4102- Chap 6A - Welded Structures.pdf

8/10/2019 4102- Chap 6A - Welded Structures.pdf

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Strength & Fracture Analysis

Chapter 6A

Welded Structures

Professor R. BellDepartment of Mechanical & Aerospace Engineering

Carleton University

2013

Chapter 6A - Welded Structures 1


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Department of Mechanical &

Aerospace Engineering

Components

Residual Stresses

Fatigue Performance Affected by:

Imperfections

Lack of Fusion

Solidification Cracks

Slag Inclusions

Stress Concentrations

Weld Imperfections



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Design Codes - Offshore Structures

A.W.S. American Welding Society

ea t a ety xecut ve

A.B.S. American Bureau of Shi in

C.S.A. Canadian Standards Assoc.



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CODES -Flaw Assessment

ASME Pressure Vessel Code- Section XI -Inspection of Nuclear Components

BS-PD6493 Guidance on Methods for

Assessing the Acceptability of Flaws in

Welded Structures

BS 7910 Guidance on Methods for Assessingthe Acceptabil ity of Flaws in Fusion Welded

Structures



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CODES -Flaw Assessment

ASME Pressure Vessel Code- Section XI -Inspection of Nuclear Components

BS-PD6493 Guidance on Methods for

Assessing the Acceptability of Flaws in

Welded Structures

BS 7910 Guidance on Methods for Assessingthe Acceptabil ity of Flaws in Fusion Welded

Structures



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Steel Fixed Jacket



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-



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-u



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u u

Semi-Submersible



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o e ruc ure

Semi-Submersible



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s or on o e e a es



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e mper ec ons



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ana an o n es s



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321

4

221

4

a e o n s u u ar o n s

Fatigue

tBtB Strength

Factor on Li e 32

3

4

Factor on Li e 22

3

4

tt

Chapter 8 - Welded Structures 17

B


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o spo ress



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o spo ress e n on



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- u v u u



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Thickness Effect

Attachment Length



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Joint T

(mm)

t

(mm)

l

(mm)

L

(mm)

A

(mm)

A 16 16 8 - 12 300 406

-

C 52 52 26 - 32 442 406

D 78 78 37 - 42 584 437

-

F 52 26 14 -18 442 406



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T-Plate ResultsDepartment of Mechanical &




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Fatigue Life Prediction of Welded Joints

a

dN C K

m Paris law

a

e ka

rac ape eve opmen

k x SP 2 09 10 6 1 95

..

S S kP N t



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Life Prediction



rac ape eve opmen o e s



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Np - k cycles from ai = 0.5 mm to aT/T = 0.5 ( = 150 MPa)

PlateThickness

LEFMPredictions

Statistical Analysisof Experimental Data

SC1 SC2 MC1 MC2 CANMET UW Combined

16

26

1880

1400

1080

890

1680

1170

1397

768

894

588

1732

715

1388

676

78103

900750

435315

470365

217170

257130*

408248*

318195*

* -

SC1 natural growth of single crack (ai/2c = 0.5)SC2 single crack with fixed aspect ratio (a/2c = 0.1)

MC1 single crack with forcing function for a/2c to account for coalescence

Chapter 8 - Welded Structures 28


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f h l


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D t t f M h i l &


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Offshore S-N Curves Design



o es or e e o n s

The S-N Curves are obtained b ex erimental tests

Welded joints are divided into classes (ref Gurney)

Tubular joints are assumed to be T class

The Design curve is defined as the mean minus two

. . .

of survival)


D t t f M h i l &


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- -






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- -



The S-N curve which corresponds to a 97.6% probability of survival is:

loglogloglog2loglog mamsaN

Where:

N = predicted number of cycles to failure

log a = intersection of the Log N axis by the S-N curveog s = s an ar ev a on o og

m = negative inverse slope of S-N curve

saa log2loglog




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- -



logloglog maN

Details of Basic S-N Curve - Air

N 107 N 107

Class log a log s log a m log a m

B 15.3697 0.1821 15.01 4.0 17.01 5.0

C 14.0342 0.2041 13.63 3.5 16.47 5.0

D 12.6007 0.2095 12.18 3.0 15.63 5.0

. . . . . .

