lcf-hcf tutorial

7/26/2019 LCF-HCF tutorial

1/85

Low Cycle Fatigue (LCF)

High Cycle Fatigue (HCF)


2/85

What is Fatigue?

The ASTM definition.....

The process of progressive localized permanent structural change

occurring in material subjected to conditions which produce fluctuating

stresses and strains at some point or points and which may culminate incrack or complete fracture after a sufficient number of fluctuations.

Translation:

!yclic damage leading to local cracking or fracture.


3/85

Time

"esign#e$uirements

Material%roperties

&istorical 'asic (ngineering

%roperties

Strength)

!reep

*+,-s / *+0-s Add ... 1atigue &!1) 2!1) TM1

2ate *+0-s Add ... "amage

Tolerance

!rack 3rowth

Requirements have evolved for Gas Turbine Engines....

Emphasis today is on Cyclic Properties...


4/85

&igh !ycle 1atigue Allowable vibratory stresses

2ow !ycle 1atigue !rack initiation life

*4*--- to small crack

!omponen

retirement

!rack 3rowth #emaining life from crack

Safety

inspection interval

5nspection

size re$uirement

Emphasis today is on Cyclic Properties...


5/85

For Crack Initiation, High Cycle Fatigue

(HCF) and o! Cycle Fatigue (CF) are

treated separately. Why?

3eneral distinction for 3as Turbines6

HCF/ 7sually high fre$uency) due to resonant

vibration. 1ailure criteria based on allowable

stresses. illions of Cycles

!CF/ 7sually low fre$uency) due to engine

start4stop or throttle cycles. Accurate life

prediction re$uired. Thousands of Cycles


6/85

Turbine Disk Design Requirements

8 Environmentally friendly

8 Fatigue cracking resistance

initiation

propagation8 Creep resistant

8 Strong

8 Lightweight

8 redi!table"#nspe!table

8 $ffordable

8 Environmentally stable

%i!kel Superalloy &alan!es $ll Requirements

'ost Severe Stru!tural Challenge: (igh stru!tural loads) fatigue) * !reep


7/85

Combustor) Turbine Componentsresent a Severe Thermal +atigue Cra!kingChallenge

8 'e!hani!al fatigue) !aused

by !y!li! thermal strains

8 (igh temperature

a!!elerates fatigue damage

8 E,a!erbated by !ra!k tip

o,idation


8/85

+atigue is a 'a-or Challenge for 'any Engine Components)

#n!luding +an &lades

8 Caused by Load Cy!ling

8 .!!urs at !y!li! loads well below the /ltimate Strength

8 (igh Cy!le +atigue 0(C+1

Caused by vibration"flutter

8 Low Cy!le +atigue 0LC+1

Caused by engine !y!ling

"atigue crack initiation site

!ompressor blade tested in

a vibratory fatigue test rig


9/85

Cyclic #s. $onotonic Cur#es% &eha#ior can 'e signi"icantly di""erent ...

1rom Metal 1atigue in (ngineering) &.9. 1uchs and #.5. Stephens) :ohn ;iley ) *+?-


10/85

Crack ie% Ho! 'ig is 'ig? ...


11/85

HCF% (*+ Cur#es ...

5nitially used to address &!1 for allowable

stress) but what about predicting actual cycles

of life@ ...

&!1 cycle prediction is more of a statistical

estimate with a large scatter allocation)

instead of an eact science


12/85

*2$ Stress Control (C+ Test $pparatus


13/85

Specimen Fully Reversed Stress/Strain Cycle S/N Plot


14/85

Alternating Stress Amplitude6

a =ma minB

Mean Stress6

-B

= +ma min

Stress #atio6 R =

min

ma

Stress #ange6

= ma min

Basic Cycle

Terms to Remember


15/85

Soderberg C7SA) *+D-E a

e

m

y" "

+ = *

3oodman C(ngland) *?++E a

e

m

u" "+ = *

3erber C3ermany) *?0FE a

e

m

u

" "+

=

B

*

#$here "eis the fully reversed endurance limit.%


16/85

Cyclic e"ormation Parameters% Fatigue loop illustration ...


17/85

Fatigue% Ho! do HCF and CF "it !ith

tress #s. i"e? ...

