Intro to ANSYS NCode DL 14 5 L11 Strain-Life Pt1

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    Introduction to ANSYS nCode DesignLife

    Customer Training Material

    Lecture 11: Strain-Life FatiguePart 1

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    Introduction to ANSYS nCode DesignLife

    L11-2ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialAgenda

    Discuss basics of Strain-Life (EN) method

    Discuss development of cyclic stress-strain curve and strain-life

    relationship

    Discuss mean stress effect

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    Introduction to ANSYS nCode DesignLife

    L11-3ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialStrain-Life Fatigue

    Strain-Life (EN)fatigue assesses fatigue damage using cyclic strainreversals and the Strain Life Relationship equation

    local plastic strains drive fatigue

    applicable to both low cycle and high cycle applications

    stresses less than or greater than yield

    uses elastic-plastic strains

    either directly calculated or adjusted from elastically calculated

    Relatively new fatigue analysis technique

    usage first began approximately 30 years ago

    difficult to implement with hand calculations limited to CAE applications

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    Introduction to ANSYS nCode DesignLife

    L11-4ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialStrain-Life Engine StepsFatigue

    Strain-Life (EN) engine follows these basic steps

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    Introduction to ANSYS nCode DesignLife

    L11-5ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialStrain-Life Fatigue

    For many real-world components nominal stresses remain elastic,but stress concentrations create local plastic strains

    contained by elastic deformation of surrounding elastic material

    behavior in the local region is strain controlled

    Cracks initiate at microscopic defects therefore fatigue behavior is

    controlled by the local plastic strains

    overall component is load (or stress) controlled

    material at notch root is deformation (or strain) controlled

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    Introduction to ANSYS nCode DesignLife

    L11-6ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialStrain-Life Fatigue

    Material behavior at notch root is analogous to small test specimenunder strain controlled conditions

    F

    F

    overall body remains elastic

    local deformation is strain controlled

    behavior of notched component with localized plasticity

    can be simulated using smooth test specimen in a

    displacement controlled test

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    Introduction to ANSYS nCode DesignLife

    L11-7ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    To employ the Strain-Life fatigue method, the stress-strainrelationship must be expressed as true stress vs. true strain

    True Stress-Strain Curve

    L

    L

    L

    Lln

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    Introduction to ANSYS nCode DesignLife

    L11-8ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    For many engineering materials, plot of true stress vs. true plasticstrain in plastic region is a straight line in log-log coordinates

    Can be represented as a power law expression

    K = strength coefficient

    n = strain hardening exponent

    True Stress-Strain Curve

    n

    pK

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    Introduction to ANSYS nCode DesignLife

    L11-9ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Power law can be rewritten as in terms of plastic strain

    for elastic-perfectly plastic material, n 0

    for ductile metals, n 0.15

    Total strain can be written in terms of the elastic strain plus the

    plastic strain

    True Stress-Strain Curve

    n

    pK

    1

    n

    KEt

    1

    Ramberg-Osgood equation

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    Introduction to ANSYS nCode DesignLife

    L11-10ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Cyclic loading past yield produces a stress-strain curve with a

    hysteresis loop

    Cyclic Stress-Strain Curve

    = total stress range

    = total strain range

    a = stress amplitude = /2

    a = strain amplitude = /2

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    Introduction to ANSYS nCode DesignLife

    L11-11ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    When subjected to cyclic stresses greater than yield, the stress-strain hysteresis loop progressively changes with each loading cycle

    ductile materials tend to harden (increase in strength)

    very hard materials tend to soften (decrease in strength)

    Deformation changes are more significant early in cyclic loading

    after a few cycles, most engineering materials stabilize to a steady-state

    hysteresis loop

    Cyclic Stress-Strain Behavior

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    Introduction to ANSYS nCode DesignLife

    L11-12ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Area inside the stress-strain hysteresis loop is the strain energy perunit volume (lb-in/in3)

    represents plastic work done on the material

    plastic work represents dislocation movement and crack initiation

    fatigue damage

    Cyclic Stress-Strain Curve

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    Introduction to ANSYS nCode DesignLife

    L11-13ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Plot of true stress amplitude vs. true plastic strain amplitude for

    cyclic loading is also a straight line in log-log coordinates that can be

    represented as a power law function

    Stabilized cyclic stress-strain hysteresis loop curve can also be

    represented by a Ramberg-Osgood relationship

    Cyclic Stress-Strain Curve

    '

    2'

    n

    K

    p

    a

    '1

    '2

    2E

    2

    2

    2

    pe

    n

    Ka

    '1

    '22

    E

    n

    K

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    Introduction to ANSYS nCode DesignLife

    L11-14ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Cyclic stress-strain equation describes elastic-plastic, stress-strain

    behavior under cyclic loading

    K and n are material parameters that best fit the measured data

    K = cyclic strength coefficient

    n = cyclic strain hardening exponent

    Cyclic Stress-Strain Curve

    '1

    '22E

    n

    K

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    Introduction to ANSYS nCode DesignLife

    L11-15ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Strain based fatigue curves are described using the Strain Life

    Relationship

    Total strain amplitude is expressed as separate elastic and plastic

    components

    both components are approximated as straight lines in log-log coordinates

    Strain Life Relationship

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    Introduction to ANSYS nCode DesignLife

    L11-16ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Elastic behavior controls in high-cycle fatigue regime

