Help Concerning Nominal Strain and Stress

[Pheijei]

Homework Statement

This is just a simple questions concerning the method of obtaining nominal stress and nominal strain. From various sources on the net they refer to nominal stress/strain as engineering strain
therefore can I just use the simple equations for stress and strain?

In this instance the material I'm looking at is rubber and I've assumed it is a hyper elastic material. I've elongated the sample piece (for simplicity's sake assume the sample is a square) until failure and recorded such data as Force,elongation,stress.

So for this instance how would I determine nominal stress and nominal strain?

Homework Equations

The Attempt at a Solution

Thanks

 

[rock.freak667]

You would use 'nominal' stress/strain once you assume that whatever you're doing the calculation on is unformly distributed.

 

[Pheijei]

For this case yes it is uniformly loaded.
So it is okay to use values obtained from the test? (Stress, Strain)
Even though during the test the cross-sectional area is decreasing from the elongation?
Thanks

 

[PhanthomJay]

You say you recorded force, elongation, and stress. The force is what it is and the strain/elongation is what it is. Engineering stress is force divided by original cross sectional area, even when the the cross section starts to neck down considerably at high loads. True stress is force divided by actual cross section area at the particular elongation point, and is thus more than the engineering stress at the high load levels. So depending on how you define stress, you get different stress -strain curves. If you are just recording force and elongation during the test, it is easier to use engineering stress, since the original cross section is known, if you are given that choice.