Fatigue Cracking (Offshore Engineering)

8/12/2019 Fatigue Cracking (Offshore Engineering)

1/14

PLEASE SCROLL DOWN FOR ARTICLE

This article was downloaded by: [ABM Utvikling STM / SSH packages]On: 21 July 2009Access details: Access Details: [subscription number 792960683]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Ships and Offshore StructuresPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t778188387

Assessment of design criteria for fatigue cracking from weld toes subjected toproportional loadingI. Lotsberg aaDepartment for Offshore Structures, Hvik, Norway

Online Publication Date: 01 June 2009

To cite this ArticleLotsberg, I.(2009)'Assessment of design criteria for fatigue cracking from weld toes subjected to proportionalloading',Ships and Offshore Structures,4:2,175 187

To link to this Article DOI 10.1080/17445300902733998URL http://dx.doi.org/10.1080/17445300902733998

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf

This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
http://www.informaworld.com/smpp/title~content=t778188387http://dx.doi.org/10.1080/17445300902733998http://www.informaworld.com/terms-and-conditions-of-access.pdfhttp://www.informaworld.com/terms-and-conditions-of-access.pdfhttp://dx.doi.org/10.1080/17445300902733998http://www.informaworld.com/smpp/title~content=t778188387


2/14

Ships and Offshore Structures

Vol. 4, No. 2, 2009, 175187

Assessment of design criteria for fatigue cracking from weld toes subjected

to proportional loading

I. Lotsberg

Department for Offshore Structures, DNV, Veritasveien 1, 1322 Hvik, Norway

(Received 20 December 2008; final version received 9 January 2009)

For fatigue design it is necessary to provide guidelines on how to calculate fatigue damage at weld toes based on S-N datawhen the principal stress direction is different from that of the normal direction to the weld toe. Such stress conditionsare found in details in different types of plated structures. Some different fatigue criteria for these stress conditions arepresented in design standards on fatigue design. Criteria used by the International Institute of Welding (IIW), Eurocode,British Standards and in the DNV (Det Norske Veritas) standards have been assessed against some relevant fatigue test datapresented in the literature. Only proportional loading conditions have been considered here. (By proportional loading it isunderstood that the principal stress direction is kept constant during a load cycle.) An alternative equation for calculation

of an equivalent or effective stress range based on stress normal to the weld toe and shear stress at the weld toe has beenproposed. The proposed methodology can be used for nominal S-N curves, which can be used together with a hot spot stressS-N curve with stresses read out from finite element analysis. The different design criteria are presented in this paper togetherwith recommendations on analysis procedure.

Keywords: design criteria; fatigue; welded plate structures; weld toe; principal stress direction; proportional loading

1. Introduction

For fatigue design it is necessary to have guidelines on

how to calculate fatigue damage at weld toes based on S-N

data when the principal stress direction is different from

that of the normal direction to the weld toe. Details of

such stress conditions are found in different types of plated

structures, such as at connections with soft brackets and

at tubulars penetrating plates in ship structures, e.g. DNV

CN 30.7 (2005) and Lotsberg (2004). Some fatigue design

standards have advised to use the largest principal stress

range within 45 to the normal to the weld toe together

with an S-N curve derived for stress ranges normal to the

weld toe for fatigue design. Reference is made to the IIW

(1996), British Standards Institution 5400 (1980), BS 7608

(1993) and DNV CN 30.7 (2005).

The International Institute of Welding (IIW) (2007) de-

cided to change the angle for largest principal stress range

direction from 45 to 60, which is now included in the

present version of the IIW fatigue design guidelines. Thesame revision was also made in DNV-RP-C203 (2005).

During actual design cases it has been found that the new

criterion can have significant impact on the design of some

special details andit is observed that designers have difficul-

ties meeting the required fatigue life at these hot spots when

using this procedure. Therefore, it was decided to make a

further assessment of recommended design criteria based

Email: [email protected]

on a review of some relevant fatigue test data from the liter-

ature. This work is presented in more detail in the following.

