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NX Nastran quality check - HEX DETJ

Creator
Creator

Dear all,

I've been looking for a similar topic on the forum, but I couldn't find a solution. I've done a simple model and meshed it using Hex Mesh solid option. Then I wanted to check the quality of the elements using Nx Nastran check element quality option and I discovered that none of my elements is actually passing the test:

qualitycheck.JPG

I know that the mesh is not perfect since it was only a trial but I didn't expect that all the elements wouldn't pass the checks.

It seems that the determinant of the Jacobian matrix is equalled to zero for all the elements. I also check the value of Jacobian using Femap check and here most of the elements pass the test:

pica2.JPG

I also run a static analysis with the Geomcheck and all the elements passed the tests. Therefore, I don't know how to interpret the results from the NX Nastran element quality check.

I would be grateful for any advice.

Karolina

13 REPLIES 13

Re: NX Nastran quality check - HEX DETJ

Creator
Creator

I have also done a model with better quality elements as seen in the picture below and have the same problem:
picture3.JPG 

The results of NX Nastran check:

 

Quality Check Number Failed Worst Value
Hex AR 0 3.55556
Hex DETJ 1280 0.
Hex Warp 0 1.

1280 Elements Failed out of 1280 Checked.

Re: NX Nastran quality check - HEX DETJ

Siemens Phenom Siemens Phenom
Siemens Phenom

What version of FEMAP are you using and is it possible to post the model?

Re: NX Nastran quality check - HEX DETJ

Siemens Experimenter Siemens Experimenter
Siemens Experimenter

The Femap Jacobian values for all element topologies are normalised from 0.0 best to 1.0 worst, with 2.0 indicating failure. This allows plotting and comparison with a single set of criteria for a mixed mesh model.

Nastran Jacobian values best and worst values vary by element type. The main citeria Nastran checks in regard to Jacobian is if value < 0. This would indicate a failed element. However, with hex mesh, that's hard to achieve.

 

Model below shows 3 1x1x1 blocks with 1 and 2 corners collapsed. Nastran Jacobian for collapsed corners does not fail.

Femap Jacobian fails with 1 or 2 corners collapsed.

 

Note: Nastran will fail with a collpsed corners, but not because of Jacobian.

 

JacobExpl.jpg

Re: NX Nastran quality check - HEX DETJ

Creator
Creator

I've got Femap v11.2.1

Please find attached both models.

 

Re: NX Nastran quality check - HEX DETJ

Creator
Creator

Dear jprokop,

Thanks for the explanation, I was already studying Femap's help description but your description together with pictures is much clearer. Anyway, I checked the Femap Jacobian in the second model I prepared and the maximum value is around 0,232 which means that elements are fine. Although doing Nx Nastran check I still receive:

 

Quality Check Number Failed Worst Value
Hex AR 0 3.55556
Hex DETJ 2560 0.
Hex Warp 0 1.

2560 Elements Failed out of 2560 Checked.

Therefore, I'm assuming there is something wrong with those elements, but I cannot find a reason. I would be grateful for some feedback.

Thanks,

Karolina

Re: NX Nastran quality check - HEX DETJ

Creator
Creator

I discovered that scaling the model to millimetres (initially it was in meters) the problem disappears... maybe it's some numerical error then? 

Highlighted

Re: NX Nastran quality check - HEX DETJ

Gears Phenom Gears Phenom
Gears Phenom

Jacobian is only one parameter. You can check Internal angles and see that another elements are distorted.Quality.pngFemap Jacobian less than 0,8 is acceptable. If femap jacobian will be more than 0,9 Nastran may stop solution due to poor elements.

Re: NX Nastran quality check - HEX DETJ

Creator
Creator

I've already checked all the parameters in both NX Nastran and Femap checks. Even performed analysis on a better quality mesh that passed all the checks in Femap check. I'm trying to understand why none of the elements passes the HEX DETJ test. As I mentioned before, a scaled model to millimetres is fine. 

Re: NX Nastran quality check - HEX DETJ

Siemens Experimenter Siemens Experimenter
Siemens Experimenter

There was a bug in v11.2 that was fixed in v11.3. It affected DetJ for all solid elements. Are you able to upgrade?