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kiruthika_pechimuthu
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PostPosted: Tue Aug 02, 2016 7:59 am    Post subject: steel Reply with quote

hi sefians,
              Can anyone explain why stress is plotted in y axis and strain in x axis? how stress is dependent on strain. we are taking the strain values after applying loads. then howcome the strain is an independent value.
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es_jayakumar
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PostPosted: Wed Aug 03, 2016 5:57 am    Post subject: Reply with quote

Er.Kiruthika,
My observation on your query is as follows :
We are carrying out a tension test or compression test using a Universal Testing Machine (UTM), that you might have used in the Strength of Materials Lab. The UTM transmits the load to the test specimen using a Rack and Pinion system. In this, it is the elongation (or strain)that is controlled using the screw type movement system, and not the load. In other words, the load (or stress) that the specimen is subjected to, is strain dependent, here. Hence, the independent variable, strain, is plotted along the X axis and the dependent variable, stress along Y axis. In a nut shell, we are increasing / decreasing the strain on the specimen and not the stress. Stress will vary in keeping with the modulus of elasticity at the particular strained level.
Other members may please post their comments on this.

E S Jayakumar
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kiruthika_pechimuthu
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PostPosted: Tue Aug 09, 2016 9:23 am    Post subject: steel reg Reply with quote

E.S.Jayakumar sir,
                Thank you sir. But still I have a doubt. We are applying the load first and then we are observing the elongation using extensometer. The elongation is dependent upon the rate of loading. In such case, How can we say strain is independent. Please reply me sir. Sorry for not understanding well sir.
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es_jayakumar
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PostPosted: Tue Aug 09, 2016 9:54 am    Post subject: Reply with quote

Please see the structure of a UTM (sketch attached). It is clear that the strain is increased or decreased in the machine using  worm gear threaded rod mechanism. Even when the crushing or yielding of the test specimen starts, the strain cannot get reduced, but the internal stress will get reduced according to the yielding property of the material. From this, it may be clear that we are varying the strain on the specimen. It is not that we are increasing the stress (load) on it by imposing incremental load.

E S Jayakumar



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vegad
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PostPosted: Thu Aug 18, 2016 12:20 am    Post subject: Reply with quote

Dear Member

Consider an easy example in terms of a working person.
Say material specification = personality of a person which is invisible;
Elasticity = Nature of a personality, which is invisible.
External circumstances = Visible Loads (Tensile, Compressive, Temperature, Vibration)
strain = Representative factor of the visible Behavior of the personality

Note that there are only two visible factors - Behavior (strain) and external circumstances (Loads and rate of load);

So a person can be upset (strained) because either there is more work demanded (Tensile loads) or tight deadline along with non-cooperative supervisor (Tensile loads due to contraction due to drop in temperature with fixed supports).

In both the cases above what is visible is the change in behavior (strain), which is further used to know the nature of the person (elasticity).

So if material gets strained it may be due to a tensile force or temperature loads. Strains can be known without knowing the cause (Loads), hence they are termed independent. You are correct they are related to loads, but to know them we do not need to know any other parameter, except change in length and original length.

In cases of strain-hardening, the person will not allow his stress to increase and try as much as possible to adjust reflected by effects on his health and family life due to work. On further increase of strain, the persons leaves the job or die of over work (it happens in far east-Karoshi)

Its funny example, and hope your queries on relations and dependency of each parameter are satisfied.

All the best.
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rkyadav
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PostPosted: Thu Aug 18, 2016 5:56 am    Post subject: Reply with quote

The Concept of stress is philosophical which can neither be seen nor measured directly. Deformation can of course be seen and measured and the stress is estimated indirectly from the deformation.
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es_jayakumar
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PostPosted: Fri Aug 19, 2016 12:49 am    Post subject: Reply with quote

We can increase the stress also. Take the example of a steel wire fixed at one end stressed by adding weights at its tip. We can measure the elongation / strain comfortably and plot the strain Vs stress graph, in the elastic range. But once the yielding starts, it will break and hence the strain-stress characteristics during yielding cannot be sensed.

E S Jayakumar
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muraharikrishna
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PostPosted: Sun Oct 16, 2016 4:05 pm    Post subject: Reply with quote

Stress  develops internally due to strain.
So Here Strain is independent and stress is dependent.

So Strain is on X-axis, Stress is on Y-Axis.



-Muraharikrishna
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vsmkumar
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PostPosted: Thu Oct 20, 2016 12:22 pm    Post subject: Reply with quote

the main doubt of kiruthika is we in UTM load is applied then why strain is an independent quantity.
i feel her doubt is valid.
1.Strain is resultant of stress hence with out application of load there is no strain- i think every body agrees.
2.strain is visible and directly measurable quantity.
3.Stress is abstract quantity i.e. we can feel it but we cannot see it.
4.Stress can be measured indirectly by its consequences only. like displacement,strain etc.
Then how come we measure stress of specimen with respect to strain in it?
1.In UTM or any other machine we can develop force or pressure by  hydraulic or mechanical means.
2. The above developed force is measured by suitable instrument and displayed as applied force.
3. The developed force or pressure not directly applied to specimen.
4.The developed force or pressure transferred to specimen support plates by screw or gear mechanism.
  ( remember car jack lift  we apply force car is lifted).
5.The screw rotations and in turn vertical displacement can be measured or strain guage measurement taken.
6. The above displacement gives strains and at any instance for this strain the applied  force as described at point-2 above  can be  
read .This force/ area =applied stress.
7.Remember the force  at point-2 is not proportional to strain at  at point-5.
  Remember the force  at point-2 is proportional to reaction/resistance of specimen.
  hence the reading of the force  at point-2 can not be controlled as as it depends on resistance of specimen.
8. but the strain at  at point-5 is progressive and can be controlled.
so we apply controlled strain and measure internal stress.
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