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

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• 8/11/2019 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

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

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

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

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

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

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

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

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

April 20133

Customer Training Material

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

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)

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

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

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

April 20133

Customer Training Material

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

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

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

April 20133

Customer Training Material

Elastic behavior controls in high-cycle fatigue regime

Plastic behavior controls in low-cycle fatigue regime

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

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

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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Introduction to ANSYS nCode DesignLife

L11-19ANSYS, Inc. Proprietary

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

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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L11-21ANSYS, Inc. Proprietary

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

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

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

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

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

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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L11-27ANSYS, Inc. Proprietary

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

April 20133

Customer Training Material

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

)

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

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

Morrow is recommended in those situations

Mean Stress Correction Comparison

Introduction to ANSYS nCode DesignLife

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