    Plastic behavior controls in low-cycle fatigue regime

    Total strain life relationship for cyclic loading is the elastic strain

    plus the plastic strain

    Strain Life Relationship

    bfN22

    f

    be ff

    NE

    22

    cp ff N22

    Basquinsequation

    Coffin-Mansons equation

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    Introduction to ANSYS nCode DesignLife

    L11-17ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialStrain Life Relationship

    be ff

    NE

    2

    2

    cp ff N22

    cbfff

    f

    NNE

    222

    Basquinsequation

    Coffin-Mansons equation

    Strain Life Relationship

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    Introduction to ANSYS nCode DesignLife

    L11-18ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Parameters in the Strain Life Relationship equation are material

    properties that best fit the measured test data

    Strain Life Relationship

    cb ffff

    NN

    E

    22

    2

    Measured Data

    = total strain range

    2Nf = number of reversals to failure

    Material Properties

    f = fatigue strength coefficient

    E = modulus of elasticity

    b = fatigue strength exponent

    f = fatigue ductility coefficientc = fatigue ductility exponent

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    L11-19ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    For large strain amplitudes (large amount of plastic work), plastic

    strain component dominates

    For small strain amplitudes (small amount of plastic work), elastic

    strain component dominates

    Strain Life Relationship

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    Introduction to ANSYS nCode DesignLife

    L11-20ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Strain-Life material properties are defined in Engineering Data

    Strain Life Material Properties

    b

    c

    f

    f

    cb ffff

    NNE

    222

    fatigue cutoff

    endurance

    limit

    I d i ANSYS C d D i Lif

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    Introduction to ANSYS nCode DesignLife

    L11-21ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Material properties for the cyclic stress-strain curve are also defined

    in Engineering Data

    Strain Life Material Properties

    '1

    '22

    E

    n

    K

    K

    n

    I t d ti t ANSYS C d D i Lif

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    Introduction to ANSYS nCode DesignLife

    L11-22ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialPredefined EN Process

    ENprocesses are defined using pre-defined Analysis Systems

    I t d ti t ANSYS C d D i Lif

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    Introduction to ANSYS nCode DesignLife

    L11-23ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialEN Properties

    DesignLifespecifies some ENparameters in the Properties menu

    StrainLife_Analysis Glyph > RMB > Properties

    I t d ti t ANSYS C d D i Lif

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    Introduction to ANSYS nCode DesignLife

    L11-24ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialEN Properties

    Other ENproperties are specified using Advanced Edit

    StrainLife_Analysis Glyph > RMB > Advanced Edit

    I t d ti t ANSYS C d D i Lif

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    Introduction to ANSYS nCode DesignLife

    L11-25ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialMean Stress Effect

    Total cyclic strain range is primary influence on fatigue life

    Mean strain that occurs during the strain cycle has a secondary

    effect on fatigue life

    tensile mean strains reduce fatigue life

    compressive mean strains increase fatigue life

    Mean stress effect is inversely related to amount of plasticity

    as plasticity increases, effect of mean stress decreases

    for short lives (low-cycle fatigue), mean stress has negligible effect

    primarily affects high cycle region

    elastic stress/strain

    mean stress effect can be included by modifying only the elastic life line

    Introduction to ANSYS nCode DesignLife

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    Introduction to ANSYS nCode DesignLife

    L11-26ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialMorrow vs SWT Mean Stress Effect

    DesignLife supports the two most common ENmean stress

    correction methods

    Morrow

    straight line correction to the elastic life line

    Smith Watson Topper (SWT)

    most common

    assumes driving force in a fatigue cycle is strain amplitude times peak stress

    Introduction to ANSYS nCode DesignLife

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    Introduction to ANSYS nCode DesignLife

    L11-27ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Morrow Mean Stress Correction

    straight line correction to elastic component

    modifies fatigue strength coefficient (f)

    tensile mean stresses are positive

    detrimental to fatigue life

    compressive means are negative

    beneficial to fatigue life

    cbm

    fff

    f

    NNE

    22

    Morrow Mean Stress Correction

    m/E

    Introduction to ANSYS nCode DesignLife

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    Introduction to ANSYS nCode DesignLife

    L11-28ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Smith Watson Topper (SWT) method uses a damage parameter (PSWT

    )

    to develop a fully reversed loading cycle that has equivalent damage

    as a stress cycle with a mean stress

    The two equations are used to find the equivalent strain amplitude

    used with the standard strain-life curve to determine the fatigue damage

    Smith Watson Topper Mean Stress Correction

    equivmax,a,equivmaxa

    SWTP

    '1

    '

    E

    max,max,

    ,

    n

    K

    equivequiv

    equiva

    a,equiv

    a

    Introduction to ANSYS nCode DesignLife

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    Introduction to ANSYS nCode DesignLife

    L11-29ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training Material

    Both SWT and Morrow predict minimal mean stress effect at large

    strain amplitudes

    large amount of plasticity

    low-cycle fatigue

    For small strain amplitudes, Morrow predicts less mean stress effect small amount of plasticity

    high-cycle fatigue

    SWT is more conservative

    most common approach

    SWT predicts no fatigue damage when maximum stress is negative

    can be unconservative for compressive loadings

    Morrow is recommended in those situations

    Mean Stress Correction Comparison

    Introduction to ANSYS nCode DesignLife

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    Introduction to ANSYS nCode DesignLife

    L11-30ANSYS, Inc. Proprietary

    2013 ANSYS, Inc. All rights reserved.Release 14.5

    April 20133

    Customer Training MaterialMean Stress Correction

    Mean Stress Correction Method is specified using Advanced Edit

    StrainLife_Analysis > RMB > Advanced Edit

    I...

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