design of fatigue strength

7/26/2019 Design of Fatigue Strength

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DESIGN OF

FATIGUE STRENGTH


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The fiber on the surface of a rotating shaft subjected to a

bending load, undergoes both tensionand compressionforeach revolution of the shaft.


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The shaft is therefore subjected tofluctuating stresses

Machine elements subjected to fluctuating stresses

usually fail at stress levels much below their ultimatestrength and in many cases below the yield point of the

material too.

These failures occur due to very large number of stress

cycle and are known asfatigue failure Fatigue failure begins with small crack, which may

develop..

-At the points of

discontinuityholes,notches,grooves etc!

-An e"isting subsurface crack

-#nternal defects


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Stress cycle


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$-% A'(AM$ ) *%&+(A%* #M#T


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#n order to study the effect of fatigue of a

material, arotating beam method is used

#n this method, a standard polished specimen

is rotated in a fatigue testing machine while

the specimen is loaded in bending

As the specimen rotates, the bending stress atthe upper fibres varies from ma"imum

compressive to ma"imum tensile while the

bending stress at the lower fibres varies fromma"imum tensile to ma"imum compressive


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#n other words, the specimen is subjected to a

completely reversed stress cycle.

This is represented by a time-stress diagram as

shown in Fig.


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#f the stress is kept below a certain value as

shown by dotted line in Fig. the material willnot fail whatever may be the number of cycles.

This stress, as represented by dotted line, is

known as endurance or fatigue limit (e)


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It is defined as maximum value of the

completely reversed bending stress which apolished standard specimen can withstand

without failure, for infinite number of cycles

(usually 107cycles)


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A typical plot of reversed stress $! against

number of cycles to fail %! is shown in figure


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The one below /01 cycles is considered as low

cycle fatigue, one between /01

and /02

cyclesis high cycle fatigue withfinite life and beyond

/02 cycles, the one is considered to be high

cycle fatigue with infinite life.


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Factor *ffecting the *ndurance limit

*ffect of oading on *ndurance imitoad

Factor!

*ffect of $urface Finish on *ndurance

imit$urface Finish Factor!

*ffect of $ie on *ndurance imit$ie

Factor!


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*ffect of oading on *ndurance imitoad Factor!

The endurance limit 3e) of a material as

determined by the rotating beam method is for

reversed bending load

There are many machine members which are

subjected to loads other than reversed bending

loads.

hus the endurance limit will also be

different for different types of loading.


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Let,

Kb= Load correction factor for the

reversed or rotating bending load. Itsvalue is usually taken as unity.

Ka= Load correction factor for the

reversed axial load. Its value may betaken as 0.8.

Ks= Load correction factor for the

reversed torsional or shear load. Itsvalue may be taken as 0.55 for ductilematerials and 0.8 for brittle materials.


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*ffect of $urface Finish on *ndurance imit$urface Finish Factor!

When a machine member issubjected to variable loads, theendurance limit of the material for

that member deends uon thesurface conditions


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*ffect of $ie on *ndurance imit$ie Factor!

#f the sie of the standard specimen is increased,

then the endurance limit of the material will

decrease. This is due to the fact that a longer

specimen will have more defects than a smaller one.


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Goodman criterion


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!oderberg criterion

design of fatigue strength

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