2. Fatigue test results for inclined welds

from literature

A literature search has been performed in order to find fa-tigue test data where the principal stress direction relative

to the weld toe has been a varying parameter. It was found

that such fatigue tests have been performed by Kim and

Yamada (2004) using test specimens as shown in Figure 1.

They presentedthe test results at an IIW conference in 2004.

Fatigue test data from specimens shown in Figure 1(b) and

test specimens in Figure 1(a), where fatigue cracking oc-

curred at the straight part of the weld toe, are selected for

assessment. The fatigue test data used in the present assess-

ment are listed in Table 1. The following notations are used

on test specimens in this table: G for gusset specimen and N

for non-load-carrying cruciform specimen. Specimen no-tation also include angle as defined in Figure 2 and the

specimen number.

Reference is made to Figure 2 for calculation of stresses

in the test specimen. Equilibrium in the loading direction

of the test specimen gives

(//sin + cos )w/ cos = 1w (1)

ISSN: 1744-5302 print/ 1754-212X online

Copyright C 2009 Taylor & Francis

DOI: 10.1080/17445300902733998http://www.informaworld.com


3/14

176 I. Lotsberg

Figure 1. Fatigue test specimens used by Kim and Yamada (2004).

Equilibrium in the transverse direction to the specimen

gives

//cos = sin (2)

From these equations the following stresses are derived:

// = 1sin cos ,

= 1cos2

(3)

The stresses as function of unit stress for varying values of

are shown in Figure 3.

3. Design procedures

3.1. Procedure in Eurocode 3 (2005)

The procedure in Eurocode 3 (2005) is a summation of

calculated fatigue damages from normal and shear stress

ranges at the weld toe. This can be presented in the form of

a design equation as

D+D= 1.0 (4)

3.2. Procedure in IIW (2007)

Two alternative procedures are presented in the latest rec-

ommendations on fatigue design by IIW (2007).

1. Principal stress direction.

2. Quadratic interaction of allowable normal stress range

and shear stress range.

The different methodologies are presented in detail in

the following sections:

Principal stress direction in Section 2.2.3.1 of IIW

(2007):

In the case of biaxial stress state at the plate surface,

it is recommended to use the principal stress which is ap-

proximately in line with the perpendicular to the weld toe,

i.e. within 60. The other principal stress may be anal-

ysed, if necessary, using the fatigue class for parallel weldsin the nominal stress approach. Reference is also made to

Figure 4.

Quadratic interaction of allowable normal and shear

stress ranges of IIW (2007):

The effects of combination of normal and shear stresses

shall be verified by

S,d

R,d

2+

S,d

R,d

2= CV (5)

whereR,dorR,dis the design resistance stress range

for the specified number of cycles and the appropriate FAT

class for normal and shear stresses at the weld toe. S,dorS,dare the corresponding design stress ranges.CVis

a comparison value, which is given as 1.0 for proportional

loading in table 4.1 of IIW(2007).

3.3. Procedure by Kim and Yamada (2004)

Kim and Yamada (2004, 2005) proposed to use the follow-

ing expression for effective stress:

Eff= 1 cos (6)


4/14

Ships and Offshore Structures 177

Table 1. Fatigue test data (from Kim and Yamada (2004)).

