The difference between strain and strain_y

[1question]

Homework Statement

I am doing a lab. My textbook defines strain as ε= l_f-l_o and E = σ/ε. In my lab, E= σ_y/ε_y. ε_y = ΔL_y/l_o

I do not understand how they can both = ε as l_f-l_o is completely different from ΔL_y/l_o. ΔL_y 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.

Homework Equations

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]

Homework Statement

I am doing a lab. My textbook defines strain as ε= l_f-l_o

that is elongation, not strain... Did you copy this down incorrectly?

and E = σ/ε. In my lab, E= σ_y/ε_y. ε_y = ΔL_y/l_o

I do not understand how they can both = ε as l_f-l_o is completely different from ΔL_y/l_o. ΔL_y 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

Homework Equations

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.

 

[1question]

Thank you, I understand now.