6.1 Problems S02fcp.mechse.illinois.edu/files/2014/07/6.1-Problems-S02.pdfJul 06, 2014 · 37...
Transcript of 6.1 Problems S02fcp.mechse.illinois.edu/files/2014/07/6.1-Problems-S02.pdfJul 06, 2014 · 37...
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6.1 Problems with weldments
before after
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Outline
nn Applied stresses not well knownApplied stresses not well known
nn Complex geometriesComplex geometries
nn Sources of inherent scatterSources of inherent scatter
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The stresses which a structure may experience in service are not precisely known.
Applied stresses
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Loading Conditions
Fatigue resistance depends upon loading condition
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Service stresses generally unknown
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Measurement of service history
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T
Strain Gage Pair
= +
Total Axial BendingT
Applied stresses
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Region of Rapidly Rising Stress
Region of Linear Stress
Weld Toe
"Nominal" Component
"Geometric" Component
"Local" Component
Strain Gauge
Stress Distribution
Strain gage location?
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σ/S
T S
x
rσ
x/T
1
2
3
0.1 0.2 0.3 0.4
r = 0.4 mm
r = 3 mm
Pure axial loading
Pure bending loading
Stress concentrating effects of weld toe persist for as much as x/t ˜ 0.3.
Less for bending and less for more generous weld toe radii.
Weld toe stress fields
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104 105 106 107 1081
10
100
Fatigue Life, N (cycles)
∆
Ideal
Nominal
2x
10x 10x
Variability in applied stresses
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Outline
nn Applied stresses not well known Applied stresses not well known
nn Complex geometriesComplex geometries
nn Sources of inherent scatterSources of inherent scatter
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Geometries
There is an endless numberof weldmentgeometries.
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Outline
nn Applied stresses not well known Applied stresses not well known nn Complex geometriesComplex geometriesnn Sources of inherent scatterSources of inherent scatternn Weld qualityWeld qualitynn Mean, fabrication and residual stressesMean, fabrication and residual stressesnn Stress concentrationsStress concentrationsnn Weldment Weldment sizesizenn Material propertiesMaterial properties
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10 710 610 510 410 310 0
10 1
10 2
10 3
failuresrun outs
Fatigue life, N (cycles)
S = 159.20 * N^-0.16648, r^2 = 0.173, s = 0.1718
logN = 6.8974 - 1.03751*logS, r = -0.2387, s = 0.4293
99% survival with 50% confidence
99% survival with 95% confidence
Scatter in weldment fatigue data
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Statistical variation
The geometry of a weldment may vary with location.
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"Nominal" Weldment
Undercut, Slag Entrapment and/or other Discontinuities
Fatigue Cracks
Base Metal
"Ideal" Weldment
Weld Metal
HAZBase Metal
Weld Metal
HAZ
0.1 in
Weld quality
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104 105 106 107 1081
10
100
Ideal: Sr = 36, S
fab = 36
Nominal: Sfab
= 36
Fatigue Life, N (cycles)
∆
Effect of weld quality
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Mulitple failure scenarios
While the weldment may be simple, many different failure scenarios may exist.
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²M²S
180 - θ
r
T
Sfab²Sa
²Sb
Remote Weld Done Last
+ +
Restraint
²M²S
σr
Mean, fabrication, and residual stresses
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Thermal contractions after welding generate yield point residual stresses
Welding Residual Stresses
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Subsequent welding may develop fabrication stresses which are superposed on the welding residual stresses.
Fabrication (residual) stresses
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104 105 106 107 1081
10
100
Ideal: Sr = 0, S
fab = 0
Ideal: Sr = 36, S
fab = 0
Ideal: Sr = 36, S
fab = 36
Nominal: Sfab
= 0Nominal: S
fab = 36
Fatigue Life, N (cycles)
∆Mean, fabrication, and residual stresses
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Mean, fabrication, and residual stresses
104 105 106 107 108
1
10
100
Ideal: T = 0.5 in., Sfab
= 0Ideal: T = 2.0 in., S
fab = 36
Nominal: T = 0.5 in., Sfab
= 0Nominal: T = 2.0 in., S
fab = 36
Fatigue Life, N (cycles)
Rem
ote
Stre
ss R
ange
, ∆S
(ksi
)
Light Industry, High Quality Welding
Heavy Industry, High Quality Welding
Light Industry, Low Quality Welding
Heavy Industry, Low Quality Welding
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²S²S
180 - θ
r
t +
Applied stress
Induced Bending Stress
L
α
SB =32
α SALt
tanh ββ
β =Lt
3 SA4E
Mean, fabrication, and residual stresses
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Weldment stress concentrations
Component with a stress-concentrating notch
Applied Stress
Notch
Unnotched Component
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Weld toe is a stress concentration
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Fatigue failure starting at weld toe
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The mavericks
Tight fit-up: K- = 3.6
Normal fit-up: 3.6 < Kt < 6.4
Loose fit-up? Kt = 6.4
Example: T-joint with a variable fit-up
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180 -θr
t
? M
? S
? M? S
180 - θr
t
? M
? S? M
? S
Longer Fatigue Life
Shorter Fatigue life
Region of high stress Same initial
size, crack grows to 2x initial length
Effect of weldment size
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1
10
100
0.1 1 10
Ideal, Sfab=36
Ideal, Sfab=0
Nominal, Sfab=36
Nominal, Sfab=0
Thickness, T (in.)
-1/3-1/2
-1/7
Ideal weldments are predicted to be more affected by the size effect than Nominal weldments.
Predicted size effect
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Base Metal (BM)Weld Metal (WM)
Heat Affected Zone (HAZ)
Fatigue cracks begin in WM or HAZ not in BM!
Material properties
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Correlation between HAZ and BM hardness
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Correlation between Syand hardness
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Correlation between hardness and strain-controlled fatigue properties
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Variation in fatigue crack growth behavior of steels
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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
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
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Summary
n The variables influencing weldment fatigue life can be thought of as being only two: n the magnitude of the notch root stresses. n the properties of the notch root material.
n In this sense, the applied stresses, the degree of bending, the welding residual stresses, the fabrication stresses, the weldment geometry, the notch root weld defects all influence the magnitude of the notch root stresses.
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Summary
n The fatigue behavior of a weldment is controlled by the local (notch root, hot-spot) stress strain history.
n For structural steel weldments: material properties are of minor importance except as they determine and limit the value of the residual stresses.