What is an Inverted Mesh Element?

[ryandias]

I am new to (Stress/Mechanical) FEA. I am trying to use Comsol Multiphysics to show stress's on a plastic connector.

My past experience is with CF Design (cfd) and I am not finding it any help trying to use the mechanical software.

I keep getting an Inverted Mesh error. What exactly does this mean? What sort of geometry would cause such errors (what should I change in the CAD, or is it a matter of changing settings)?

Earlier post in the wrong section

http://www.eng-tips.com/viewthread.cfm?qid=229673&page=1

 

[spongebob007]

In the end, it is kind of hard to tell what is really going on without looking at the model itself. I have found that STEP files are extremely reliable as far as producing good quality geometry. IGES files are the worst. However it has been my experience that bad geometry will cause meshing failures.

If your model is undergoing extremely large defelctions then you might need to remesh the model at different solution steps.

One example of this problem would be metal forging/forming analysis. The deformed shape is so different than the original shape that the elemnents will distort during the solution phase. Is it possible this could happen during a non-linear simulation of an elastomer? I don't have much experience with FEA of rubber parts.

 

[ryandias]

image attached illustrates the model/simulation.
Materials properties are as on the PBT gf 30 spec sheet.
Force is as in the image.

 

[spongebob007]

Your geometry could be the problem. It looks to me like there are some very thin sections on the part. If you mesh this part as-is with a tetrahedral mesher I would expect that you will get some high aspect ratio (distorted) elements in some of the thinner sections. For example, say the wall thickness is 0.1" and you set an element length of 0.5". The mesher will fill that solid with distorted elements that might lead to solution failures. Of course the brute force approach is to specify .1" elements. However this may lead to a model with an unwieldy number of degrees of freedom.

Your overall element quality will be a function of how robust the mesher is that you are using. I find that for some codes, the "auto mesh" functionality often chooses element sizes that are too large for the geometry. The mesher can compound this problem by trying to fit elements of all the same size to the geometry. Some codes have smart meshers that will identify small features and adjust mesh sizes and element size transitions accordingly.

You might have to resort to using local mesh controls to get a good quality mesh. Sometimes splitting up the solid into multiple bodies is what you are going to have to do. If possible, defeature the geometry. Remove all small features that you feel don't contribute to the overall response of the model. This is easier said than done when you are working with imported geometry. Many of the higher end preprocessors can delete surfaces and heal imported geometry even without model history.

 

[ryandias]

ok.

So this brings up another question then. What is an appropriate mesh.

Is the attached mesh ok, or is this causing my problems?

there are some larger tetrahedrals in the middle areas, and very fine ones near the holes.

I have tried refining the mesh, and only resulted in longer run/fail times.