Improving Fatigue Life
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Transcript of 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
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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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
Crack length / main plate thickness, (a/T)
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
Crack length / main plate thickness, (a/T)
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 Obvious things to do
n Problems the weld toe
n Fatigue life ImprovementStrategies
n Light and heavy industry weldments
n 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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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 Obvious things to do
n Problems the weld toe
n Fatigue life Improvement Strategiesn 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
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
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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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
Crack length / main plate thickness, (a/T)
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