# Determining elastic limit for a nonlinearly elastic material, to be used for calculating chord modulus (i.e. Young's modulus) in tensile testing

I have a protein material that is nonlinearly elastic; from the get-go, the stress-strain curve shows quite an immediate curvature.

I have to use chord modulus (reference to ASTM E11-04).

The problem! How should I choose the 2nd point to get the chord modulus, by plotting from origin to this 2nd point? The 2nd point should be the point of elastic limit; how do I find it?

My solution:

1. I calculate the chord modulus at increment of 1% strain;
2. I compare the mod calculated using 1% strain as elastic limit, and the mod calculated using 2% strain as elastic limit;
3. if the difference is <5%, then I continue my increament to 3%, get the mod, and compare this new mod to the mod calculated using 1% strain;
4. I keep doing this until the difference is >5%; then that strain will be my elastic limit.

I ran this for quite a few samples and most of them have elastic limit (determined by my method) to be either 4 - 6 %.

Is this the right way to go about it? Does anyone has reference, papers, standards etc that show how to determine the elastic limit of nonlinearly elastic materials? Or how should I explain it?

• You cannot estimate the elastic limit without running loading-unloading experiments. Those tests have been run extensively for common metals/alloys and standards developed (so you can get estimates for a loading curve). But that's not the case for new materials. Sep 30, 2021 at 19:55