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VII Improving the fatigue life of weldments

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Outline

n Obvious things to don Problems the weld toen Fatigue life Improvement Strategiesn Light and heavy industry weldmentsn 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

n Obvious things to do

n Problems the weld toen Fatigue life Improvement Strategiesn Light and heavy industry weldmentsn Improving the “bad” weldments

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Weld toe is a stress concentration

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Slag entrapments at toe?

Virtually eliminates fatigue crack initiation life NI

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Welding procedure A Welding procedure B

Cold-lap defects at weld toe

Cold laps virtually eliminate the fatigue crack initiation life (NI)

Such weldments may have an appreciable fatigue crack initiation life (NI)

“Nominal”“Perfect”

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Cold lap at a weld toe

Loading Direction

D

Weld Metal

Base MetalHeat Affected Zone

Weld Toe Location Without Cold-Lap Defect

Curved PathVertical Path

θ

rφ

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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, M

K CG, vertical path from

cold-lap defect

K

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

2-D FEM

Wel

d ge

omet

ry c

orre

ctio

n fa

ctor

, MK

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Effect of flank angle

1

10

0.001 0.01 0.1

θ = 30o

θ = 45o

θ = 60o

K

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

2-D FEM

D = 1.0 mm

Wel

d ge

omet

ry c

orre

ctio

n fa

ctor

, MK

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Effect of cold root radius

1

10

0.001 0.01 0.1

D = 0.00 mm, weld toe locationD = 2.00 mm, r

cl = 0.146 mm

D = 2.00 mm, rcl

= 0.025 mm

K

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

Wel

d ge

omet

ry c

orre

ctio

n fa

ctor

, MK

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Recent study on rail welds

Toe Radius, R

Weld Reinforcement

Height, H

Base Metal Thickness, Tb

Weld Collar Width, W

Flank Angle, θ

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Geometric Parameters

Fin Length, Lf

Cold Lap Length, Lcl

Fin Thickness, Tf

Flank Angle, θ

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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 90Flank Angle, θ (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)

Ela

stic

Str

ess

Con

cent

ratio

n Fa

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 Tests R=3mm and

θ=30°

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Trends in “Ideal”1.0-in plate thickness, non-load carrying cruciform weldments fatigue strength.

• R = 0

• Welding residual stresses = 50% of SYBM

• Sfab ˜ SYBM

Predicted effect of SuBM

0

10

20

30

40

50

60

0 50 100 150 200

Fatig

ue S

tren

gth

at 1

E+0

7 cy

cles

, 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 Obvious things to don Problems the weld toe

n Fatigue life Improvement Strategies

n Light and heavy industry weldmentsn Improving the “bad” weldments

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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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TWI suggestions as to weld improvement procedures

Improvement Strategies

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100

10

θ15°30°45°

15°30°45°

15°30°45°

1 10001

100Fa

tigue

Stre

ngth

(ksi

) at

106

cycl

es

Base metal UTS (ksi)

Plain plate

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 Obvious things to don Problems the weld toen Fatigue life Improvement Strategies

n Light and heavy industry weldments

n Improving the “bad” 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 1081

10

100

Toe Ground (radius = 0.1 in.)Over StressedStress RelievedWeld Profile (flank angle 20 o)As Welded

Fatigue Life, NT (cycles)

Rem

ote

Stre

ss R

ange

, ∆S

(ksi

)

t = 0.5-in. (12mm); R = 0Without 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 StressedStress Relieved

Weld Profile (flank angle 20 o)As Welded

Fatigue Life, NT (cycles)

Rem

ote

Stre

ss R

ange

, ∆

S (k

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

With Fabrication Stresses

Nominal

Ideal

Heavy industry weldments

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Outline

n Obvious things to don Problems the weld toen Fatigue life Improvement Strategiesn Light and heavy industry weldments

n Improving the “bad” weldments

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Weldment with a transverse attachment

Transverse attachment

Good weldment

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Weldments with longitudinal attachments have a low fatigue resistance because of the presence of weld terminations. Starts and stops introduce 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-PL-P

LP

C

CC

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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 attachmentwith 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

Wel

d ge

omet

ry c

orre

ctio

n fa

ctor

, M K

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

105

106

107

0.002 0.02 0.2

L.A.L.A. with stress diffusersTransverse attachment

Fatig

ue C

rack

Pro

paga

tion

Life

, N P

(cyc

les)

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 2L.A., Series 3 L.A. with stress diffusers, Series 1 L.A. with stress diffusers, Series 3

Rem

ote

Stre

ss R

ange

, ∆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 much improved; whereas, the fatigue strength of “Nominal”weldments cannot.

• Weld toe grinding or weld profile control works best for “Ideal” weldments at short lives. Beware of corrosion pitting.

• 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 are dissimilar.

• Stress relief annealing and over-stressing works best for “Ideal”weldment at long lives. Beware of compressive overloads.

• Fatigue behavior of weldments and effective life improvement methods depends upon weldment size and weld quality

• Stress-diffuser can substantially improve the fatigue life of terminations without post-weld processing.

Summary