F 12.2370 0.2183 11.80 3.0 15.00 5.0

F2 12.0900 0.2279 11.63 3.0 14.72 5.0

G 11.7525 0.1793 11.39 3.0 14.32 5.0W 11.5662 0.1846 11.20 3.0 14.00 5.0

T 12.6606 0.2484 12.16 3.0 15.62 5.0 Tubular




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S-N Curves for Welded Joints in Seawater






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S-N Curves for Welded Joints in Seawater



Details of Basic S-N Curve

Seawater and Cathodic Protection

o

(MPa)

B 15.3697 0.1821 15.01 4.0 48

. . . .

D 12.6007 0.2095 12.18 3.0 20

E 15.5169 0.2509 12.02 3.0 18F 12.2370 0.2183 11.80 3.0 15

F2 12.0900 0.2279 11.63 3.0 13

G 11.7525 0.1793 11.39 3.0 11

W 11.5662 0.1846 11.20 3.0 10

T 12.6606 0.2484 12.16 3.0 19

8

Tubular


0 -



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-

p f





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-

p f





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-

lolololo mtm

aN

S-N Curves for Other Joints relate to a wall thickness of 22 mm

loglogloglog mtm

aN




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S-N Curve CSA CodeAerospace Engineering



A i i


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Weld Im rovement - Toe Grindin


European

o e



A E i i


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Weld Im rovement ResultsAerospace Engineering





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Weld Im rovement - a vs NAerospace Engineering





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Life Prediction Multiple Crack Model


a vs N





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t/Tmm/mm

StressRange

ExptNp

kCycles

MC2Np

kCycles

SC1Np

kCycles

MSC1Np

kCycles

52/52 300 71 78 163 84

52/52 250 169 162 280 186

52/52 200 349 400 544 -

19/19 300 270 146 304 175

31/31 250 137 223 340 184

31/31 200 506 582 663 360





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Multiple Crack Model - a vs NAerospace Engineering





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TAPS Trans Alaska Pipeline Service





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p g g

Failure study published in 1990

ee accoun e or o e ee

These vessels accounted for 59% of hulls fractures

members

Conclusion that these TAPS tankers suffered a

disproportionately higher number of structuralfailures when compared to vessels in other trades


TAPS T kDepartment of Mechanical &



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TAPS Tankerp g g

170,000 ton





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TAPS Tanker

82 000 ton





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The TAPS study concluded that the hull cracks

Poor design of structural details, resulting in hard spots andextreme stress raisers

were genera y a r u e o :

Poor weld workmanship including fabrication and fit-up during theconstruction of the vessel resulting in stress raisers in butt and

seam welds Undercuts, Lack of penetration, Wrong amperage, Misalignment

Quality control and inspection

Detectable crack size - 3 in (visual) 2 in (ultrasonic)

Exposure to the harsh environment in the Gulf of Alaska



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T W b F




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Transverse Web Frame





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The TAPS study concluded that the hull cracks

Use of high tensile steel (HTS) either completelyconstructed from HTS or in combination with mild steel

were genera y a r u e o :

No account taken of fatigue behaviour of HTS

Fatigue strength of HTS equal to that of mild steel

Advanced analytical techniques optimized structures

The reduction of scantl ings based on the use ofpro ec ve coa ngs n e an s

Lack of maintenance of corrosion control systems





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Allowable stress

increases as HTS

yield strength

Fati ue stren th ofHTS weldments is

about the same as





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-

requen s ruc ura nspec ons

Critical area inspection plan

Redesi n of critical details

Stress analysis and repair of cracked details

Repair of less severe cracks Grinding of poor weld contours

Hammer peening

Dril led holes as crack arresters

Voyage planning severe storms are avoided

Improved ballasting procedures





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TAP Tanker tructural Failures

References:

J.D. Snipes, Trans-Alaska Pipeline Service (TAPS) TankerStructural Failure Study, June 1990, Office of Marine Safety,

,Guard, Washington, D.C. 20593-001

Trans-Alaska Pi eline Service TAPS Tanker Structural Failure

Study Follow-Up Report, May 1991, Office of Marine Safety,Security and Environmental Protection, United States CoastGuard, Washington, D.C. 20593-001





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References:

T. Gurney, Fatigue of welded Structures, Cambridge UniversityPress, 2nd Ed. 1979.

I. Lotsberg and H. Andersson, Section, Fatigue Design Handbook(Ed. A. Almar-Naess), Tapir Publishers 1985.

AWS d1.1 (1983), Structural Welding Code, American WeldingSociety


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