midsurfaces Edge & Face IDs changing/disappearing.

[joe_duffyW8496]

I've been trying to do some FEA using surface bodies and it's been a very frustrating experience. I'm using multi-body parts and creating midsurfaces in Nastran. Everything seems fine at first but inevitably I will get an error along the lines of “geometry has been lost due to part modification” or “Geometry not applied at a valid FE mesh location”.

Eventually I may discover that the mesh has disappeared from 1 or more faces. When investigating the midsurface command for that surface I will see that the midsurface was applied to “face 2”. When I put the mouse over the location of “face 2” I see that we now have “face 399” in that location and no option to click on a “face 2”. Essentially those faces no longer exist and new faces have been created in their place. Unfortunately, the mesh (and loads, constraints, ect.) do not transfer to this new face.

This issue has occurred in every single FEA I’ve ever done with Nastran.

Please guys tell me I’m doing something wrong here? Has anybody successfully done FEA on a multi-body part using midsurfaces?

Thanks in advance.

I've attached the latest model with this issue for anyone who is interested.

I’m using Inventor 2023

[John_Holtz]

Hi @joe_duffyW8496 

I do not know about your specific case, but I have found that "Manage > Update > Rebuild All" is helpful when things get out of synch.

I tried it on your model and exited and re-entered. So far it worked okay.

John

[joe_duffyW8496]

Hi John Holtz. Thanks for the reply! Haha. I've read so many Autodesk threads I feel like I'm talking to a celebrity!
I appreciate the suggestion and I'll give it a try if the error happens again. I've actually had a good deal of correspondence with the tech support team (shoutout to Marcin Kościółek for all the help) and we have discovered a workaround. The error of the missing geometry seems to happen whenever the part file is saved while in the Nastran environment. If the part is large enough, you can actually see the CAD bodies reappearing and turning back into midsurfaces. After this save many of the faces & edges will have a new ID number. Then all of the constraints, loads, contacts, meshes, etc that have been allocated to the old face/edge IDs are no longer valid and the analysis will not run. This error has been replicated by myself & the Autodesk tech support team in Nastran 2021, 2022 & 2023. I’m frankly shocked that this issue wasn’t brought to light earlier as it has been ongoing for years and stopped my & my team from completing any FEA using surfaces.
The workaround that I previously mentioned is to exit the Nastran environment before you save the part. You can NEVER save in the Nastran environment of else this error will occur. So far this process has worked flawlessly.
Now that we have found a workaround the issue has been transferred from the tech support team to the developers who will take ownership of the active issue and hopefully develop a fix that allows saving in the Nastran environment.

[joe_duffyW8496 · Accepted solution]

I'd thought I'd create a reply to my own post because I can mark this issue as sloved due to a workaound we've found. Hopefully this will help anyone who has experienced the same issue. Below is a bit of a copy/paste from other conversations in this thread but it does contain the solution:

I've actually had a good deal of correspondence with the tech support team (shoutout to Marcin Kościółek for all the help) and we have discovered a workaround. The error of the missing geometry seems to happen whenever the part file is saved while in the Nastran environment. If the part is large enough, you can actually see the CAD bodies reappearing and turning back into midsurfaces. After this save many of the faces & edges will have a new ID number. Then all of the constraints, loads, contacts, meshes, etc that have been allocated to the old face/edge IDs are no longer valid and the analysis will not run. This error has been replicated by myself & the Autodesk tech support team in Nastran 2021, 2022 & 2023. I’m frankly shocked that this issue wasn’t brought to light earlier as it has been ongoing for years and stopped my & my team from completing any FEA using surfaces.

The workaround that I previously mentioned is to exit the Nastran environment before you save the part. You can NEVER save in the Nastran environment of else this error will occur. So far this process has worked flawlessly.

[br.sbrava]

Thank you so much for updating this post, I see the same issues here. I will avoid saving while in the Nastran environment to see if that helps.

We have the following behavior in several machines here, which is a total disaster:

- Simply closing and opening the file can sometimes break everything.