Number of Stress rangeSpecimen cycles (MPa) Comments

G0-01 216,000 190G0-02 237,000 190

G0-03 1,564,000 120G0-04 3,428,000 98G30-03 603,000 190G30-04 608,000 190G45-05 1,447,000 190G45-06 735,000 204G45-07 1,278,000 190G45-08 982,000 190G45-09 2,270,000 152 Run-outN0-01 198,000 206N0-02 170,000 203N0-03 470,000 160N0-04 556,000 160N0-05 1,415,000 136N0-06 630,000 136

N0-07 990,000 136N0-08 2,788,000 113N0-09 6,764,000 113 RunoutN15-01 360,000 206N15-02 324,000 203N15-03 479,000 161N15-04 867,000 160N15-05 760,000 160N15-06 1,577,000 136N15-07 1,739,000 136N15-08 984,000 136N15-09 2,366,000 123N15-10 4,860,000 123 Run-outK-30-01 502,000 206K-30-02 389,000 203

K-30-03 1,264,000 174K-30-04 2,053,000 159K-30-05 1,620,000 159K-30-06 6,449,000 138K-30-07 10,000,000 138 Run-outK-30-08 10,000,000 123 Run-out

where the stress 1 and the angle are defined in

Figure 2.

3.4. Alternative procedure

A combined stress range (or effective stress range), takinginto account the stress normal and the shear stress along the

weld toe can be expressed in the following form:

Eff=

2 +

2// (7)

where the stress components are explained in Figure 2. The

S-N category will depend on the type of detail in relation

to the normal stress. This will result in different values

as presented in Table 2. The combined stress range should

Figure 2. Definition of symbols and stress components.

be used together with an S-N curve that is selected as if this

stress was acting normal to the weld toe.

The details tested in Figure 1 are classified as E follow-

ing DNV-RP-C203 for small thicknesses of the attachments

and F for larger thicknesses when = 0. The test results for

= 0 are presented in Figure 5 together with the E-curve.

(Reference is made to Table 3 for relation between notations

on S-N curves used in DNV-RP-C203 (2005), IIW (2007)

and Eurocode (2005)).

For presentation of mean S-N curves it is assumed that

a standard deviation in logarithmic format is 0.20. (The

design curve is defined as mean minus two standard devia-

tions assuming the test data to follow a normal distribution

in a logarithmic format.)

S-N category C2 may be used for continuous shear

stress in a full penetration weld according to Table A.8 in

DNV-RP-C203. Assuming that the shear stress is classified

as C2, the following equation for combined or effective

stress is derived from Table 2 when the effective stress is

combined with S-N curve E:

Eff=

2 + 0.64

2// (8)

The basis for this equation is also illustrated in Figure 6

where 1 = // at = 45. The stress components in

Equation (8) are to be combined with different S-N curves

as shown in Figure 6. (When1is acting normal to a weld

toe, it is classified as an E detail or FAT 80. When the detail

is subjected to shear along the weld, S-N curve C2 or FAT

100 should be applied.)


5/14

178 I. Lotsberg

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

Angle (deg)

Factor

Tau

Sigma normal

Figure 3. Stress as function of unit stress for varying value of.

4. Comparison of design procedures with fatigue

test data

4.1. Eurocode (2005)

The detail shown in Figure 1 is classified as FAT 80 fol-

lowing Eurocode 3 (2005) and IIW (2007) for = 0. This

is the same as the E-curve in DNV-RP-C203. The test re-

sults for = 0 are presented together with the E-curve in

Figure 5.

The S-N curves for stress range normal to the weld toe

and shear stress can be presented as

N = am

N// = a//m//

(9)

For stress normal to the weld the design S-N curve is FAT

80 withm = 3.0. The design S-N curve for shear stress in

Eurocode 3 and IIW is FAT 100 with m = 5.0.

Figure 4. Figure from IIW (2007) showing stress to be used for fatigue analysis.


6/14


Table 2. Values for based on S-N curves in DNV-RP-C203(2005).