G (ists in theor onl


18/85

CF% (*+ Cur#es ...

1atigue Strength is the Maimum Stress that can

be repeatedly applied for a specified number of

cycles Ctypically *-0E without failure. Titanium

alloys are curve fit to *-+

cycles.


19/85

CF% +otes on -pproaches ...

Soderberg is highly conservative and seldom

used

Actual test data usually falls between3oodman < 3erber !urves

This is not a large difference in the theories

when the mean stress is small in relation to

the alternating stress.

%


20/85

HCF% - Christienson ,iagram Contains all o"

this in"ormation ...


21/85

HCF% -n eample o" Pratt/s 0oodman

diagram !hich com'ines (tress -mplitude and

$ean (tress E""ects ...

The discontinuous slope on the /ais modifies

for the yield value instead of the ultimate as

re$uired by a traditional 3oodman "iagram.


22/85

HCF% Cyclic limits ...

*-

0

cycles / Most other alloys*-+cycles / Titanium) certain =ickel 'lade

Alloys*-

+cycles / @@@@@ C%roposed following the

&!1 5nitiativeE

$hy no actual &'(Testing)

%resent fre$uency capability is B-- &z)

which is *., yearsHH

Assuming BI tests on two machines) this is

B- years to characterize a single material HHH

Target now is B--- &z for coupon testing)

which is B months for a single test.


23/85

HCF% Elastic tress*i"e 1elationship ...


24/85

HCF +otches% Parameters o" Interest ...

%arameter "escription

Jt (lastic Stress

!oncentration

Jf 1atigue =otch

1actor CJfJtE

Material constantCrelated to grain sizeE

r =otch radius

$ =otch sensitivity


25/85

HCF +otches% +eu'er proposed the

"ollo!ing relationship ...

**

rf

t= +

+*

*

* 4

q

*

* r

f

t=

= +

*

*

*

* 4

$here+

SeCnotchedE

KSeCunnotchedE

4 Jf

5n the previous e$uations) the notched value

would then be substituted.


26/85

CF 2esting% 3eri"ication ...

Three primary ways of verification testing6

Subcomponents

Spin %it

1erris ;heel


27/85

*2$ Strain Control LC+"T'+ Test $pparatus


28/85

CF 2esting% 2ypical set*up in#ol#es

uniaial loading ...


29/85

Strain #ange /

Stress #ange / = %4A K ma

/ min

Ma. Tensile Stress /

T

Mean Stress / m

K -.IGCma

L min

E

5nelastic Strain / i)

p

Temperature / T

Cyclic Fatigue% 2esting Parameters o" Interest ...


30/85

(lastic Modulus) CmonotonicE or CcyclicEEe

=

e

Stress #atio) R=

min

ma

tot elastic inelastic

= + inelastic plastic creep

= +where

Ma. Stress) ma = +mean

B

Min. Stress)

min=

mean

B

Cyclic oading% 4ey 1elationships ...


31/85

Total Strain 3 Elasti! Strain Range 4 lasti! Strain Range

tot e p

= +

;here andE

p

n

*=

B

B

*

totE *

n

= +

BB

*


32/85

CF% Pratt 5 Whitney e"inition ...

=ucleation to detectable crack.

5nitiation is a *4DB crack along the surface.

The acceptable probability of occurrence of

an 2!1 crack as * crack occurring in a

sample size of *--- C*4*--- or '.*E havinga *4DB inch long crack at the predicted

minimum life.


33/85

CF% Characteristics ...

1rom stress4strain cycling in the plastic

range at significantly higher stresses than for&!1.

The stress4strain cycles that cause 2!1

cracking are produced by significant engine

power level changes.

Microscopic changes in a material that has

been subjected to 2!1 cycling may be seen

after only a few cycles.Microscopic dislocations in the crystal

structure.The dislocations link up to form

cracks."epends on the stresses and

orientation of the individual grain.

&i hl statistical in nature.


34/85

CF% What are the parameters? ...


35/85

CF% $ean tress E""ects must 'e included ...

Simple approach by :. Morrow6

( ) t

u mf f f

" "

E, ,=

+ D F - *B - , - ,. . . .

Alternative approach by Smith) ;atson

lcf-hcf tutorial

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