caef 7 strain life

7/21/2019 CAEF 7 Strain Life

1/11

Explaining the differences from Stress-Life Fatigue


2/11

The Strain-Life Method

(Local Strain, -N, or Crack Initiation)

*Relates local strain to fatigue damage at that point

*Useful when cycles have some plastic strain component

*Suitable for predicting life in components which aresupposed to be defect free, i.e., not structural joints,sharp features, etc.

(Low Cycle Fatigue LCF)


3/11

MSC Fatigue Features

*Based on Local Strain Concepts

*

Mean Stress Correction*Elastic-Plastic Conversion

*Statistical Confidence Parameters

*Palmgren-Miner Linear Damage

*

User Defined Life

*Cyclic Stress-Strain Modeling

*Surface Conditions

*Factor of Safety Analysis

*Biaxiality Indicators

*Multiple Loads

Strain-Life (-N)

1/2cycle

1cycle

1/2cycle

1cycle

1cycle

1/2cycle

Strain

Time


4/11

Cyclic Testing - Hysteresis Loops and End Point Definition

2Nf

StabilizedHysteresis Loop

Stress

Strain


5/11


6/11

-NFatigue Tests*Specimens are subjected to constant

amplitude strain using an

extensometer in the servo loop

*

N test controls plastic strain, theparameter that governs fatigue

*The number of cycles to crack

initiation is plotted against the total

strain on a log-log plot and the best fitcurve computed

( ) ( )cff

b

f

f

a NNE

22`

`+=


7/11

Elastic & Plastic Components(Due to Basquin, Coffin and Manson)

(2Nf)b + f

(2Nf)c

f

E

2=

Elastic

(Basquin)

Plastic

(Coffin-Manson)


8/11

Neubers Rule

The strain concentration factor,

and the stress concentration factor,

after plastic yielding.

Neither are known but Neuber suggested that the square root of theproduct of the stress and strain concentration factors was equal to

Kt . Hence Neubers Rule is simply:

Re-arrangement of this Rule gives a useful equation:

eK

e

=

SK

=

( )2.Te

KKK =

( ) =SeKT

2


9/11

Another re-arrangement gives:

in which the LHS is known. This can be solved with the cyclic stress

strain curve equation simultaneously to derive and

Topper simply replaced Ktby Kf to make Neubers Rule applicable infatigue analysis for local stress strain tracking.

Use of Kfin Neubers Rule

( ) =EeKT

2


10/11

1

2

3

Kf

e

s

Cyclic Stress Strain Curve

Neuber

Equation

Solution

point

Graphical Solution for Local and

1

K

3yclic Stres

Solution

2


11/11

Morrow Life Plot

StrainAmplitude(M/M)

Life (Reversals)nCode nSoft

STW Life Plot

STWP

arameter(MPa)

Life (Reversals)nCode nSoft

Morrow Smith-Topper-Watson

( ) ( )

2

2 2=

+f mf

bf f

c'' ( ) ( )

c+bfff

2bf

2f

max 2''2'

2

+

=

Correcting for Mean Stress

caef 7 strain life

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