lec 3 fatigue failure 031004_for_students.ppt
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Chapter 6 Fatigue Failure
Theories
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Fatigue Failure Occurs when stresses are changing
throughout the life of a part.
_______________________ starts with a
crack that propagates until a catastrophic
failure occurs.
This usually begins at a manufacturing defector stress concentration that is subjected to
tensile stresses for part of its load cycle.
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Fatigue-Failure Models Stress-Life - cyclic stresses are kept
below fatigue strength or endurance
limit. Most widely used.
Strain-Life - complicated not often used.
_________________ - sometimesused gaining fa!or especially in the
aircraft and aerospace industry.
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"atigue Stresses
-#$$$$
-%$$$$
%$$$
#$$$$
#%$$$
&$$$$
&%$$$
'$$$$
0 2 4 6 8 10
Load (ycles
)pp
liedStress-ps
ia
a
a
m
m
"ully *e!ersed *epeated "luctuating
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Fully Reversed Failure
Criteria Most data comes from *. *. Moore
rotating-beam test.
+ighly polished specimen of $.' inches in
diameter is subjected to pure bending
stresses that are alternated by rotating
the specimen. *otation is at #,&% rpm.
Takes #& day to reach #$cycles.
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Endurance Limit and
Fatigue Strength /ndurance Limit of a Material 0Se12 -
stress below which fatigue failure does
not occur regardless of the number of
stress cycles.
____________________Strength of a
Material 0Sf12 -stress below which fatigue
failure does not occur for a specified
number of stress cycles.
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Typical S-N Curve for Non-Ferrous Metals
#$$$$
#$$$$$
#.$$/3$' #.$$/3$4 #.$$/3$% #.$$/3$ #.$$/3$, #.$$/3$5 #.$$/3$6
Cycles to Failure
FatigueStrength-
psi
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Estimating Seor S
f
"or ____________7
Se18 $.%Sut for Sut9&$$ ksi 0#4$$ M:a2
Se18 #$$ ksi 0,$$ M:a2
for Sut;&$$ ksi 0#4$$ M:a2
"or ______________7
Se18 $.4Sut for Sut9$ ksi 04$$ M:a2
Se18 &4 ksi 0#$ M:a2
for Sut;$ ksi 04$$ M:a2
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Estimating Seor S
fcontd
"or __________________7
Sf1
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Correction Factors to
Endurance Limit andFatigue Strength Se8 (load(size (surf(temp(reliab Se1
Sf8 (load(size (surf(temp(reliab Sf1
Se -corrected endurance limit for a part
Sf -corrected fatigue strength for a part
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Correction Factors to
Endurance Limit andFatigue Strength (load - load factor see e=. .,a pg '45
(size - si>e factor see e=1s. .,abc
pg '45-'46
(surf - surface factor see "ig1s. -&
.&, and e=. .,e pg '46-'%&
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Correction Factors to
Endurance Limit andFatigue Strength (temp- temperature factor see e=. .,f
pg '%&-'%'
(reliab - reliability factor see Table .4
pg '%'
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Stress Concentration ?f8 #3=0?t3 #2
?t - geometric stress concentration factor
?f -___________________________
= - notch sensiti!ity factor
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Fluctuating Stresses
Modified-@oodman Aiagram
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Failure Lines for Fluctuating
Stress
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!ugmented" Modi#ed-
$oodman %iagram
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Stress Concentration)pply ?f to the ______________
components of stress.
"or __________ materials apply ?tto
the mean components of stress.
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Stress Concentration -
contd Bf ?fCmaxnomC 9 Sy then7 ?fm8 ?f
Bf ?fCmaDnomC ; Sy then7
?fm8 0Sy - ?fanom2 CmnomC
Bf ?fCmaxnom- minnomC 9 &Sy then7 ?fm8 $
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Factor of Safety for Fluctuating
Stress - Case &
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Factor of Safety for Fluctuating
Stress - Case '
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Factor of Safety for Fluctuating
Stress - Case (
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Factor of Safety for Fluctuating
Stress - Case (
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Factor of Safety for Fluctuating
Stress - Case )
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Factor of Safety for Fluctuating
Stress - Case )
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Fluctuating Stresses
MultiaDial Stresses in "atigue
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Fully Reversed Simple
Multia*ial Stresses
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Fluctuating Simple Multia*ialStresses
Eon Mises Method
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$eneral !pproach to +igh
Cycle Fatigue %esign @enerate Modified @oodman Aiagram (alculate alternating and mean components
of stress at areas of concern on the part.Bnclude appropriate stress concentrationfactors.
(on!ert alternating and mean appliedstresses to alternating and mean Eon MisesStresses.
:lot these stresses on the Modified @oodmanAiagram and find the factor of safety.
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E*ample The figure pertains to the shaft of a disk sanderthat is made from steel ha!ing Su86$$ M:a andSy8,%$ M:a. The most se!ere loading occurs
when an object is held near the periphery of thedisk with sufficient force to de!elop #& F-m0which approaches the stall tor=ue of the motor2.
)ssume a coefficient of friction between theobject and the disk is $..
Ghat is the factor of safety with respect toe!entual fatigue failure of the shaftH
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E*ample