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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



Factors in Weld Fatigue Life Prediction

LIFEFABRICATION

VARIANCE

DESIGN

LIMIT STATE

TRUE

LIMIT STATE

ANALYSIS

UNCERTAINTY



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.



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



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.



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.



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.



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



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



FEA Evaluation of Geometric Stress:

Continuum Models

• Examples of Hot Spot -

Plane Strain Evaluationof Condition (1 of )




Continuum Models

• Examples of Hot Spot -Plane Strain Evaluation

of Condition (2 of 3)



FEA Evaluation of Geometric Stress:Continuum Models

• Examples of Hot Spot -Plane Strain Evaluation

of Condition (3 of 3)




Shell Element Models

• A fair amount of Literature and Current Work on the Subject:

Neimi, Radaj, Hobbacher - IIW




Shell Element Models



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



FEA Evaluation of Geometric Stress:Shell Element Models: Issues

• Shell Element Cross Section Singularity(1 of 2)




Shell Element Models: Issues

• Shell Element Cross

Section Singularity

(1 of 2)



FEA Evaluation of Effective Notch Stress:Continuum Models

• Plane Strain for SCF

• Solid

• Resolution to 1 mm radius of sharp features



FEA Evaluation of Effective Notch Stress:SCF

• Plane Strain Determination of SCF-Shell Element Models

-Classical Calculations

-Determination of Improvement.

71911

2923

..

.=

≥

OldLife

NewLife



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

Applied Nominal Axial Load: 1 MPa

Weld at Break in Profile

******** END COMMENT BLOCK *********/

@INPUT{K1_1_Membrane

K1_2_MembraneK1_1_BendingK1_2_Bending

}

Str_Mem = (Sjj_1 + Sjj_2)/2Str_Bend = Sjj_1 - Str_Mem

Notch_Str_M1 = Str_Mem*K1_1_MembraneNotch_Str_M2 = Str_Mem*K1_2_Membrane

Notch_Str_B1 = Str_Bend*K1_1_BendingNotch_Str_B2 = Str_Bend*K1_2_Bending

Notch_Str_1 = Notch_Str_M1 + Notch_Str_B1Notch_Str_2 = Notch_Str_M2 + Notch_Str_B2

@IF(Notch_Str_1 >= Notch_Str_2){Notch_Str_Max = Notch_Str_1

}@ELSE{Notch_Str_Max = Notch_Str_2

}

@STORE{

Notch_Str_Max{description = "Worst Case Transverse Notch Stress, Sides 1 and 2"plotsummarize max unsigned

}

Notch_Str_1{description = "Transverse Notch Stress, Side 1"summarize max unsigned

}


}

Notch_Str_M1{ "Transverse Notch Stress, Side 1, Membrane Load" }


Notch_Str_B1{ "Transverse Notch Stress, Side 1, Bending Load" }


Str_Mem{ “Transverse Structural (Geometric) Membrane Stress" }

Str_Bend{ "Transverse Structural (Geometric) Bending Stress" }}

0

0.5

1

1.5

2

2.5

3

3.5

4

0.00 0.50 1.00 1.50 2.00

Notch_Str_Max, Worst Case Transverse Notch Stress, Sides 1 and 2

-2

-1

0

1

2

3

4

0.00 0.50 1.00 1.50 2.00

Notch_Str_1, Transverse Notch Stress, Side 1

Notch_Str_2, Transverse Notch Stress, Side 2



FEA Evaluation of Effective Notch Stress:Solid Elements

• Example with Radiused, Ground Special Quality Weld on Heavy

Weldment - Not too many degrees of freedom required here because

of the smooth geometry.



FEA Evaluation Solid Models

• Lack of Fusion Must Be Modeled. Done here in CAD.Would be a nice FEA Meshing Tool.



