Lec 3 Fatigue Failure 031004_for_students.ppt

7/25/2019 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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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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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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Stress - Case '


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Stress - Case (


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Stress - Case (


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Stress - Case )


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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

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Lec 3 Fatigue Failure 031004_for_students.ppt