What is the equation of this stress-strain curve?

[wh88]

For this nonlinear hardening stress strain curve, how do you express strain in terms of stresses ?

 

[Nidum]

Arguably you already have .That plot tells you everything .

If you mean though how to write it down using algebraic functions then you have to do it in sections . For each of the line segments on that plot identify the start and end values of ε for that segment and find a linear function which is valid for that line segment .

You then have a list of linear functions matched with a list of the intervals over which they are valid .