Fatigue of Welds
Transcript of Fatigue of Welds
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Fatigue Analysis
This one is easy
This one is difficult
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Fatigue Analysis
Material Data
Component Geometry
Service Loading
Analysis Fatigue Life Estimate
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Nominal Stress
Nominal stress approaches are based on extensive tests of welded joints and connections. Weld joints are classified by type , loading and shape. For example, a transversely loaded butt weld. It is assumed and confirmed by experiments that welds of a similar shape have the same general fatigue behavior so that a single design SN curve can be employed for any weld class. The designer need only determine the nominal stress and select a weld class. There is no need to directly consider the stress concentration effects of the weld.
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Structural Stress
Structural stress approaches are often referred to as "hot-spot methods". The structural stress includes the macroscopic stress concentrating effects of the weld detail but not the local peak stress caused by the notch at the weld toe. There are various methods used to determine the structural stress. They involve extrapolating the computed or measured stresses from two points near the weld to a structural stress at the weld toe. This method works in situations where there is no clear definition of the nominal stress.
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Local Stress Strain
Local stress or strain approaches include both the macroscopic stress concentration due to the weld shape and the local stress concentration at the weld toe. To apply traditional methods of fatigue analysis to welds, an appropriate value of the stress concentration factor and residual stress must be selected. Although the smallest radius produces the largest stress concentration factor, its effect in fatigue is smaller because of the gradient effect. As a result there is a critical radius for fatigue that can be used to compute the fatigue notch factor.
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Crack Growth
Many weld details have planar lack of fusion defects. This is particularly true of fillet welds. In this case fracture mechanics models for crack growth are the most appropriate fatigue technology.
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Similitude
Local stresses and strains control the fatigue life
Lifetime to about a 1mm crack
Crack initiation
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Similitude (continued)
Nominal stresses and crack Length control the fatigue life
Crack propagation
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Vehicles Are Frequently Overloaded
Occasional plastic deformation → strain life analysis
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Strain-Life Fatigue Analysis
Material Data
Component Geometry
Service Loading
0 ε
σ
'1
'
n
E Kσ σε = +
( ) ( )'
'2 22
∆= +
b cff f fN N
Eσε
ε
σe/E
log
(∆ε/
2)
log (2Nf)
σf/E
εf
0 2Ne
2N
ε
σ
3
2,2'
4
5,5' 7,7'
6
8
1,1'
∆σ
σ
ε
∆ε p ∆ε e= ∆σ/Ε
∆ε
0
Cyclic stress strain curve
Strain-life curve
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Strain-Life Fatigue Analysis
Material Data
Component Geometry
Service Loading
Gradient Effects
Neuber’s Rule
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Strain-Life Fatigue Analysis
Material Data
Component Geometry
Service Loading
Analytical
Experimental
Structural Loads
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Crack Growth Fatigue Analysis
Material Data
Component Geometry
Service Loading
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Crack Growth Fatigue Analysis
Material Data
Component Geometry
Service Loading
σ(x)
a a
Stress distribution along crack path in an un-cracked body
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Crack Growth Fatigue Analysis
Material Data
Component Geometry
Service Loading
Analytical
Experimental
Structural Loads
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Why Are Welds Difficult to Analyze?
This one is easy
This one is difficult
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Welds Have Distortions
What is the real stress at a weld toe?
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Loading Conditions
How is the weld loaded ?
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Many Possible Failure Locations
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What is KT?
Tight fit-up KT = 3
Loose fit-up KT = 7
?
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Fatigue Analysis of Welds
Material Data
Component Geometry
Service Loading
Uncertain, but unimportant
Uncertain, but very important
Uncertain, but important
How do we deal with these uncertainties?
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Analyzing Welds
Nominal Stress Structural or Hot Spot Stress Local Stress Strain Crack Growth
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Nominal Stress Weld Classifications
D E
F2 G
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BS 7608 - Steel
100
200
300
400
B
C
D
E
F F2 G W 0
105 106 107 108
Fatigue Life, Cycles
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IIW Classification
( ) ( )
( )
m
m 6
1 1m m
16 m
N CC (FAT) 2 10m 3
C N
2 10FATN
−
∆σ =
= ×=
∆σ =
×∆σ =
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Japan Society of Steel Construction
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Crack Growth Data
( ) 0.312 mMPaK109.6dNda
∆×= −
( ) 25.210 mMPaK104.1dNda
∆×= −
( ) 25.312 mMPaK106.5dNda
∆×= −
Ferritic-Pearlitic Steel:
Martensitic Steel:
Austenitic Stainless Steel:
Barsom, “Fatigue Crack Propagation in Steels of Various Yield Strengths” Journal of Engineering for Industry, Trans. ASME, Series B, Vol. 93, No. 4, 1971, 1190-1196
5 10 100
10-7
10-6
10-8
Cra
ck G
row
th R
ate,
m/c
ycle
∆K, MPa√m
σyield 252 273 392 415
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0
25
50
75
100
125
105
B
C
D E
F
106 107 108
Nominal Stress - Aluminum
Fatigue Life, Cycles Sharp, “Behavior and Design of Aluminum Structures”,McGraw-Hill, 1992
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Crack Growth Data
1 10 100
Cyclic Stress Intensity, MPa√m
Cra
ck G
row
th R
ate
m/c
ycle
A533B m/cycle
2024-T3 m/cycle
10-2
10-4
10-6
10-8
10-10
10-12
3X
Steel welds are 3 times stronger than aluminum
1
3
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Residual Stress from Welding
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Weld Toe Residual Stress
Yield stress
Maximum stress at the weld toe is nearly the same for any cycle
∆ε
ε
σ
∆ε
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Mean Stress Effects
As welded structures usually have the maximum possible mean stress
Stress relief, peening, etc. will have a substantial effect on the fatigue life
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Butt and Fillet Weld Test Data
99% survival with 95% confidence
1000
Stre
ss R
ange
, MPa
100
10
103 104 105 106 107
Fatigue Life, Cycles
Failures Run outs
The good welds
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Weld Terminations 1000
Stre
ss R
ange
, MPa
100
10
103 104 105 106 107
Fatigue Life, Cycles
Failures Run outs
99% survival with 95% confidence
The bad welds
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Sources of Inherent Scatter
Weld quality Mean, fabrication and residual stresses Stress concentrations (geometry) Weldment size Material properties
Opportunities for Improvement !
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The Good and Bad
Good weld design
Poor weld design
Local stress concentration from weld toe
Macroscopic stress concentration from a geometry change
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Nominal Stress ?
Solution: use structural stress approach
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Weld Toe
Microcracks form during welding process
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All Welds Contain Microcracks
100
200
300
400 B
C D
E
F F2 G W 0 105 106 107 108
Fatigue Life, Cycles
Same slope means same mechanism, crack growth
10-12 10-11 10-10 10-9 10-8 10-7 10-6 1
10
100
Crack Growth Rate, m/cycle
mKCdNda
∆=
m ~ 3
∆K, M
Pa√
m
m ~ 3
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Fracture Mechanics Modeling
Driving force is crack depth, a, not length, c
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Stress Intensity Solution
( )
f
o
a
ma
max min
max applied residual
daNC K
K K KK K K
=∆
∆ = −= +
∫
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Weld Improvement
Reduce stresses Residual Distorsion fabrication
Reduce KT Weld toe Macroscopic Shape Weld starts and stops
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Gradual Change in Stiffness
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Stress Diffuser Improvement
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Things Worth Remembering
Local weld toe stresses, geometry and flaws control the life of weldments
There are many ways to improve the fatigue strength of welded structures.