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Low Cycle Fatigue (LCF)
High Cycle Fatigue (HCF)
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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.
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Time
"esign#e$uirements
Material%roperties
&istorical 'asic (ngineering
%roperties
Strength)
!reep
*+,-s / *+0-s Add ... 1atigue &!1) 2!1) TM1
2ate *+0-s Add ... "amage
Tolerance
!rack 3rowth
Requirements have evolved for Gas Turbine Engines....
Emphasis today is on Cyclic Properties...
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&igh !ycle 1atigue Allowable vibratory stresses
2ow !ycle 1atigue !rack initiation life
*4*--- to small crack
!omponen
retirement
!rack 3rowth #emaining life from crack
Safety
inspection interval
5nspection
size re$uirement
Emphasis today is on Cyclic Properties...
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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
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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
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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
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+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
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Cyclic #s. $onotonic Cur#es% &eha#ior can 'e signi"icantly di""erent ...
1rom Metal 1atigue in (ngineering) &.9. 1uchs and #.5. Stephens) :ohn ;iley ) *+?-
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Crack ie% Ho! 'ig is 'ig? ...
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HCF% (*+ Cur#es ...
5nitially used to address &!1 for allowable
stress) but what about predicting actual cycles
of life@ ...
&!1 cycle prediction is more of a statistical
estimate with a large scatter allocation)
instead of an eact science
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*2$ Stress Control (C+ Test $pparatus
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Specimen Fully Reversed Stress/Strain Cycle S/N Plot
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Alternating Stress Amplitude6
a =ma minB
Mean Stress6
-B
= +ma min
Stress #atio6 R =
min
ma
Stress #ange6
= ma min
Basic Cycle
Terms to Remember
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Soderberg C7SA) *+D-E a
e
m
y" "
+ = *
3oodman C(ngland) *?++E a
e
m
u" "+ = *
3erber C3ermany) *?0FE a
e
m
u
" "+
=
B
*
#$here "eis the fully reversed endurance limit.%
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Cyclic e"ormation Parameters% Fatigue loop illustration ...
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Fatigue% Ho! do HCF and CF "it !ith
tress #s. i"e? ...
G (ists in theor onl
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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.
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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.
%
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HCF% - Christienson ,iagram Contains all o"
this in"ormation ...
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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.
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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.
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HCF% Elastic tress*i"e 1elationship ...
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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
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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.
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CF 2esting% 3eri"ication ...
Three primary ways of verification testing6
Subcomponents
Spin %it
1erris ;heel
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*2$ Strain Control LC+"T'+ Test $pparatus
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CF 2esting% 2ypical set*up in#ol#es
uniaial loading ...
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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 ...
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(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 ...
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Total Strain 3 Elasti! Strain Range 4 lasti! Strain Range
tot e p
= +
;here andE
p
n
*=
B
B
*
totE *
n
= +
BB
*
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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.
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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.
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CF% What are the parameters? ...
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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
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