Stress direction parallelwith the weld

Stress direction normalto the weld toe C C1 C2

D 0.518 0.646 0.810E 0.409 0.510 0.640F 0.322 0.402 0.504F1 0.254 0.316 0.397F3 0.201 0.250 0.314G 0.160 0.199 0.250

A scaling of the stress ranges in the fatigue test data is

performed such that the test data can be presented forn =

106 cycles. The following scaling of stress are made for

comparison with Eurocode 3 and IIW (m = 3.0 for normal

stress and 5.0 for shear stress):

106 = test

ntest106

1/3.0//106 = //test

ntest106

1/5.0 (10)

From the equation for summation of damages in Equation

(4) and (9) the following expression for shear stress resis-

tance for Eurocode 3 is derived:

R,d= a// 1

n

3.0S,d

a

1/5.0

(11)

Table 3. Relations between notations in DNV-RP-C203and IIW and Eurocode 3.

DNV-RP-C203 IIW and Eurocode 3

B1 160B2 140

C 125C1 112C2 100D 90E 80F 71F1 63F3 56G 50W1 45W2 40W3 36

This equation together with fatigue test data is shown inFigure 7. It is observed that this figure shows a rather small

interaction effect between normal and parallel stress ranges

at = 45.

4.2. IIW(2007)

The IIW quadratic interaction equation on stress reads

S,dR,d

2

+ S,dR,d

2

= 1.0 (12)

10

100

1000

Number of cycles

Stressrange(MPa)

G0

N0

E design

E mean

100,000 1,000,000 10,000,000

Number of cycles

Stressrange(MPa)

G0

N0

E design

E mean

Figure 5. Test data for principal stress normal to the weld.


7/14

180 I. Lotsberg

1

// C2 (FAT 100)

E (FAT 80)1

// C2 (FAT 100)

E (FAT 80)

Figure 6. Illustration of stresses1 = //when = 45 that are

combined with different S-N curves.

From Equations (9) and (12) the following expression for

shear stress resistance for IIW is derived:

R,d=a//

n

1/5 1 2S,d

n

a

2/3(13)

This equation together with test data is shown in Figure 8.

It is observed that the fatigue test data, also at = 45,

are in good agreement with the mean line for quadratic

interaction on stress as shown.

4.3. Present proposal

For comparison of Equation (8) with fatigue test data, the

fatigue test data are scaled with respect to stress range to

correspond to 106 cycles. The following scaling of stress

range is made for comparison with DNV-RP-C203 (inverse

negative slope of S-N curve m = 3.0):

106 = test

ntest106

1/3.0//106 = // test

ntest106

1/3.0 (14)

The test data for different principal stress range directions

are presented in Figure 9. It is observed that there is a good

correspondence between the test data and the proposed de-

sign equation for effective stress.

4.4. Kim and Yamada (2004) and the present

proposal

A comparison using different equations for effective stress

is presented in Figure 10. The effective stress from Equation

(6) (Kim and Yamada 2004) is compared with the present

proposal from Equation (8). As the present proposal fits the

test data well, it may be concluded that the procedure by

Kim and Yamada (2004) is slightly on the conservative side.

From Figure 10 it is observed that the effective stress is

reduced bya factor0.63at anangle =45. This means that

using theprincipal stress withinan angle45 to thenormal

to the weld toe becomes conservative for large angles.

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

Normal stress (MPa)

Para

llelstress(MPa)

Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01N30-01

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

Normal stress (MPa)

Para

llelstress(MPa)

Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01N30-01

Figure 7. Test data presented in format of interaction equation in Eurocode (2005).


8/14


0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

Normal stress (MPa)

Parallelstress(MP

a)

Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01

N30-01

0

20

40

60

80

100

120

140

160

0 50 100 150 200 250

Normal stress (MPa)

Parallelstress(MP

a)

Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01

N30-01

Figure 8. Test data presented in format of IIW quadratic interaction equation on stress range components.

5. Comparison of design procedures

and recommended approach

A review of fatigue test data considering principal stress

direction relative to the weld toe geometry has been per-

formed. Based on this assessment one may reconsider the

text related to Figure 4 from IIW (2007).

The IIW method with the calculation of allowable stress

ranges for stress normal to the weld and shear stress sep-

arately and using a quadratic interaction equation on these

is considered to fit test data very well. A methodology with

adding the damages from these stress components together,

which is used by Eurocode (2005), is not that good.

