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Mesh update after geometry changes

I am trying to perform a structural analysis on 2 different alternate of an assembly where I only want to change some dimensions, for example a fillet radius.

This is what I have done: after obtaining the solution on the original model I have opened the lower level file (prt of the model) and choose to do a "Save as". In this way I have been able to save as all the files involved in the analysis: sim, fem, fem_i and of course the part which I want to change.

After changing the dimensions in the expression panel, I have saved and updated both the part and the idealized part. Going back to fem file of course I have noticed that all the model cleanup actions have gone lost (for example circular imprint used for bolt connection mesh) and so I had to recreate them.

When I then tryed to update the mesh (2D and 3D) the result is completely different from the original one with coarse mesh where before it was fine and most of all with a lot of RBE3 1D elements between faces previously coupled with a mesh mating condition. Also some parts were not able to 3D mesh.

What is wrong with my method? What is the best way to do a comparing analysis between 2 different version of a model?
Thank you in advance,
GC
8 REPLIES

Re: Mesh update after geometry changes

Innovator
Innovator
Dear Giuseppe,
You have learned a good lesson: the idealized part is associated to the master part, then any geometric change in the master model will affect finally the FEA model, as desired!.

I suggest to make actions like washer imprints in the idealized part, not in the FEM part, this way your mesh will update correctly. Is true that the idea with NX developpers is to create "geom recipes" for everything like circular imprints, stitch edges, etc.. to perform in the FEM environment, then the mesh will update correctly in case of editing the master model, this is new in NX8, good job!!.

Also, when dealing with RBE2/RBE3 elements, my suggestion is to define the mesh based in the geometry always, not in the mesh. For instance, instead to use "node-to-node" to define an spider, use better "point-to-edge", this way if you update the hole diameter the mesh will update correctly. Etc...

Best regards,
Blas.

Re: Mesh update after geometry changes

Thank you Blas.
The main problem remains the fact that mesh mating conditions existing on the original model, after the dimension changes turn into RBE3 elements connecting the surfaces. I have tried to delete theme and manually recreate mmc but it is not possible anymore to use free coincident option, only glue coincident is allowed (tried to reverse too). The result is that on coincident faces I have different mesh size. Also on other surfaces not affected by dimension changes, mesh change considerably. I think this is a problem if I want to compare results between the 2 models.
I thought it was easier to compare 2 different design of a component to decide which one to choose.
Any other suggestion?
Thanks,
GC

Dear Giuseppe,
You have learned a good lesson: the idealized part is associated to the master part, then any geometric change in the master model will affect finally the FEA model, as desired!.

I suggest to make actions like washer imprints in the idealized part, not in the FEM part, this way your mesh will update correctly. Is true that the idea with NX developpers is to create "geom recipes" for everything like circular imprints, stitch edges, etc.. to perform in the FEM environment, then the mesh will update correctly in case of editing the master model, this is new in NX8, good job!!.

Also, when dealing with RBE2/RBE3 elements, my suggestion is to define the mesh based in the geometry always, not in the mesh. For instance, instead to use "node-to-node" to define an spider, use better "point-to-edge", this way if you update the hole diameter the mesh will update correctly. Etc...

Best regards,
Blas.

Re: Mesh update after geometry changes

Innovator
Innovator
Dear Giussepe,
Do not use mesh mating conditions when its cause the use of RBE2/RBE3 elements, I only use this feature when full coincidence exist between solid parts that will cause simply merging nodes in a natural way, if not do not use it.

The solution is simply: instead please prescribe GLUE contact surface-to-surface in the SIM environment, this is my preferred method, a lot of times better than using RBE2 elements to define rigid joints, you can compare stress results in the joint and you will realize that RBE2 elements will cause stress concentrations with high values that are artificial, instead the GLUE face-to-face and GLUE edge-to-face contact is simply excellent, very soft stress gradients values, congratulations to the NX NASTRAN developers, the have done a good job.
Best regards,
Blas.

Re: Mesh update after geometry changes

Pioneer
Pioneer
As Blas mentioned the Surface-to-Surface gluing method is very convenient, especially when you create groups of surfaces in the underlying fems, then even when you have multiple levels of afms, you can select those groups in the sim easily. As long as your groups are correctly defined and refreshed after a geometry change, the sim will have no problems.

Now, if you must insist on using MMC that is fine. However you run into geometrical problems related to the simplification process from CAD solid body to FEM polygon body. If the MMC doesn't want to refresh itself properly, then you must re-create it. Occasionally the bodies are corrupted so you must fix them.

To do this you must delete all meshes related to the two (or more) polygon bodies joined with the MMC. Then you must use the Reset command (located in the drop-down list with all the polygon body simplification tools) on both bodies. Then re-do the MMC. If it still fails then after running the Reset command, you must go back to the Modeling application, change the geometry of the bodies (ie. move any face 1 mm) then go back to Advanced Simulation, allow the polygon bodies to update themselves, then go back to Modeling and delete the geometry change from the tree. Then go back to Adv Sim and re-create the MMC. This is like a manual body Reset.

If this last technique doesn't work, then you have extremely corrupted polygon bodies, in which case the only solution is to create a brand new fem. HTH.

Re: Mesh update after geometry changes

hweiss1, thank you for the explaination. I will use these steps in case of further difficulties.

Re: Mesh update after geometry changes

Siemens Phenom Siemens Phenom
Siemens Phenom
If you do feel the need to force an update of a polygon body, you can use an alternative path to editing the CAD solid/sheet body. The FEM lets you edit the bodies that it uses.

1. Make the FEM the work part
2. Select the FEM in the Simulation Navigator and MB3 Edit
3. Edit bodies to use
4. Deselect the body you want to update

This removes the body from the FEM.

5. Repeat the process to add the body back in the FEM

NX 8.0.0 has MB3 options on the polygon bodies. It lets you delete bodies from the FEM. It lets you recreate the bodies and recreate with update (update will recreate MMCs and auto stitch operations that the body participates in).

Regards,
Mark

Re: Mesh update after geometry changes

Thank you very much.
Unfortunately at the moment I am using NX 7.5 but if in the future I will be able to install NX 8 I will try.

Re: Mesh update after geometry changes

Siemens Phenom Siemens Phenom
Siemens Phenom
Giuseppe,

In older versions, the step by step instructions I provided apply. NX 8.0.0 simply makes the process easier with additional capabilities (i.e. update options).

Regards,
Mark