Soem models do not mesh

[Sebastian]

I am having trouble meshing some models imported as step. The mesher starts but either crashes (Netgen/Tetgen needs to close) or the 'OK button' window just disappears with no meshing having been done.

Sometimes this happens with stl too.

What could be the reason for this?

[Sebastian]

I want to expand a bit on the above post...

As a test I modelled a kiddies chair, a tad over a 500mm in height:

small.png

(160.52 KiB) 2626-mal heruntergeladen

It is a very simple model. I exported it as step and stl. The step file can be imported into Z88 but cannot be meshed. The netgen mesher calculates for a while and then crashes (window opens up that says Netgen need to shut down). The tetgen mesher doesn't do anything, the window to start meshing opens but once I press OK it closes without doing any meshing. I don't understand why this is...

Strangely the stl files loads into Z88 but at 25Mb it is much larger than the step, which is only 388kb. This means during the import process i get constant messages about increasing MAX*whatever*. By the time the model is loaded the RAM usage is so high the PC is pretty unusable.

Why is a simple step file like this not meshing?

For interest sake, this PC is a Core 2 Duo 1,5GHz 4Gb RAM running Windows 7 Ultimate x64. My other PC is a Core i7 2,88GHz 8Gb RAM also running Windows 7 Ultimate x64.

[auroraIco]

Simple is in the eye of the beholder. This part has ample length ratios, very little curves and long edges. The import step will work with the open source library Open Cascade. It may be that the structure will be imported, but it cannot meshed. Try to export the STL file so that the triangles are generated equilateral. This is achieved by setting the aspect ratio, internal angles, etc. Depends on your CAD system. Consider also whether you need all model’s little rounding for the calculation. The skill in FE calculations is to simplify the model that it remains computationally, but the simplifications are tolerable for your result.

regards,
aurora_ico

[Sebastian]

I only just realised that it outputs a log file Tried another quadratic tetgen and got this report:

Opening z88i1.stl.
Constructing Delaunay tetrahedralization.
Delaunay seconds: 0.359
Creating surface mesh.
Jettisoning redundants points.
Recovering boundaries.
Error: Invalid PLC! Two subfaces intersect.
1st (#3379): (4928, 84, 5140)
2nd (# 162): (84, 79, 80)
Program stopped.

Two subface intersect... what doe this mean and how do I find them?

[auroraIco]

This means your CAD File is corrupt. Please improve the import structure! You need to convert your geometry into a set of triangulated surfaces and export it. The tricky bit is that the surfaces you export must represent proper solid objects i.e. they must be orientable, closed, manifold and non self-intersecting surfaces.

aurora_ico

[Sebastian]

Thanks.

Could it be possible that a corrupted file can be imported, meshed but not be able to calculate?

[auroraIco]

No. If there´s a mesh, it works.

[MNeidnicht]

Sometimes a well created mesh cannot be used for analysis. In this cases there are usually very small tetrahedrals or tetrahedral with very small angles. Therefore, there is an extra function in z88aurora: The mesh check.

But firstly the mesher must be executed without errors. As long as this is not happening, the input-file has to be processed.

Best regards
Martin Neidnicht

[Sebastian]

I will try checking the mesh. Does it fix the mesh automatically? How do I proceed if there is an error?

[Sebastian]

Did mesh check. Result:

mesh check.png

(10.13 KiB) 2549-mal heruntergeladen

Still doesn't work though. I select the constraints and load, set the material, solve. Then in postprocessor is says no stresses found.

Obviously if the mesh is fine then its can' t be that...

[MNeidnicht]

Hi Sebastian.

The mesh check looks good. If the solver produces an error, it must have other causes. Please check the console. (the second little window, that is currently minimized or in the background) We have done our best, that the most important errors are displayed there.

Are the constraints sufficient? Did you choose a stress-calculation mode?

Best regards
Martin Neidnicht

[Sebastian]

Are the constraints sufficient? Did you choose a stress-calculation mode?

This may seem like a silly question (I am a beginner) what exactly do you mean by stress calculation mode?

[auroraIco]

Dear Sebastian,

it took us a long time to write and translate the manuals of Z88Aurora. The program is freeware, the support too, but we can ask you kindly to use the manuals for such basic questions!

Best wishes, aurora_ico

[Sebastian]

I understand. Admittedly I have only speed-read it thus far. Thank you for all your help.

[auroraIco]

You are welcome and if there is any problem with bugs, etc. Feel free to ask. However your team is pretty small, only 7 men at the moment. We have to push the new version forward and so time is in short supply. I hope the handbooks give you answers,

best wishes, aurora_ico