# Another one about Yield Stress...

Hi Forum,
I´m happy I found this forum...
I hope someone can answer my (simple) question:
According to this thread https://www.physicsforums.com/threads/calculating-0-2-proof-stress-of-aluminium.206648/ on page 2 the stress/strain diagram is not the same as force/elongation.
Due to the constant area surface and the constant start length, the graphs look the same.

Now the question:
Am I wrong when I use the force/elongation graph for further studies? (e.g. Yiel point 0.2% determination). Or did I miss another influence?

Background:
We build bending machines and need to find out the Yield point of mainly Aluminium material. The parameters we have can gather are:
- Force
- position (length)
- Time (Speed)
- We can input length and areal surface in a programmed mask

Thank´s a lot and excuse my maybe too simple question...
Jens

## Answers and Replies

PhanthomJay
Science Advisor
Homework Helper
Gold Member
Hi Forum,
I´m happy I found this forum...
I hope someone can answer my (simple) question:
According to this thread https://www.physicsforums.com/threads/calculating-0-2-proof-stress-of-aluminium.206648/ on page 2 the stress/strain diagram is not the same as force/elongation.
that is correct, they are not the same
Due to the constant area surface and the constant start length, the graphs look the same.
they both are generally linear, but the slopes of the 2 graphs are quite different, however.
Now the question:
Am I wrong when I use the force/elongation graph for further studies? (e.g. Yield point 0.2% determination). Or did I miss another influence?
in determining the yield stress of the material at 0.2 percent strain, you can use the force - elongation graph, and determine the elongation at 0.2% length, then find the force at that elongation. The force must then be divided by the cross section area to get the yield stress at .2 % strain
Background:
We build bending machines and need to find out the Yield point of mainly Aluminium material. The parameters we have can gather are:
- Force
- position (length)
- Time (Speed)
- We can input length and areal surface in a programmed mask

Thank´s a lot and excuse my maybe too simple question...
Jens
I assume you are measuring force and elongation in a tensile testing apparatus? And it is not surface area, it is cross section area you want to use.

that is correct, they are not the same they both are generally linear, but the slopes of the 2 graphs are quite different, however. in determining the yield stress of the material at 0.2 percent strain, you can use the force - elongation graph, and determine the elongation at 0.2% length, then find the force at that elongation. The force must then be divided by the cross section area to get the yield stress at .2 % strainI assume you are measuring force and elongation in a tensile testing apparatus? And it is not surface area, it is cross section area you want to use.

Thank you very much for your reply.
The test happens in a production machine, where we have the possiblitiy to measure force (calibrated suitable for our needs) and displacement (elongation).
Of course I refered to cross section area, that had been lost in translation ;-)
We have input possibilities of the cross section area of the used profile.

So I can go on with the programming.
Thanks again