loads n failures

7/30/2019 Loads n Failures

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Things To Be Done Today:

- Types of Loads- Cyclic Stresses


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Significance of Todays Lecture

- Various ways of failure

- To avoid failure


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Types of Loads are :

1. Static Load

2. Dynamic Load

i. Cyclic Load

ii. Impact Load


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Static Load :-

- Does not vary in magnitudeor direction w.r.t time .

- It is gradually applied and once

it is applied then remains constant

.

- Ex. Dead weights of machinery, tightening up loads on bolts and

fasteners .


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Dynamic Load :-

- Varies in magnitude or in direction

or in both w.r.t time.

Types of Dynamic Loads:

i. Cyclic Load :-

- Varies in magnitude in a repetitive

cyclic manner , either completely

reversing itself from tension to

compression or oscillating about some

mean value


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- Pattern of load variation w.r.ttime is repeated again and again

for thousands of cycles .

- e.g forces induced in gear teeth

,forces acting on the compression

spring of the valves in internalcombustion engine.


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ii. Impact Load :-- applied suddenly at a high

velocity .

- energy transferred during theimpact is converted into heat .

- load with time reaches to a

static value.

-e.g punch press ,rivet gun etc.


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Static Load

YieldingPlastic deformation

after yield point

fracture

No plasticdeformation and

fail by suddenfracture


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Fatigue Failure :* Failure may occur when the load

cycles are repeated several million

times even though the stress isbelow the elastic limit.

- failure begins with a crack athighly stressed point


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* Pattern of stress variation due to

dynamic loads is irregular and

unpredictable .* 80% of mechanical failures are

due to fatigue resulting from

cyclic loads .


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3 mathematical models of cyclic

loads are :

- fluctuating or alternating loads- repeated loads

-reversed loads


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FLUCTUATING OR ALTERNATING LOAD :

- VARIES IN A SINUSOIDAL MANNERW.R.T TIME

-POSSESS SOME MEAN VALUE AND

AMPLITUDE VALUE-FLUCTUATES BETWEEN TWO LIMITS i.e

MAXIMUM AND MINIMUM LOAD

-LOAD CAN BE TENSILE ORCOMPRESSIVE OR PARTLY TENSILE OR

PARTLY COMPRESSIVE


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Repeated Load :-

-varies in sinusoidal manner w.r.t

time

-variation is from 0 to somemaximum value

-minimum load is 0 in this case

-amplitude load and mean load areequal


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Reversed Load :- varies in asinusoidal manner w.r.t. time

-it has 0 mean load

-half portion of the cycle consists

of tensile load and remaining half

of compressive load .-mean load is 0 .


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m = ( max + min) a = 1/2( max min )

* In fluctuating stresses , tensile stress

is taken as +compressive stress is taken as

* Repeated stress and reversed stress

are special cases of fluctuating stresswith min=0 and m=0 respectively.

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