Hi Dhiviya,
Here the answer for your question.
What is stress? Every one say Load by Area. Actually it is internal resistance per unit area. Question is not about the applied load? How it was resisted.After attending interview your parents will not ask what they have asked? How you have answered? So, for calculating stress we must take internal resistance only. But there is no device to measure the value of internal resistance. So we assume that the internal resistance is equal to applied external load. Since this assumption is disqualifying the stress to put in x-axis.
Let us assume the cross section of test bar is 10 unit. We are applying the the load 1 kg, 2 kg like that. What will the stress at 1 kg? You will say 1 by 10. So the stress is 0.1. At 2 kg will it be exactly 0.2. No. Because after applying 1 kg load the dia will be slightly less than 10. It may be 9.99. Like that while going on increasing the load the dia will get reduce. But we are calculating just divided by area 10 and say 0.1,0.2,0.3,0.4 etc. This is an error.
So this error is disqualifying the stress to put in x-axis.
So we applying the load to get strain. The strain can be measured directly from out side by more than one person with the help of some scale fitted in the machine. So , there will be no error or assumption in measuring the strain. Actually we are not measuring strain, measuring change in length and calculating strain.
So, here strain is independent and placing in x axis. From the strain we are calculating stress.
For any further doubt contact me vps_kanna@yahoo.co.in. 10 yrs. experiance in material testing in Defence Research organisation.
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OK
VP Srinivasan
Ambattur
Chennai

Answered by VP SRINIVASAN at Jul 16, 2011 08:41 AM

Hai Divya,

According to me, X - axis is horizontal, and Y- axis is vertical.Generally CAUSE is plotted on x-axis, and EFFECT is on y-axis.and also I think we can interchange the axis. In the experiment, it is possible that controlling the stress and measure strain. But the diagram maybe changed.

If u have still now doubts,mail me to monoiyappanmani@gmail.com.

Answered by manoiyappan at Mar 25, 2011 06:38 AM

In any graph on horizontal axis we keep the independent variable and on vertical axis the dependent variable. similarly in stress strain diagram the strain is independent variable and stress is dependent variable so due to this reason strain is kept on x-axis and stress is kept on y-axis.

We take stress on the Y-axis and strain on the X-axis. On X- axis we take independent variable and on the Y -axis, the dependent variable i.e y is dependent on x, Means, you change a quantity on X and its effect is seen on Y. Here stress is dependent on strain. Actually here stress means the inter-atomic force between the atoms. This interatomic force depends on the distance between the atoms which will change by change in length ie by strain. So the stress is dependent on strain. So Stress is a dependent variable and should be taken on Y- axis with strain the independent variable on X- axis.

Though it is also said that taking stress on y and strain on x axis is conventional and we are so used to it that the convention seems more of a rule now.

See also: True Stress & True Strain | Engineering Stress-Strain Curve

Answered by Haseeb Jamal at Mar 25, 2011 11:11 PM