# Another one about Yield Stress....

• Jens Wensing
In summary: JensIn summary, according to this thread, the stress/strain diagram is not the same as force/elongation. The force/elongation graph is linear, but the slopes of the 2 graphs are quite different. The yield stress at 0.2 percent strain can be determined using the force - elongation graph, and the elongation at 0.2 percent length can be found using the force at that elongation.
Jens Wensing
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

Jens Wensing said:
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.

PhanthomJay said:
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 referred 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

## What is yield stress?

Yield stress is the amount of stress a material can withstand before it begins to permanently deform or yield.

## How is yield stress determined?

Yield stress is typically determined through mechanical testing, such as tensile or compression tests, where the material is subjected to increasing amounts of stress until it begins to yield.

## What factors affect yield stress?

The yield stress of a material can be affected by various factors, including its chemical composition, microstructure, temperature, and strain rate.

## Why is yield stress important?

Yield stress is an important mechanical property of materials as it determines their ability to withstand loads and stresses without permanent deformation or failure. It is also used in design and engineering calculations.

## Can yield stress be changed?

Yes, the yield stress of a material can be changed through various methods, such as heat treatment, alloying, and cold working. These processes can alter the microstructure and chemical composition of the material, thus affecting its yield stress.

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