Elasto Plastic Yield Point / Youngs Modulus


When inserting the values for the true stress vs strain curve for elasto plastic analysis should the youngs modulus be equal to the materials youngs modulus in the plastic zone?
I.e when inserting the excel file, Simscale will calculate it automatically based on the yield point (first row).
When looking at a stress strain curve the yield points Youngs Modulus will be greatly different compared to the Youngs modulus of the material.

Using Ramberg-Osgood equation from website: Stress-Strain Curve Calculator | MechaniCalc this can be seen / calculated.

So my question is, is the yield point youngs modulus the correct value?

Hello btaylo201,

Please have a look at this page on the definition and assignment on elastoplastic materials for SimScale:

Best regards

I have been using this page but still unsure.
Should σyield /ϵyield = E (at yield)? Rather than the traditional Hookes Law area?

from what i would see I would use the youngs modulus provided by the Hooke’s law.
Also, I would think that the difference will not be that huge since most of the explanatory diagrams are mostly over-exaggerating the part between the yield point and the linear point.

Best regards

I would also think to use the modulus provided by Hook’s Law, but when you insert the excel file with the first point being your yield point, Simscale automatically calculates the Young’s Modulus.

For example,

Defined elastic modulus = 28000000 psi
At yield point elastic modulus = 70,000 / 0.004500 = 15,555,555.5 psi
I know i would still need to convert these values from engineering stress to true stress but would produce similar result.

Would the modulus described as the E=σyield / ϵyield not be described as the secant modulus?

Hi there!

Here to add my two cents: notice how your last picture illustrates the situation

Bear in mind that the proportionality limit and the yield point are not the same. There is a region of non-proportional elasticity before the yielding. In other words, the deformation follows a process where first the stress is proportional to the strain (Hook’s law), then it is not anymore, although upon release of the load, there will not be residual strain. This behavior ends at the elastic limit, and after this point there will be residual strain.

That being said, the Elasto-Plastic model in our solver assumes that the proportionality limit and the elastic limit are the same, which differs from the ramber-osgood model that your are using, where an offset is assumed between this points. This is why the first point in the plastic region curve must coincide with the ( \sigma = E \varepsilon ) line, and the solver even enforces this.

The consequence is that the first point in the curve should be the proportional limit and that the small region of non-proportional elasticity is not captured, as after this first point the yielding starts.


Thanks for the response. I think this has cleared it up. I plan to use the below formula but exclude the 0.002 to find the associated strain at the yield point to give me the correct youngs modulus value.

And the following equation for the strain value at ultimate.

I can then insert these into simscale, although less accurate will hopefully provide similar result.
Let me know agrees or disagrees but hopefully im on the right track now :slightly_smiling_face:

Sounds good to me!

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