Fatigue Estimation of Welds with FEA: Modeling, Criteria, Approaches, and Issues • Introduction • We ld Fatigue and Physi ca l Influencing Fact ors • Me thods of Analysi s and Pr ediction and Application of FEA • FEA Tool Development Specific t o Weld Fatigue • Concluding Remarks WEAVERE GINEERING Seattle, Washington. http://www.weavere ng.com Presentation to SAE Fatigue Committee, Mike Weaver, October 2003, Cedar Rapids, Iowa
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Fatigue Estimation of Welds with FEA:Modeling, Criteria, Approaches, and Issues
• Introduction• Weld Fatigue and Physical Influencing Factors
• Methods of Analysis and Prediction and
Application of FEA
• FEA Tool Development Specific to Weld Fatigue
• Concluding Remarks
WEAVER E GINEERINGSeattle, Washington. http://www.weavereng.com
Presentation to SAE Fatigue Committee, Mike Weaver, October 2003, Cedar Rapids, Iowa
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Factors in Weld Fatigue Life Prediction
LIFEFABRICATION
VARIANCE
DESIGN
LIMIT STATE
TRUE
LIMIT STATE
ANALYSIS
UNCERTAINTY
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Weld Fatigue and Physical Influencing Factors• Material State Variations: Mill Heat,
Electrode, Moisture
• Material Damage Due to Welding -
Hydrogen Cracking, Hot Short Cracking,
Lamellar Tearing, Other Base Metal Damage
• Fit-up and Joint Preparation.
• Process and Position
• Operator and Machine Variations
• Starts and Stops
• Sequence, Restraint, and Residual StressState
• Heat Affected Zone - Grain Structure, Local
Brittle Areas, Strength Mismatch
Impoverishment, Overaging, etc
• As Welded Profile - Local Stress
Concentrations• Flaw Density and nature.
• Load History and Environmental
Uncertainties -
Multi-axial Loading, Non-Proportional
Loading
Improvements:
- Mechanical: Burr Grinding, Machining, Peening
- Thermal: PWHT, TIG Dressing, Selective Spotand Line Heating.
- NDT: Improves distribution by truncating tail.
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Methods of Analysis and Prediction
Per IIW Guidelines, Four
Categories:• Nominal Stress Method - Classical
Analysis
• Geometric (Structural, Hot-Spot)Stress Method
• Effective Notch Stress
• Fracture Mechanics … Fitness for
Purpose
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Methods of Analysis: Nominal Stress Method
• P/A … Mc/I• Structural Load Path Variations in Criteria
• Weld Notch Effect in Criteria
• Joint Performances Tabulated and Classified invarious Codes, Design Guides, etc.
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Methods of Analysis: Geometric Stress
• A.K.A. Structural Stress Method, HotSpot Stress
• Structural Load Path Determined by
Analysis or Physical Measurement
• Weld Notch Effect in Criteria• Joint Performances Classified based on
Weld Notch Geometry and Weld Quality.
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Methods of Analysis: Effective Notch Stress
• Geometry of Weld Modeled to 1 mm Resolution
• Sharp Features Rounded with 1 mm radius to allow for fatigue notchsensitivity.
• One S-N curve. The Most Refined Stress Based Approach.
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Methods of Analysis: Fracture Mechanics• da/dN material evaluations with∆K and R
determined by Analysis.
• Detailed and Simplified Methods
• Tabulated (Simplified) Equivalent Stress
Categories for S-N Evaluation
• Fitness for Purpose Evaluations, As Fabricated
Quality Level, Joint Design
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Application of FEA to Prediction Methods
• Nominal Stress: Beam Element Models
• Geometric Stress, Shell and Continuum Models
• Effective Notch Stress - Continuum Models or Shell Models with SCF
• Fracture Mechanics: Continuum Models with Flaws Modeled or FEA
Combined with Classical Fracture Mechanics
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FEA Evaluation of Geometric Stress:
Continuum Models
• Examples of Hot Spot -
Plane Strain Evaluationof Condition (1 of )
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FEA Evaluation of Geometric Stress:
Continuum Models
• Examples of Hot Spot -Plane Strain Evaluation
of Condition (2 of 3)
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FEA Evaluation of Geometric Stress:Continuum Models
• Examples of Hot Spot -Plane Strain Evaluation
of Condition (3 of 3)
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FEA Evaluation of Geometric Stress:
Shell Element Models
• A fair amount of Literature and Current Work on the Subject:
Neimi, Radaj, Hobbacher - IIW
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FEA Evaluation of Geometric Stress:
Shell Element Models
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FEA Evaluation of Geometric Stress:Shell Element Models: Issues
• Nodal Stress Averaging
WELD
FREEEDGE
Correct TerminatedPart Element Selection
The Offending Elementfor Incorrect TerminatedPart Element Selection
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FEA Evaluation of Geometric Stress:Shell Element Models: Issues
• Shell Element Cross Section Singularity(1 of 2)
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FEA Evaluation of Geometric Stress:
Shell Element Models: Issues
• Shell Element Cross
Section Singularity
(1 of 2)
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FEA Evaluation of Effective Notch Stress:Continuum Models
• Plane Strain for SCF
• Solid
• Resolution to 1 mm radius of sharp features
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FEA Evaluation of Effective Notch Stress:SCF
• Plane Strain Determination of SCF-Shell Element Models
-Classical Calculations
-Determination of Improvement.
71911
2923
..
.=
≥
OldLife
NewLife
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FEA Evaluation of Effective Notch Stress:
SCF
Tensile Load, Toe Bending Load, Toe.
Tensile Load, Root. Bending Load, Root.
K TENSION K BENDING
TOE 1.59 1.36
ROOT 2.45 -2.16
6 mm Sheet Metal Formed and Welded Hollow FrameModeled with Shell Elements