Mike Weaver

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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

    WEAVERE 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 withK 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.

    KTENSION KBENDINGTOE 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_2{description = "Transverse Notch Stress, Side 2"summarize max unsigned

    }

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

    Notch_Str_M2{ "Transverse Notch Stress, Side 2, Membrane Load" }

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

    Notch_Str_B2{ "Transverse Notch Stress, Side 2, 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

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    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.

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    FEA Evaluation Solid Models

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

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    FEA Evaluation: Plane Strain Stress Intensity

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    FEA Tool Development Specific to WeldFatigue

    Production Analysis Computations

    Automation and Data Management

    FEA Systems Interface

    Flexibility and User Input Ease

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    FEA Tools: Production Analysis

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    FEA Tools: FEWeld

    Mathematics Data Management

    Data Input

    Results Presentation

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    FEA Tools: FEWeld Shell Element Mechanics

    Resolution of Weld Loads, Node 340:

    tb.3

    8in Base Material Thickness

    t.19560 psi Normal Stress at Top of Joint

    b.7884 psi Normal Stress at Bottom of Joint

    zx_avg.390.2 psi Average Shear Stress in Joint

    yz_av

    .2530 psi .1210 psi

    2

    avg zx_avg2

    yz_avg2

    = avg 1910 psi

    Joint Normal Load:

    P . t b

    2tb =P 5146

    lbf

    in

    Joint Bending Load:

    M . t b

    2

    tb2

    6=M 136.8

    .inlbf

    in

    Joint Shear Load:

    V . avg tb =V 716.4lbf

    in

    tb

    t

    b

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    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_B