# The difference between strain and strain_y

## Homework Statement

I am doing a lab. My textbook defines strain as ε= lf-lo and E = σ/ε. In my lab, E= σyy. 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.

See above.

## The Attempt at a Solution

I can only conclude I am either missing something or the equations mean the same thing, somehow.

PhanthomJay
Science Advisor
Homework Helper
Gold Member

## Homework Statement

I am doing a lab. My textbook defines strain as ε= lf-lo
that is elongation, not strain... Did you copy this down incorrectly?
and E = σ/ε. In my lab, E= σyy. 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.
correct. Lf -Lo = ΔL = elongation, and ΔL/Lo = ε = σ/E

See above.

## The Attempt at a Solution

I can only conclude I am either missing something or the equations mean the same thing, somehow.
correct the definition for strain in your text. The y subscripts then refer to specific values of stress and strain etc at yield. Without the subscripts, these are the general equations within the proportional limit.

Thank you, I understand now.