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ENGINEERING DESIGN

BATCH 9

GURU PRASATH(10A213)LAKSHMAN(10A222)

PRASANTH(10A234)

SHREE RAMAN(10A247)

SUKI(10A253)

SRINATH(10A445)

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QUESTION

Design of component for fatigue lifecompletely reversed stress subjected to

axial load with a notch sensitivity factor for

finite life

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Understanding the question

Keywords:Fatigue life

Fatigue failure

Completely ReversedStress

Axial Load

Notch

Notch Sensitivity Factor

Endurance Limit

Strength.

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FATIGUE LIFE AND FATIGUE FAILURE

FATIGUE LIFE:

The number

of cycles of stress that

the component canwithstand before

failure(fatigue crack).

FATIGUE FAILURE:

Fatigue failure

is defined as time

delayed fracture undercyclic loading.

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Completely Reversed Stress

The completely reversed stress varies in a

sinusoidal manner with respect to time, but it

has zero mean stress. In these cases, halfportion of the cycle consists of tensile stress

and the remaining half of compressive stress,

the mean stress is zero here.

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AXIAL LOAD:

The stress applied along the length of bar eithercompressive or tensile along the axis

TORSION:

The equal and opposite force applied in abar which causes a turning moment in the bar.

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Notch

A notch is a straight or angular cut in thespecimen. With notches the strength of the

specimen is found because its useful in all

practical applications.

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Notch sensitivity factor:

Notch sensitivity is defined as the susceptibility of

a material to succumb to the damaging effects of

stress raising notches in fatigue bending.

q= increase of actual stress over nominal stress

increase of theoretical stress over nominal stress

q =Kf 1

Kt 1

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Endurance limit strength

Endurance limit of a material is defined as the

maximum amplitude of a completely reversed

stress that the standard specimen canwithstand without failure

(Se)axial=0.5*(Se)bending

Se=Ka.Kb.Kc.Kd.Se

Ka=surface finish factor

Kb=size factor

Kc=Reliability Factor

Kd=Modifying factor

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REAL LIFE EXAMPLE

A Shaft of a washing machine motor

Sut=500N/mm^2.

Notch Sensitivity Factor:0.8

Endurance Limit:200 N/mm^2

Realiabitlity:90%

Dia of the Rod:30 mm

Machining Process :Turning.

Load:300N

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Procedure for Designing

Step 1:CALCUTAION OFENDURANCE LIMIT STRENGTH

Se=Ka.Kb.Kc.Kd.Se

Ka=surface finish factor

Kb=size factorKc=Reliability Factor

Kd=Modifying factor

Se=0.5(Se)(for steels)

Step 2:CALCULATION OF PERMISSIBLE STRENGTH(Se)permissible=Se/F.O.S

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Contd

Step 3:CALCULATION OF DIAMETER

(Se)permissible=P/A

where , A=(pi*d*d)/4.

From this we can find the diameter.

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S-N Curve