Expt 05 Fatigue Test

8/13/2019 Expt 05 Fatigue Test

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Experiment- 5:

To Study the Fatigue Behavior of Mild Steel

Objet of the experiment

To study the effect of fluctuating stress normally encountered in the cyclic loading ofmaterials in service.

!e"uirement# for the experiment

a) Specimen with the correct design

b) Vernier calipers

c) Dead weight as loadd) Wrench for tightening the bolt of specimen holder

5$ Brief de#ription of the e"uipment%mahine

The schematic diagram of the fatigue testing machine is shown in Fig.1. t

consists of a !"phase motor with #$%% rpm speed. The machine is designed to carry outtesting of two specimens simultaneously. The samples for fatigue test can be of three

types as shown in Fig.# depending upon the loading scheme provided by the machine.The specimens can be either cyclically loaded in the a&ial manner 'Fig.# (a) or in a

rotating manner 'Fig.# (b) and (c)

Fig$& Shemati diagram of Fatigue te#ting mahine

*oad +n and +ff

Switches

Sample holder ,otor



Fig$' (oading #heme# for laboratory #ale fatigue te#ting: )a* +xial loading of the

#peimen, )b* #ingle-end rotating antilever te#ting mahine and )* Four-point

rotating antilever te#ting mahine

mportant .arameter# and E"uation#

- fluctuating stress cycle can be considered to be made up of two components

a mean or steady stress /m and an alternating or variable stress /a. We must first consider

the range of stress /r . -s can be seen from Fig. !a 0 !b the range of stress is thealgebraic difference between the ma&imum and minimum stress in a cycle. Thus

/r /ma& 2 /min

The alternating stress is one half of the range of stress.

/a #

//

#

/minma&r

−=

The mean stress is the algebraic mean of the ma&imum and minimum stress in the cycle.

#

///

minma&

m

+=

Two other parameters are also used for representing fatigue data3

Stress 4atio (4) ma&

min

/

/-mplitude ratio (-)

4 1

4 1

/

/

m

a

+

−=



For a fully reversed stress cycle as shown in Fig.! (a) the Stress 4atio 4 is "1

and if the stresses are partially reversed 4 becomes a negative number less than 1. f the

stress is cycled between a ma&imum stress and no load the stress ration becomes 5ero. fthe stress is cycled between two tensile stresses the stress ratio becomes a positive

number less than 1.

Fig /$ Typial fatigue #tre## yle#$ )a* !ever#ed #tre##0 )b* repeated #tre##0 )*

irregular or random #tre## yle

The results of fatigue crac6 initiation tests are usually plotted as ma&imumstress minimum stress or the stress amplitude on (y"a&is) against the number of cycles to

failure 7 (on the &"a&is). The number of cycles to failure is generally plotted on the

logarithmic scale while stress is plotted either on the linear or logarithmic scale.

The regime in which the pea6 load is above the yield strength of the material is

referred to as the low cycle fatigue. 8omponents usually endure 91%: cycles during lowcycle fatigue. n contrast when the pea6 cyclic stress is below the yield strength of the

material the component undergoes more than 1%: cyclic reversals and the regime is

referred to as the high cycle fatigue. Fig.: depicts some of the general characteristics of

fatigue.



Fig5$ Some of the important harateri#ti# of fatigue

The pea6 stress in case of cantilever bar testing is obtained by the following formula. For

the four"point cantilever bending the pea6 stress /ais given by

!a

;d

!#,b/ =

Where ,b is the bending moment <=# d is the diameter of the sample < is the appliedload * is the length of the sample.

For the single"end cantilever testing

!a

;d

!#<&/ =

Where & distance along the length from the fi&ed end and ma&imum value of & is 1

Experimental .roedure

a) <olish the sample surface as smooth as possible and observe for any surface

defects and deep scratch=machining mar6s. 4e>ect the sample if you find any

defects. b) ,easure dimensions of the given specimen of mild steel.

c) Fit the specimen is in the sample holder such that it passes through the opening

provided in the rod on which the loads are seated.d) -fter fitting the sample 6eep the desired load on the seat provided for the loads.e) Switch on the instrument to conduct the fatigue test and record the time for the

failure when it occurs.

f) 7ote the appearance of the fractured surface in each case.

Experimental 1ate 2olletion and .re#entation

Expt 05 Fatigue Test

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