Improving Fatigue Life

7/29/2019 Improving Fatigue Life

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VII Improving the fatigue

life of weldments


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Outline

n Obvious things to do

n Problems the weld toe

n Fatigue life Improvement Strategies

n Light and heavy industry weldments

n Improving the bad weldments


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Crude! (bad)


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Better


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Bad - planar weld discontinuities


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Outline







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Weld toe is a stress concentration


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Slag entrapments at toe?

Virtually

eliminates

fatiguecrack

initiationlife NI


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Welding procedure A Welding procedure B

Cold-lap defects at weld toe

Cold laps virtually eliminate thefatigue crack initiation life (NI)

Such weldments may have

an appreciable fatigue crackinitiation life (NI)

NominalPerfect


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Cold lap at a weld toe

LoadingDirection

D

Weld Metal

Base MetalHeat Affected Zone

Weld Toe LocationWithout Cold-Lap Defect

Curved PathVertical Path

r


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Effect of cold lap depth

1

10

0.001 0.01 0.1

D = 0.0 mm, MK CG

, curved path from

weld toe locationD = 1.0 mm, M

K CG, curved path from

cold-lap defectD = 1.0 mm, MK CG

, vertical path from

cold-lap defect

K

Crack length / main plate thickness, (a/T)

2-D FEM

Weld

geometrycorrectionfactor,MK


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Effect of flank angle

1

10

0.001 0.01 0.1

= 30o

= 45o

= 60o

K


2-D FEM

D = 1.0 mm

Weldgeometrycorrectionfactor,MK


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Effect of cold root radius

1

10

0.001 0.01 0.1

D = 0.00 mm, weld toe location

D = 2.00 mm, rcl

= 0.146 mm

D = 2.00 mm, rcl

= 0.025 mm

K


Weldgeometrycorrectionfactor,MK


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Recent study on rail welds

Toe Radius, R

Weld

Reinforcement

Height, H

Base Metal

Thickness, TbWeld Collar

Width, W

Flank

Angle,


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Geometric Parameters

Fin Length, Lf

Cold Lap

Length, Lcl

Fin Thickness,

Tf

Flank

Angle,


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Weld with a Fin and a Cold Lap


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Deformed Shape


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Nominal Weld Geometry

1

1.5

2

2.5

3

3.5

4

4.5

0 10 20 30 40 50 60 70 80 90

Flank Angle, (Deg.)

R=.5mm

R=1mm

R=3mm

R=6mm


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1

2

3

4

5

6

7

0 2 4 6 8 10 12 14

Fin Length, Lf (mm)

Elastic

StressConcentrationFa

ctor,

Kt(-)

Fin Thickness = .5mm

Fin Thickness=1mm

Fin Thickness=2mm

Fin length


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Fins and Cold Laps

1

2

3

4

5

6

7

-4 -2 0 2 4 6 8

Cold Lap Length, Lcl (mm)

Cold Lap; NO Fin

Cold Lap with Fin 0.5 mm Thick

Cold Lap with Fin 1 mm Thick

Cold Lap with Fin 2 mm Thick

Defect Free

For All TestsR=3mm and

=30


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Trends in Ideal1.0-in platethickness, non-loadcarrying cruciform

weldments fatiguestrength.

R = 0

Welding residualstresses = 50% of

SYBM Sfab SYBM

Predicted effect of SuBM

0

10

20

30

40

50

60

0 50 100 150 200

FatigueStrengthat1E+07cycle

s,Sa(ksi)

Base Metal Ultimate Strength, SuBM

(ksi)

As Welded, Sfab

= 0

As Welded, Sfab

= SyBM

Over Stressed

Hot Rolled Steel Q & T Steel

Stress Relieved


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Outline



n Fatigue life ImprovementStrategies




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Good - grind off reinforcement


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Good - burr grind weld toe


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Very good - full face grinding


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Shot peened weld toe


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Remelted weld toe (laser)


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TWIsuggestions

as to weldimprovement

procedures

Improvement Strategies


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100

10

153045

153045

1530

45

1 10001

100

Fatigue

Strength

(ksi)

at106

cycles

Base metal UTS (ksi)

Plainplate

As Welded

Stress Relieved

Over Stressed

Improvement strategies

Shot Peen

Residual stress

Geometry

Base metal strength


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Example

IP model predictions

and experimental

data.