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

180.00

0 50 100 150 200 250

Normal stress (MPa)

Parallelstress(MPa) Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01N30-01

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

180.00

0 50 100 150 200 250

Normal stress (MPa)

Parallelstress(MPa) Design

Mean

G0-01

G30-03

G45-05

N0-01

N15-01N30-01

Figure 9. Test data for principal stress having different angles with the normal to the weld toe compared with proposed effective stressrange.


9/14

182 I. Lotsberg

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50

Angle between normal to the weld and principal stress direction

Effectivestress

Present proposal

Kim and Yamada (2004)

Figure 10. Comparison of effective stress.

An alternative design equation has been proposed that

combines the stress normal to the weld and the shear stress

at the weld toe into an effective stress range that can be

entered into a single S-N curve for calculation of number

of cycles to failure. This design approach is considered to

be efficient for use together with stresses read out from

finite element analyses.

The general expression for effective stress is derived

from Equation (7) where values are derived from Table 2.

If the hot spot stress is derived by extrapolation of stresses to

the weld toe or to the intersection line from read-out-points

t/2 and 3t/2 as explained in DNV-RP-C203, this hot spot

stress should be combined with S-N curve D. This means

that = 0.81 in Equation (7) for calculation of effective

stress range. If the hot spot stress is based on a read-out-

point at t/2, the hot spot stress should be combined with the

E-curve and = 0.64 in Equation (7).

It is realised that the present classification of details

with guidance on S-N curve is not refined enough for some

special details in DNV-RP-C203 (2005). Therefore, a more

detailed classification is proposed as shown in Figure 11

and Table 4.

Figures 11(a and b) are intended to be used for nominal

stress analyses. The selection of the E and F curves depends

on the thickness of attachment as presented in Table A.7 of

DNV-RP-C203.

Figure 11(c) is intended to be used in special cases

when using the hot spot stress methodology as presented in

DNV-RP-C203 (2005). Figure 11(c) can be used together

with the hot spot stress methodology in general.

The stress range in both the two principal directions

should be assessed with respect to fatigue. Here a design

criterion for within an angle 45 to the normal to the

weld has been assessed against fatigue test data. For a prin-

cipal stress direction 45 < 90, anS-N curve for stress

direction parallel with the weld can be used due to the ef-

fective stress reduction factor of 0.63 at = 45 as was

given in Section 4.4.

Table 4. Classification of details and selection of S-N curve.

Anglein Figure 11

Detail classified asF for stress direction

normal to the weld

Detail classified as E forstress direction normal

to the weld

S-N curve when usingthe hot spot stress

methodology

030 F E D3045 E D C24560 D C2 C26075 C2 C2 C2

7590 C2 C2 C2

A higher S-N curve may be used in special cases. See Table A-3 in DNV-RP-C203 for further information. http://webshop.dnv.com/global/category.asp?c0=2624&c1=2627.


10/14


C2C2

F FEE

DD

Principal stressdirection

Weldtoe

Section

C2C2

F FEE

DD


Weldtoe

Section(a)

(b)

(c)

C2C2

E EDD


Weldtoe

Section

C2C2

E EDD


Weldtoe

Section

C2C2

D D


Weldtoe

Section

C2C2

D D


Weldtoe

Section

Figure 11. Classification of details and selection of S-N curve: (a) Detail classified as F for stress direction normal to the weld, (b) Detailclassified as E for stress direction normal to the weld, (c) S-N curve when using the hot spot stress methodology.

Different design criteria and interaction equations are

presented in Figure 12 and Figure 13 for comparison of

design criteria at 106 and 107 cycles respectively.

6. Derivation of hot spot stress using finite element

analysis

Two alternative methods can be used for hot spot stress

derivation in the revised DNV-RP-C203 (2008). These are

described as follows:

Method A:

For modelling with shell elements without any weld

included in the model, a linear extrapolation of the stresses

to the intersection line from the read-out points at 0.5t and

1.5t from the intersection line can be performed to derive

hot spot stress.

