Shell stress validation

Hi Karamba-community,

Has anybody yet validated the stress results of static simulations with shell elements by analytical calculations or other FEM-software?

I'm trying to validate the displacement and stress of a plate that is supported at two opposite sides (rotational degrees of freedom are allowed) and gravity load is applied. By now I can only verify the displacement of the plate with a deviation of less than 3 % using ANSYS Workbench. Kirchhoff's plate theory as an analytical approach gives a similar result with 10 % deviation.

The van Mises stress and Principal stress results in Karamba are approximately 200 times higher than the results in ANSYS and the analytical results. I tried to find the mistake for several days now and would appreciate any help or similar problems with validating the shell stresses.

Here are the values of the plate:

length: 1 m, width: 0.2 m, thickness: 0.01 m

Material: Steel 'S235' (standard)

resulting gravity load: 0.157 kN

displacement in Karamba: 0.000583 m

stress in Karamba: 116 kN/cm² (=1160 MPa = 500 % utilization!)

stress in ANSYS: 0.57 kN/cm² (=5.7 MPa)

The utilization of 500 % for a steel plate under its own dead weight makes we wonder what is wrong... See the grasshopper definition and the picture attached.

Best regards and thanks for any help,

Robert

Attachments:

shell_validation.gh, 19 KB
shell_validation.jpg, 359 KB

Replies to This Discussion

Permalink Reply by Hannes Löschke on December 2, 2013 at 11:05am

We have seen some strange behaviour of KARAMBA shells preparing our parametric design workshop here at HTWK Leipzig. Since Clemens Preisinger will attend, we just collected our observation an will bring it forward this week. If nobody else will pick this up, I will ask Clemens and come back to you.

Permalink Reply by Robert Naguschewski on December 17, 2013 at 9:25am

Hi Hannes,

belated thanks for your answer. Are there already any news on the validation issue?

Permalink Reply by Hannes Löschke on December 17, 2013 at 11:20am

Ah, sorry, we talked a bit but couldn't instantly find an answer to this specific problem. I'll make Clemens aware of this topic.

Permalink Reply by Karamba3D on December 23, 2013 at 12:32am

Hi Robert,

thank you for your bug report. Sorry for my late reply - I haven't seen your discussion right away because I get only an automatic E-mail notice when someone starts a discussion in the Karamba-group.

In Karamba 1.0.4 the position of the shell layer for calculating stresses is taken absolute instead of relative (i.e. '-1' is taken as the layer at a distance of 1m from the middle plane instead of the upper edge). This bug will be removed in Karamba 1.0.5. This bug also affects the material utilization because it gets calculated as the maximum at position -1/1.

Best,

Clemens

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