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ASTM A 36 butt weldment


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ASTM A 514 butt weldment


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Outline



n Fatigue life Improvement Strategiesn Light and heavy industry

weldments



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Light industry weldments are presumed to be

fabricated from 1/2 or smaller plate and not

to have large fabrication stresses.

Heavy industry weldments are presumed to

be fabricated from larger than 1 thick plates

and to possess large fabrication stresses.

Light, heavy industry weldments


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104

105

106

107

108

1

10

100

Toe Ground (radius = 0.1 in.)Over StressedStress Relieved

Weld Profile (flank angle 20 o)

As Welded

Fatigue Life, NT

(cycles)

RemoteStressRange,

S(ksi)

t = 0.5-in. (12mm); R = 0

Without Fabrication Stresses

Nominal

Ideal

Light industry weldments


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104

105

106

107

108

1

10

100

Toe Ground (radius = 0.1 in.)Over Stressed

Stress Relieved

Weld Profile (flank angle 20 o)As Welded

Fatigue Life, NT

(cycles)

RemoteStressRange,

S(ksi)

t = 2.0-in. (50 mm); R = 0

With Fabrication Stresses

Nominal

Ideal

Heavy industry weldments


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Outline







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Weldment with a transverse attachment

Transverse attachme

Good weldment


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Weldments with longitudinal attachments have a low fatigue resistancebecause of the presence of weld terminations. Starts and stopsintroduce weld discontinuities. Residual stresses very high. 3-D stress

concentrations effects

C

Longitudinal attachment

Bad weldment


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Examples of terminations

C

C

a

b

C

L-P

L-P

L

P

C

C

C


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Longitudinal attachment

Longitudinal attachment with stress diffusers

Stress diffuser

Placement of stress diffuser


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3-D FEM modeling


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Effectiveness of a stress diffuser

Longitudinal attachmentLongitudinal attachment

with stress diffuser


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Effect on MK and NP

1

2

3

4

0.001 0.01 0.1

L.A.

L.A. with stress diffusers

Transverse attachment

Weldgeometrycorrectionfactor,M

K


105

106

107

0.002 0.02 0.2

L.A.

L.A. with stress diffusersTransverse attachment

FatigueCrackPropagationLife

,N

P(cycles)

Initial crack length / main plate thickness, (ao

/T)


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Fatigue test results

40

60

80

100

300

500

105 106 107

L.A., Series 1

L.A., Series 2

L.A., Series 3

L.A. with stress diffusers, Series 1

L.A. with stress diffusers, Series 3

R

emoteStressRange,S(MPa)

Fatigue Life, N (cycles)


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40

60

80

100

300

500

105 106 107

Transverse attachments, databaseLongitudinal attachments, databaseLA specimens, Procedures 1 and 2LA specimens, Procedure 3SD specimens, Procedure 1SD specimens, Procedure 3

Fatigue Life, N (cycles)

Fatigue test results


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Summary

The fatigue strength of Ideal weldments can be muchimproved; whereas, the fatigue strength of Nominalweldments cannot.

Weld toe grinding or weld profile control works best forIdeal weldments at short lives. Beware of corrosionpitting.

Smaller Ideal weldments are more susceptible to

improvement than larger weldments.

Fabrication stresses are critically important.


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The behaviors of light and heavy industry weldments aredissimilar.

Stress relief annealing and over-stressing works best for Idealweldment at long lives. Beware of compressive overloads.

Fatigue behavior of weldments and effective life improvementmethods depends upon weldment size and weld quality

Stress-diffuser can substantially improve the fatigue life ofterminations without post-weld processing.

Summary

Improving Fatigue Life

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