For modelling with three-dimensional elements with

the weld included in the model, a linear extrapolation of the

stresses to the weld toe from the read-out points at 0.5t and

1.5t from the weld toe can be performed to derive hot spot

stress.


11/14

184 I. Lotsberg

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

Normal stress (MPa)

Parallelstress(MPa)

Present proposal

IIW (2007)

Eurocode (2005)

Figure 12. Comparison of design equations at 106 cycles.

Thenotations for stress components are shown in Figure

14 and Figure 15.

The effective hot spot stress to be used together with

the hot spot S-N curve D (FAT 90) is derived as

Eff= max

2 + 0.81

2//

1 |2|

(15)

where

= 0.90 if the detail is classified as C2 with stress

parallel to the weld at the hot spot (ref. Table A-3 in DNV-

RP-C203 (2008)).

= 0.80 if the detail is classified as C1 with stress

parallel to the weld at the hot spot (ref. Table A-3).

= 0.72 if the detail is classified as C with stress

parallel to the weld at the hot spot (ref. Table A-3).

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60

Normal stress (MPa)

Parallelstress(MPa)

Present proposal

IIW (2007)

Eurocode (2005)

Figure 13. Comparison of design equations at 107 cycles.


12/14



Weld

toe

Section

Fatigue crack

//

//

Principal stress

direction

Weld

toe

Section

Fatigue crack

//

//

//

//

Figure 14. Fatigue cracking along weld toe.

The principal stresses are calculated as

1 = +//

2+

1

2

//

2+ 4 2//

2 = +//

2

1

2

//

2+ 4 2//

(16)

The first equation for effective stress (Equation (15)) is

made to account for the situation with fatigue cracking

along a weld toe as shown in Figure 14 and the second and

third equations are made to account for fatigue cracking

when the principal stress direction is more parallel with

the weld toe as shown in Figure 15.

Principal stressdirection Weld

toe

Section

Fatigue crack

//

//

Principal stressdirection Weld

toe

Section

Fatigue crack

//

//

//

//

Figure 15. Fatigue cracking when principal stress direction is more parallel with weld toe.


13/14

186 I. Lotsberg

Method B: For modelling with shell elements without

any weld included in the model the hot spot stress is taken

as the stress at the read-out point 0.5t away from the inter-

section line.

For modelling with three-dimensional elements with the

weld included in the model the hot spot stress is taken as

the stress at the read-out point 0.5t away from the weld toe.The effective hot spot stress to be used together with

the hot spot S-N curve D (FAT 90) is derived as

Eff= max

1.12

2

+ 0.81 2

//

1.12 1

1.12 |2|

(17)

where,1and2are explained under Method A.

The first equation for effective stress (Equation (17))

is made to account for the situation with fatigue cracking

along a weld toe as shown in Figure 14 and the second andthird equations are made to account for fatigue cracking

when the principal stress direction is more parallel with the

weld toe as shown in Figure 15.

7. Conclusions

The purpose of the present assessment has been to arrive

at guidelines on how to calculate fatigue damage at weld

toes based on S-N data when the principal stress direc-

tion is different from that of the normal to the weld toe.

Some different fatigue criteria have been assessed together

with fatigue test data from the literature. Only proportionalloading has been considered here.

The method used by Eurocode (2005) is to calculate the

fatigue damage due to stress range normal to the weld toe

and the damage due to the shear stress at the weld toe and

then adding the damages together. This sum should be less

than 1.0. It is observed that this method shows somewhat

low interaction effect between normal stress andshear stress

when compared with the test data.

IIW (2007) presents two methods for proportional load-

ing. The first one is to calculate the principal stress at the

weld toe on a nominal basis. If the angle between the prin-

cipal stress and the normal to the weld toe is less than 60,

this principal stress is used together with S-N curves for a

detail with stress acting normal to the weld toe.