FEA Evaluation: Plane Strain Stress Intensity



FEA Tool Development Specific to Weld Fatigue

• Production Analysis• Computations

• Automation and Data Management

• FEA Systems Interface

• Flexibility and User Input Ease



FEA Tools: Production Analysis



FEA Tools: FEWeld

• Mathematics• Data Management

• Data Input

• Results Presentation



FEA Tools: FEWeld Shell Element Mechanics

Resolution of Weld Loads, Node 340:

t b.3

8in Base Material Thickness

σ t.19560 psi Normal Stress at Top of Joint

σ b.7884 psi Normal Stress at Bottom of Joint

τ zx_avg.390.2 psi Average Shear Stress in Joint

τ yz_av

.2530 psi .1210 psi

2

τ avg τ zx_avg2

τ yz_avg2

=τ avg 1910 psi

Joint Normal Load:

P .σ t σ b

2t b =P 5146

lbf

in

Joint Bending Load:

M .σ t σ b

2

t b2

6=M 136.8

.inlbf

in

Joint Shear Load:

V .τ avg t b =V 716.4lbf

in

tb

σ

σt

b



FEA Tools: FEWeld Computations******** END COMMENT BLOCK *********/

@INPUT{K1_1_MembraneK1_2_MembraneK1_1_BendingK1_2_Bending

}

Str_Mem = (Sjj_1 + Sjj_2)/2

Str_Bend = Sjj_1 - Str_MemNotch_Str_M1 = Str_Mem*K1_1_MembraneNotch_Str_M2 = Str_Mem*K1_2_Membrane

Notch_Str_B1 = Str_Bend*K1_1_BendingNotch_Str_B2 = Str_Bend*K1_2_Bending

Notch_Str_1 = Notch_Str_M1 + Notch_Str_B1Notch_Str_2 = Notch_Str_M2 + Notch_Str_B2

@IF(Notch_Str_1 >= Notch_Str_2){Notch_Str_Max = Notch_Str_1

}@ELSE{Notch_Str_Max = Notch_Str_2

}@STORE{

Notch_Str_Max{description = "Worst Case Transverse Notch Stress, Sides 1 and 2"plotsummarize max unsigned

}


}

Notch_Str_2{

description = "Transverse Notch Stress, Side 2"summarize max unsigned

}





Str_Mem{ “Transverse Structural (Geometric) Membrane Stress" }

Str_Bend{ "Transverse Structural (Geometric) Bending Stress" }}



FEA Tools: FEWeld Data Management



FEA Tools: FEWeld Overview



FEA Tools: FEWeld FEA Interaction

FEWeld GUI Interaction with Cosmos

(Same for Ansys)



FEA Tools: FEWeld Generalized Data Layout (Future)



FEA Tools: FEWeld Future Scripting

MATERIAL E70_ELECTRODE{Fut 485 MPa

}

MATERIAL ASTM_A572GR50{Fut 485 MPaFy 345 MPa

}

LOAD_GROUP EXT_LOADS{ 01 02 03 04 }LOAD_GROUP FAT_LOADS{ 11 - 30 }LOAD_GROUP ALL{ EXTREME FATIGUE 05 - 09 }

WELD_TEMPLATE DOUBLE_SIDED_PREP{CALCULATION EXTREME_THROAT_SHEAR {

MATERIAL E70_ELECTRODEFORMULATION DPF-FVPARAMETERS{

E = 4 mmSa = Fut * .3

}LOADING{ EXT_LOADS }

}

CALCULATION FATIGUE_DAMAGE {

MATERIAL ASTM_A572GR50FORMULATION SoderbergPARAMETERS{

(mean, alt) = mean_alt( FATIGUE )C_Xverse 100 MPaDesign_Life 100e6

}}

}

WELD_SET CONFIG_00{DESCRIPTION "Original Configuration"FEA_MODEL_UNITS{ F=lb, L=in, T=s }WELD 01{

DESCRIPTION "Weld between parts 150Cand 148C"

ELEM COMPONENT BRACE_150CNODE LINE LIST{ 42 43 62 62 }

TEMPLATE DOUBLE_SIDED_PREP}WELD 02{

DESC "Weld between boom and yoke"ELEM AREA LIST{ 7 14 21 28 35 42 49

56 63 70 77 84 91 98 }

NODE LINE COMPONENT YOKE_JOINT

TEMPLATE DOUBLE_SIDED_PREP}

}

WELD_SET CONFIG_01{COPY SET CONFIG_00DESC "Modified Boom wall to 0.625"

}



Fatigue Estimation of Welds with FEA:Modeling, Criteria, Approaches, and Issues

THANK YOU

WEAVER E GINEERINGSeattle, Washington. http://www.weavereng.com

Presentation to SAE Fatigue Committee, Mike Weaver, October 2003, Cedar Rapids, Iowa