack Growth i n S t r u c t u r a l S t e e l Immersed i n Sea Water", I n t e r i m Tech. Report UKOSRP 3/02, Department of Energy, U. K .

38. S. J . Maddox, Welding Research I n t e r n a t i o n a l , vol . 6 ( 5 ) , 1976, pp 1-34.

39. E . G. Signes , Bri t ish Welding Journa l , vo l . 1 4 ( 3 ) , 1967, pp. 108.

40. S. J.. Maddox, I n t e r n a t i o n a l Journa l o f F rac tu re , vol 11, 1973, pp. 221-243.

33



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~

STD.API/PETRO 12-ENGL 1980 0732290 0579334 5 5 1 9

41. P. M. Scott and D. R. V. S i l v e s t e r , "The In f luence of Sea Water on Fa t igue Crack Propagat ion Rates i n S t r u c t u r a l S t e e l " , I n t e r i m Tech. Report 3/03, Department o f Energy, U. K. Offshore S t e e l s Research P r o j e c t , Dec. 19 , 1977.

42. O. Vosikovsky, "Fatigue-Crack Growth i n an X-65 Line Pipe S t e e l a t Low Cycl i c Frequencies i n Aqueous E n v i ronments , C1 osed Loop , vol . 6( i) , 1976, pp. 3-12.

43. E. Barda l , "The Effect of Corrosion and Cathodic P r o t e c t i o n on Fa t igue Crack Growth i n S t r u c t u r a l S t e e l a t Low S t r e s s Ranges and Low Loading Frequencies i n Art i f ic ia l Sea Water", Paper no. 121 p resen ted a t 7 th I n t e r n a t i o n a l Congress on M e t a l l i c Corros ion , Rio de J a n e r i o , Oct. 4-11, 1978.

44. E. Bardal , J . M. Sondenfor and P . O. Ga r t l and , "Slow Corrosion Fa t igue Crack Growth i n a S t r u c t u r a l S t e e l i n A r t i f i c i a l Sea Water a t Different P o t e n t i a l s , Crack Depths and Loading Frequencies" , paper no. 16 presen- t e d a t S e l e c t Seminar European Offshore S t e e l Research, Nov. 27-29, 1978, Cambridge, U. K.

45. O. O. Sprowls , M.B. Shurnaker and J . D. Walsh, "Evalua t ion o f S t ress -Corro- s i o n Cracking S u s c e p t i b i l i t y Using F r a c t u r e Mechanics Techniques" , Fina l Report, Part 1, G. C. Marshall Space F l i g h t Center Con t rac t No. NAS 8 - 21487, May 31, 1973, pp.93-100.

vo l . 2( 5) , 1970. 46. P. C. Paris, "Tes t ing for Very Slow Growth o f Fa t igue Cracks", Closed Loop,

47. P. C. P a r i s , R. 3 . Bucci , E. T. Wessel , W. G. Clark and T . R. Mager, ASTM Spec. Tech. Pub. 513, 1972, pp. 141-176.

48. T. R. Gurney, Welding Research I n t e r n a t i o n a l , vol . 6 , 1976, p . 40.

34



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~ ~- ~~ ~~

STD*API/PETRO 12-ENGL 1980 0 7 3 2 2 9 0 0577115 4 9 8 W

TABLE I

Listing of Tests and Test Variables for the Present Program

Test Number of Stress Range, Frequency, Temperature, Corrosion Number Specimens N / m m 2 (ksi ) Hz . OC. State*

2

3

4

5

6

7

8

9

10

1

2

2

4

4

2

2

2

2

2

97 (14)

138 (20)

138 ( 20 )

69(10)

69( 10)

69 (10)

138( 20)

138 (20)

138(20)

138( 20)

3

3

3

0.5

0.5

0.5

0.5

ambient

ambient

ambient

ambient

ambient

ambient

ambient

4

4

4

f . c .

f .c .

-o, a 5 ~ .

f .c .

-0 .85~.

-1. oov.

- 0 . 8 5 ~ .

f .c.

-o. a 5 ~ .

-1.oov.

* f.c. - f reely corroding. All potent ia ls referenced t o Cu-CuSO,.



--`,,,`,`,,,``,`,``,`,``,`,,`,,-`-`,,`,,`,`,,`---

~

STD.API/PETRO 12-ENGL 1960 D 0732290 05771Lb 324 W

TABLE I I

MECHANICAL PROPERTIES OF STEEL PLATE STOCK5

Yield Strength Tensi l e S t r eng th El ongation Tranverse Charpy MN/m2 (ksi) MN/m2 (ksi) 56 i n 20.3 cm. Value

390 (56.6) 536 ( 7 7 . 7 ) 38 42 Joules a t -10°C.



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m = r ; J w m a l m I

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TABLE IV

Specimen Number N, - N i x 10-6 ( N f - Nilavg, x 10'6

I

Comparison o f Cycles fo r Crack Propagation ( N f - Ni) for Specimens Where T h i s Data is Available

4

7

8

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Figure 4 . Photograph of test specimen with bath.



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Figure 5 . Photograph of fatigue machine with specimens and related instrumentation.



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Figure 7 . Results of a finite element- stress analysis f o r the specimen employed in the present experiments



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F i m r e 10. Strain range and strain asymmetry versus number of cycles for Specimen 13.



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Figure 14. Appearance of a typical s ec t ion through the cracked portion of specimen 17. Note undercutting a t w e l d toe.



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Figure 18. Schematic representation of the proposed fatigue analysis.



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