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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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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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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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Customer Training MaterialStrain-Life Engine StepsFatigue
Strain-Life (EN) engine follows these basic steps
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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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L11-6ANSYS, Inc. Proprietary
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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L11-25ANSYS, Inc. Proprietary
2013 ANSYS, Inc. All rights reserved.Release 14.5
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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
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L11-26ANSYS, Inc. Proprietary
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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
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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
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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
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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
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Customer Training MaterialMean Stress Correction
Mean Stress Correction Method is specified using Advanced Edit
StrainLife_Analysis > RMB > Advanced Edit
Introduction to ANSYS nCode DesignLife
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L11-31ANSYS, Inc. Proprietary
2013 ANSYS, Inc. All rights reserved.Release 14.5
April 20133
Customer Training MaterialStrain-Life Summary
Strain-Life fatigue assesses fatigue damage using cyclic elastic-
plastic strain reversals and the Strain Life Relationship equation
applicable to both low cycle and high cycle applications
relatively new fatigue analysis technique
usage first began approximately 30 years ago
CAE approach - difficult to implement with hand calculations
Strain Life Relationship equation contains the combined effect of
separate elastic and plastic components
both effects are approximated as straight lines in log-log coordinates
Basquinsequation represents plastic behavior (low-cycle)
Coffin-Manson equation represents elastic behavior (high-cycle)
Introduction to ANSYS nCode DesignLife
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2013 ANSYS, Inc. All rights reserved.Release 14.5
April 20133
Customer Training MaterialStrain-Life Summary
Total cyclic strain range is primary influence on fatigue life
Mean strain that occurs during the strain cycle is a secondary effect
tensile mean strains reduce fatigue life
compressive mean strains increase fatigue life
Most common mean 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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Customer Training MaterialWorkshop 9
Workshop 9: Constant Amplitude EN Analysis
restore WB archive
solve Mechanicalmodel
edit DesignLi fe
specify None for mean stress correction
solve
post-process
specify Smith-Watson-Topper mean stress method
solve
post-process