As elongation is produced due to load in σ-ε curve then Why stress,σ is plotted on Y-axis& strain,ε on X-axis?

Question

I want to know that why stress is usually plotted on Y-axis and strain on X-axis. In actual we draw the curve between taking load on Y-axis and extension on X-axis. if we see carefully then we will find that extension is produced after the load has been applied to a material. so that means load is independent varaible and extension is dependent variable. AS indenpendent variable is always taken on X-axis and the dependent variable is taken on Y-axis then it means stress should be plotted on X-axis and strain on Y-axis but in actual it is reversed. Why ????

Answer 1

Neither variable can unequivocally be called independent or dependent. One way stress vs strain is measured in a testing lab is to clamp the ends of a sample in (what used to be called) a "Scott tester" which gradually pulls the clamps apart. The strain is being applied, and the operator reads the stress from a force gauge in series with the tension.
It's analogous to measuring resistance; you can apply a constant current and measure the voltage, or a constant voltage and measure the current, or neither (say, a voltage in series with an internal resistor) and measure both voltage and current. What's the independent variable in this case?
In situations like the above, where there is no clear cause-and-effect relationship, either may be considered the independent variable, although the natural tendency would be to use the variable actually manipulated by the experimenter.
But in an unambiguous case, like weight of impact fragmentation pieces vs drop height, the experiment can only be carried out one way. Then it would look and feel unnatural to consider the impact results as the independent variable. (Especially if the experimenter were to write "... we varied the impact particle weight over the range ... and measured the resulting drop height" !)