- Moving anything or adding/removing a part will always break everything. Not only references are lost, but the mid-surfaces also get misplaced, as if they are positioned according to a UCS of another part.

This support page indicated that this might be due to a file having been saved with an old Inventor version, which is not the case.

https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Face-has-been-lost...

This page has "fix" for the misplaced surfaces, but it is a convoluted way of saying "delete everything one by one and start again":

https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/mid-offset-surface...

Ultimately, it might be better to simply not use shell elements.

[Shuzzone]

Hi @br.sbrava, I'm having exactly the same issues as you since yesterday. I've spent all morning today scouring the forums for this issue, and I also landed on the same posts that you linked, sadly with the same outcome too.

To add to your links/posts, I have found the following links too:

The support page below tells you to delete all FEA files, then rebuild your model (which is beyond unhelpful):

https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Face-has-been-lost...

The forum post below unfortunately ended up unresolved, and the last replies point to just deleting the entire analysis altogether.

https://forums.autodesk.com/t5/inventor-forum/midsurface-rebuild-error/td-p/9940064

I've attached below what's been going on on my end. I need to check what moving some seams and welds will do to the structure, sometimes just a matter of 20mm, and despite me not removing any faces or parts, I will still end up with my midsurfaces displaced, and basically at a worse point than just having to start from scratch (considering it takes times to delete all mid-surfaces by hand from the model tree and I need to redefine all contact, load and constraint faces).

When using the solution from this support page (hyperlinked in text) (which you linked) I ended up having an entire different problem too, which was that I couldn't even select the body or the midsurface as that was not appearing at all, which sparked another research.

This support page and this one too tell you to hide, then unhide, all affected parts from the model tree, and if you still don't see them, then to exit Inventor, then re-enter it right away.

Then what happens with me is that I can see my model above my midsurfaces, so I need to uncheck CAD bodies in Display -> Object Visibility, then check it again to see only my midsurfaces (so I can select the correct faces).

This whole issue is SO frustrating, that it seems like shells and midsurfaces are just plain unusable in Inventor Nastran.

I hope someone that can fix this issue will see it, and otherwise I am posting this so that hopefully someone else who is experiencing this will save themselves some time.

[peter.roman]

I get a similar issue in the plain stress analysis environment, shells and associated contacts will break and throw errors.

I haven't gone back to check if it's due to saving the file in the environment like this issue.

Currently on 2025

[dnunezHSLXH]

When I want to use shell elements, I never use the offset/ midsurfacing feature due to the GUI's poor handling of them.  I either:

1.  Manually select the faces of the 3d part to create the shell idealization.
2.  Create a surface(s) in the Inventor modeling environment and select the surface(s) in Nastran create the shell.

So far, I have been able to save within the Nastran environment.  My opinion is to just not use the offset/ midsurfacing feature until its fixed. 

[Shuzzone]

@peter.romanI it might be from the saving, but it could also happen if you made some edits to your model in inventor, and when going back into Nastran that causes for your shell midsurfaces (I assume you used those) to break, and then break contacts etc.

@dnunezHSLXHthose are the only ways to currently use shells to be sure that your model won't break, especially the latter. I made a thread about this type of thing a couple of months ago, and John_Holtz and I came to that conclusion. The offset/midsurface function is queued to hopefully be fixed in 2026 version, although I wouldn't hope for it.

[peter.roman]

I assume the plain Inventor Stress Analysis environment would have the same workaround with the "offset" command, and manually entering the thickness; and just out of luck with generated "midsurface" shells with detected thickness?

I've noticed the issue even when no geometry changes have been made, something will update or refresh and various things are broken all of a sudden, (or the icons refresh to show errors as I mouse over them), (in the Inventor Stress Analysis environment, not nastran).

[br.sbrava]

Excellent tip @dnunezHSLXH !

Took me a while to find out how to do this since it's not with the surface tools, but very well hidden in the Thicken/Offset tool.

This is the best workflow I found for this on sheet metal parts.

If your part is not in sheet metal, you'll need to pick the faces one by one.