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Question about Units, and more...

sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
edited July 2022 in Simulation
Hello everyone, 

I'm a mechanical engineer from Germany and I use N, mm, kg as units in my simulations.
I just started using the Simulation EVP and I have a few questions.

I created a simple part and fixed the left face and applied a downward force of 10000N on the right face:

When I type in the 10000N it automatically converts it to lbf. To see the original value, I have to select it again.

  • Can I prevent the conversion or is this normal because my document units are set to "mm"?

When I run the simulation I get some stress two red lines in the radius with the highest stress:

So my first guess is that the elements are not small enough to calculate the area with the highest stress.

  • Is there a way to see the mesh or the element size?
  • The units in the legend are missing. I guess it is (psi), correct?

In the next step I split the surface in Onshape to force the simulation to use smaller elements:

Now the stress distribution looks very good to me!

I also compared the result with SimSolid with the highest settings (which took much longer to run than Onshape):

  • Is there a way to change the element size or influence the mesh convergence in Onshape? 

I know that these are a lot of questions at once  :)
I really like how the Simulation is integrated in Onshape, it looks very good and is easy to use!


  • sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
    I just provided feedback with this button:

    Is it necessary to create a support ticket to share the document?
  • sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
    I just ran a simulation in Ansys Mechanical with mesh convergence. After each run the mesh is refined and if the change of max. Stress is bigger than the criterium the mesh is refined. 

    For this simulation I set the convergence to be lower than 0.5% change. The final stress is 525.58MPa which is 76229psi.
    The number of nodes was increased a lot from 1190 to 50131. Even the third run with 14561 nodes and a max stress of 525MPa would be more than accurate enough. 

    It would be great if we could see the convergence plot in Onshape. :smiley:
    That way we could estimate how close we are at the "excact" solution.

    It is also interesting that the results of Ansys and SimSolid differ that much...  :confused:

    What is the convergence criterium of Onshape Simulation?
  • sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
    I just tried and split the bottom face. This increases the stress in the radius and we get pretty close to the 525.58MPa (76229psi):

    The difference is only 2%, which is great!!!

    My guess is, that the bottom face has to big elements which increase the stiffness of the mesh. With the smaller elements, the part gets more "flexible" and the the tension stress in the radius is increasing.

  • sebastian_glanznersebastian_glanzner Member, Developers Posts: 397 PRO
    I had to change my units to mm, Newton, MegaPascal... in my account settings. Wasn't aware of the new options after joining the EVP
  • GregBrownGregBrown Member, Onshape Employees Posts: 93
    Sebastian, very interesting comments and examples you provided. Thank you! We are still tuning the solver, so you will probably see some variations to your results presented above - and hopefully they will be getting better and better as we optimize our algorithms.

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