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1
Best Practices for Fatigue Calculations on FE Models
Presented by:
Dr.-Ing. Stephan Vervoort
Senior Application Engineer Hottinger Baldwin Messtechnik GmbH, nCode Products
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Agenda
• HBM nCode Products
• Fatigue Analysis Process
• Modeling Recommendations for FE Models
• Defining the Loading Environment
• Defining Material Properties
• Conclusion
3
HBM-nCode Products
• Comprehensive analysis to reporting
• Graphical, interactive & powerful
• World leading fatigue analysis capabilities
• Search, query and reporting data through secure web access
• Analyze, trend and understand using configurable processing
• Powerful fatigue analysis technology
• Integrated reporting and processing
• Fast, expandable, and scalable
Data Processing System for
Engineers
Streamlining the Virtual Fatigue
Engineering Process
Web-based Processing for
Engineering Data
5
Inputs
Fatigue Analysis Process
Geometry
Loading
Environment
Material
Properties
Fatigue Analysis Fatigue Results
•Stress Life
•Strain Life
•Crack Growth
•Etc
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The Inputs
• Geometry
Accuracy of surface stresses are important
±10% stress @ ±100% life
Current structural FE modeling is generally sufficient
• Loading Environment
Large effect on fatigue life
Must be correctly characterized
• Material Properties
Material fatigue properties are relatively inexpensive to obtain
Materials Assurance Service available from nCode laboratory
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Modeling Recommendations for FE Models
• Fatigue cracks usually initiate at free surfaces
• Fatigue damage increases exponentially with stress
±10% stress @ ±100% life
• Recommend using node on element or averaged node on element
Check for convergence
OK for load path &
natural modes
Required for
Fatigue
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Shell Model
• Stresses calculated at Gauss points and extrapolated to node
• Node has separate stress result from each element
• Most FEA uses average nodal stress
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Solid Model
• Stresses calculated at Gauss points – but Gauss points are not on surface
• Option 1 – skim surface with membrane or thin shells
Resolves stresses to surface plane
Uses element stresses from the shells as before
• Option 2 – use surface node results
Resolves stresses to the surface
Uses node on element or averaged node on element on surface nodes only
x’
y’
z’
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Different Types of Loading Environments
• Linear static superposition
Linear static superposition is an efficient FE analysis technique
This process can also be used for modal superposition
• Time step
Time step is computationally intensive but allows for non-linear analysis and dynamic analysis
• Harmonic response
Harmonic analysis is very efficient for steady state random loading
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Linear Static Superposition
C = FE Stress tensor for Unit
Load Cases
sA
L1=1
L2=1
C1
C2
C1 x
+
=
C2 x
Real Load L1
Real Load L2
Stress time signal at element
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Modal Superposition
Geometry
Loading
Environment
Material
Properties
Fatigue
Analysis
Fatigue
Results
Modal
Transient
Loading
Histories
Modal Stresses
Modal
Coordinates
SN or EN
Curve
SN or EN
Analysis
Fatigue
Results
L2
L1 L1
L2
f1
f2
s1A* f1(t) + s2A* f2(t) + ... = sA(t)
Mode 1 Mode 2
sA
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Time Step
Stress for combined loads
calculated by FE point by point
L2
sA L1
L1
L2
For long time histories, issues with solution time
and disk space requirements
Stress time signal at element
Load time signals processed by FE
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Harmonic Response Transfer Function
C(f) = Complex transfer tensor for
unit Load Cases L(f)
sA
=
Real Load L1
Stress PSD at element
2
C f
C f
1L f
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Non-linear Contact
L1
L2
Rea
l lo
ad L
1
Rea
l lo
ad L
2
Com
bin
ed l
oad
cas
e
t
t
t
• Non-linear contact is modeled as 2 linear static load cases
• Load cases scaled by real loads
• +ve loads applied to load case 1
• -ve loads applied to load case 2
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• The fatigue life is often described with a single regression curve through data points at which 50% of samples have failed
• More data points result in better confidence, but range of life is also important
• The distribution of points allows for different Certainty of Survival levels
• The distribution of points also allows for the comparison of different batches or supplier
Defining Material Properties
19 4 2 0 2 4 6 8
Supplier A
Supplier B
Supplier A Test Data
Supplier A Regress ion fit
2 s igma range
2 s igma range
Supplier A Des ign curve
Supplier B Tes t Data
SN Fatigue T est Analysis
Number of Cycles to failure N
Str
ess R
ang
e S
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Conclusion
• Accuracy of damage results depends on accurate surface stresses
• There are three basic types of loading environments
Linear static superposition
Time step
Harmonic response
• Determine if the loading is dynamic or quasi-static
If max frequency in PSD < 1/3 f1 then use static analysis
• The fatigue life curve, certainty of survival, and range of life are all important when characterizing material properties
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