The second method in IIW (2007) is to calculate allow-

able stress ranges for stress normal to the weld and shear

stress separately and use a quadratic interaction equation

on these.

From the present assessment it is found that the first

method is considered to be conservative and it should be

explained that this approach is conservative for large angles

and that the document includes more accurate alternatives

that can be recommended to be used. The second method is

found to be in good agreement with fatigue test data and is

the preferred methodology based on comparison with test

data.

An alternative equation for calculation of an effective

stress range based on stress normal to the weld toe and shear

stress has been proposed. The equation for effective stress

range reads

Eff=

2 +

2// (18)

where

= stress normal to the weld,

// = stress parallel with the weld,

= factors from Table 5.

The -factor is derived from the S-N curve constants

in the design standard such that the calculated fatigue life

using this equation for effective stress equals that using

nominal S-N curves for stress parallel with the weld toe.

Equation (18) is considered to be efficient for calcula-

tion of fatigue life when used together with the hot spot

stress concept (or structural stress concept) with stresses

read out from finite element analysis. This methodology

can also be used for nominal stress S-N curves.

The new alternative design approaches are included in

a revision of DNV-RP-C203 that was issued in April 2008.

Table 5. Recommended -factor for design.

-factor

S-N curve for stressnormal to the weld

S-Nclassification

C2 (FAT100) for pureshear stress

S-Nclassification

C1 (FAT112) for pureshear stress

D (FAT 90)When used as nominal S-N curve

or hot spot S-N curve byextrapolation of stress to the

weld toe or the intersectionline from read-out points 3t/2andt/2 (t= plate thickness)

0.81 0.64

E (FAT 80)When used as nominal S-N curve

or hot spot S-N curve by stressfrom read-out pointst/2 (t=plate thickness) from the weldtoe or the intersection line

0.64 0.50

F (FAT 71) 0.50 0.40F1 (FAT 63) 0.40 0.31F3 (FAT 56) 0.31 0.25G (FAT 50) 0.25 0.20


14/14


References

British Standards Institution (1980). BS 5400:steel, concrete andcomposite bridges. Code of practice for fatigue. London: BSI.Part 10.

British Standards Institution. (1993). BS 7608: Code of practicefor fatigue design and assessment of steel structures. London:BSI.

DNV CN 30.7. 2005. Fatigue assessment of ship structures. Oslo(Norway): DNV.

DNV-RP-C203. 2005. Fatigue design of offshore steel structures.Oslo (Norway): DNV.

DNV-RP-C203. 2008. Fatigue design of offshore steel structures.Oslo (Norway): DNV. Available from: http://webshop.dnv.com/global/category.asp?c0=2624&c1=2627

Eurocode 3. (2005). EN-1993-1- 9. Design of steel structures,part19: fatigue. ICS 91.010.30

International Institute of Welding. (1996). Fatigue design ofwelded joints and component. Recommendations of IIW jointworking group XIII-XIV. Edited by A. Hobbacher. AbingtonPublishing.

International Institute of Welding. (2007). Recommendations forfatigue design of welded joints and components. De GaulleCedex (France): IIW. (Document XIII-2151r1/XV-1254r1-

07).Kim IT, Yamada K. 2004. Fatigue behaviour of fillet welded jointsinclined to a uniaxial load. Document No. XIII-2021-04. TheInternational Institute of Welding.

Kim IT, Yamada K. 2005. Fatigue life evaluation of welded jointsunder combined normal and shear stress cycles. Int J Fatigue27(6):695701. (Pergamon-Elsevier Science Ltd. SCI).

Lotsberg I. 2004. Fatigue design of welded pipe penetrations inplated str uctures. Mar Struct. 17(1):2951.

Documents

Fatigue Cracking (Offshore Engineering)