# Plane Stress Problem

I have a question about plane stress condition in calculix. I am trying to create simple holed plate tension analysis in calculix for plane stress condition. I setted a section to body with a thickness of 5mm. And applied 10MPa uniform pressure. When the section thickness is changed, the results that I have obtained from FEA result is changing. I use uniform pressure command, does this command account for the change in thickness? When I created same analysis in Ansys, change in thickness doesn’t cause a change on stress results.

Units are crucial. If you apply the pressure as force per surface area then with the change in thickness, the surface area also changes so there’s a different total force.

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of course it changes with thickness if you load using pressure, if you do not want it to change use surface traction in prepomax for example, in cgx you need to take into account actual thickness to convert pressure into edge load (Force/Length)

We are already applying pressure. When we do the same analysis in Ansys, why does it not affect the result?

pretty sure you aren’t applying pressure in Ansys, you are applying edge load (Force/Length). Check the manual.

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In my model, I use the pressure boundary condition for the plane stress situation in ansys. In this case, can we say that Ansys and Calculix apply the pressure boundary condition differently ? Additionally, does Ansys handle this situation automatically?

Pressure load ? Ansys uses units so it should be pretty clear how the load is applied. If not by looking at the input field in Workbench then by checking the documentation (especially if you are using APDL).

You could just model this in 3D with 2 different thicknesses and check the results. Unless the load in Ansys is a special type for 2D but load in force per unit length units is more common for beams.

This is independent of the unit. When we pull a plate with 100 MPa, we already read 100 MPa as a reaction. You can see this analysis by doing it in Ansys. Additionally, when you change thickness for plane stress in the same analysis, the results do not change in Ansys, but calculix also changes.

I don’t use Ansys, only Abaqus on which CalculiX is based. Can you share some screenshots from
Ansys showing the setup and results ?

Please, tell us what ansys interface at yo using and the command you use to apply loads. As you are dealing in ansys with true 2d elements you are probably applying edge load even if it’s described as pressure in the interface

I found the image on the internet. I give a pressure boundary condition as here. In my analysis ı have 2d plate only.

I agree with ademyilmaz it should not change.
Check if your BC are compatible with Plane Stress.
Ccx has an special way to deal with Plane stress that differes from real shell elements.
so user should pay extra care to asure the expansion is freely done.
Ask for OUTPUT 3D to check if thats the case in your model.

EDITED: Recomendation. Keep the thickness as small as possible to be closer to fullfill the Plane Stress Condition.

Do you have access to full Ansys documentation ? It’s not as comprehensive as Abaqus one but there should be some clear info about the usage of the pressure load in 2D case.

The issue we need to discuss here is how it is applied in Calculix and why the results change. In Prepomax, when you fix a simple plate on the left side and apply uniform pressure on the right side, you get different results when you enter 0.2 mm and 20 mm for different section thicknesses. In addition, the stresses in the Z direction should be 0 as it is a plane stress condition, but unfortunately this is not the case in Calculix. I’m investigating the reason for this.

and what interface are you using in CCX? CGX or prepomax? or directly writing CCX keywords?

I m using *DLOAD keyword. I have pressure value. In addition, I experimented with both Prepomax and the codes I wrote myself.

You can always check the total reaction force at the fixed side to know what total force is applied.

the manual is not clear in this point but I believe that DLOAD applied to a plane stress element is pressure as well (Force/Length^2) so if you change the thickness you increase/decrease the area for the same pressure so the total load changes. I do not think DLOAD applied to plane stress elements should be understood as Force/Length

Yes, I’m sure it works like this.

Edge loading is only provided for shell elements. Its units are force per unit length.

The underwater pressure at one-meter depth is the same everywhere no matter if we measure it in a pool or the ocean.(Assuming same surface elevation and water density)
Same pressure=same stress, no matter the surface.

Regarding the reason of Stress difference. Poisson’s ratio.
Set up your shell as ortothropic with zero Poisson ratio in the z direction and szz (your problem) will disappear.