1. The problem statement, all variables and given/known data I am doing a lab. My textbook defines strain as ε= lf-lo and E = σ/ε. In my lab, E= σy/εy. εy = ΔLy/lo I do not understand how they can both = ε as lf-lo is completely different from ΔLy/lo. ΔLy is "elongation at yeild" according to my lab. Why are they both labelled E if they are different? The y subscript, according to the lab, has to do with the yield point. I was under the impression that there was only 1 elastic modulus as after the yield point, stress and strain are no longer proportional and plastic deformation occurs. 2. Relevant equations See above. 3. The attempt at a solution I can only conclude I am either missing something or the equations mean the same thing, somehow.