Design for fluctuation loads

DESIGN AGAINST FLUCTUATING LOADSPresenting by-

Deepak Dolakasharia Deepak Dolakasharia (107)(107)Rijuan Hussain Rijuan Hussain (134)(134)Niraj Kakati Niraj Kakati (142)(142)Rupjyoti Baruah Rupjyoti Baruah (263)(263)Feroz Ahmed Mazumder Feroz Ahmed Mazumder (017)(017)

for the subject-

Don Bosco School of TechnologyDon Bosco School of Technology11..

Stress concentration and factor, Notch sensitivity.Stress concentration and factor, Notch sensitivity. Reduction of stress concentration & Fluctuating stresses.Reduction of stress concentration & Fluctuating stresses. Endurance limit.Endurance limit. Soderberg, Goodman lines, Gerber equations & Modified Soderberg, Goodman lines, Gerber equations & Modified

Goodman diagrams.Goodman diagrams.

Wednesday, May 3, 2023

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Introduction


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What is Stress What is Stress Concentration Concentration ??

Technique called Photo-elasticity.

Localisation of high stress due to irregularities and abrupt change in size.

Stress concentration factor is used, Kt or Kc


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Why Stress Why Stress concentration concentration ??

Variation in properties of materials,

Load applications, Discontinuities in

the component, Machining

scratches, Abrupt change in

cross sections,


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How to reduce stress How to reduce stress concentration concentration ??

—By providing a specific geometric shape to the components through understanding the flow analogy.

Additional notches and holes in tension members.

Fillet radius, undercutting and notch for bending member.

Drilling additional holes for shaft.

Reduction of stress concentration in threaded members.


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Fluctuating stressFluctuating stress Forces subjected are not static but, vary with time called fluctuating stress, which

causes fatigue failure. For design analysis stress-time relationship is used, like sine curve.

or


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Fatigue failureFatigue failure Observed that materials fail under

fluctuating stress at stress magnitude, lower than ultimate tensile strength.

Even lower than yield stress sometimes.

Magnitude of stress causing Fatigue failure decreases as the no. of stress cycle increase.

This phenomenon results in fatigue failure.

Is defined as time delayed fracture under cyclic loading.

It depends upon no. of cycle, mean stress, stress amplitude, corrosion, creep etc

Played with a small wire during childhood, but didn't know Played with a small wire during childhood, but didn't know the reasonthe reason


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Fatigue Fatigue testtest

Beam of circular c-s subjected to bending moment.

The amplitude of this cycle is given as-


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Endurance limitEndurance limit Definition. 10,00,000 cycles is sufficient

for endurance limit. Fatigue life. Fatigue failure testing done

in Rotating Beam testing machine.

S-N curve. S-N curve for steel. Endurance limit not exactly a

property of ultimate tensile strength.


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Low & High cycle Low & High cycle fatiguefatigue

Earlier S-N curve drawn was for small cycles.

There are two main regions.

Fatigue failure at less than 1000 cycles is low cycle fatigue and

& at more than 1000 is high cycle fatigue.

Failures like setscrews, studs on truck, etc are e.g. of low cycle and spring, ball bearings are high cycle.

Low cycle involves plastic yielding.


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Notch SensitivityNotch SensitivityNotches are stress riser

Elastic stress concentration factor,

the susceptibility of a material to succumb to the damaging effects of stress raising notches in fatigue loading.

when the material has no sensitivity to notches, q=0; Kf = 1 andwhen the material is full sensitive to notches, q=1; Kf = Kt

oror


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Endurance limit Endurance limit calculationcalculation

SSe e = K= Ka a * K* Kb b * K* Kc c * K* Kd d * S* See''

Se' = Endurance Strength of material specimen under laboratory conditionSe = Endurance Strength of material specimen under actual running condition

Two separate notations are used for endurance limit,

Ka = Surface Correction factor Kb = Size Correction factor Kc =Reliability Correction factor Kd = Temperature Correction factor

Size factor Ka

Kd is calculated as

Se’ has different value for different materials


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Design for finite & Design for finite & infinite lifeinfinite life

Design for completely reversed stress are divided into finite and infinite life fatigue.

For infinite life endurance limit is the criterion.

Amplitude stress < endurance limit for longer no. of cycles.

For finite life S-N curve is can be used.


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Soderberg, Goodman, Soderberg, Goodman, GerberGerber

GERBER LINE- A parabolic curve joining Se on the ordinate to Sut on the abscissa is called Gerber line.

SODERBERG LINE- A straight line joining Se on the ordinate and Syt on the abscissa is called the Soderberg line.

GOODMAN LINE- A straight line joining Se on the ordinate and Sut on the abscissa is called Goodman line.

It is seen that mean stress has an effect on fatigue failure when with stress amplitude.Using straight line

equations Soderberg, Goodman equations and Gerber will be-


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Modified Goodman lineModified Goodman lineComponents subjected to fluctuating axial or bending stresses. Components subjected to fluctuating torsion

shear stresses.


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Materials select & an Materials select & an ExampleExample

Steel and Cast ironAluminum alloysPlastics

T H A N K

‘U’


EVERY ONE

Engineering

Design for fluctuation loads