Connection of hex and tet elements

[Megidd]

FE model

I have this FEA model on the inp file:

hex20tet10.inp.zip

Parts

For this model, 4 components are reported by PrePoMax:

• 2 parts are disconnected elements.
  • Makes sense
• 2 parts are just internal elements.
  • I don’t understand why we have these parts

Screenshots: parts reported by PrePoMax

hex and tet element count

PrePoMax reports the element count for hex and tet as follows:

Part element count

Also, PrePoMax reports the element count for the 4 parts as follows:

Element count comparison

We have 19120 elements of hex type. One part has exactly 19120 elements. So, one part is exactly all the hex elements.

We have 39318 elements of tet type. The total element count for three parts is:

39318 == 39290 + 20 + 8

Conclusion

The element count comparison indicates that the hex and tet elements are grouped into separate parts/components. Hinting that they are not connected properly.

Question

hex elements are grouped as a separate part from tet elements. Does it mean the connection of hex elements to tet elements is problematic? How can I fix that?

[xyont]

@Megidd

hex elements are grouped as a separate part from tet elements. Does it mean the connection of hex elements to tet elements is problematic? How can I fix that?

depending how the algorithm of mesh being used, another solver FE provide transition of them by pyramids element/

hex-dominant mesher in GMSH build and generate from tetrahedral first. so, the continuity in transition of both element type can be guaranteed no mater is a 1st or 2nd order element.

to my understanding it’s only visualization artifact which need to be fixed, PrePoMax and CGX has the same behavior at this.

above pictures shown, many internal groups of tetrahedral element shown thicker boundary edges even the part names are the same. to me, this is something odd behavior.

[xyont]

may i correctiiing previous comment regarding to 2nd order element of tetrahedral and hexahedral.

mid point of tetrahedral element will be placed at the center face of hexahedral, even solvable it’s actually disconnected, so only 1st order is guaranteed.

[Disla]

Are those quads shown on the surface hex elements?
I have found sometimes a nice hex appearance on the surface which was a mess inside. Gmsh can sometimes do that kind of things. Be sure to slice your model and look inside.

[xyont]

no surface element, only hexahedral element and small fraction volumes of tetrahedral. it’s clearly shown on my inp files.

[Disla]

@xyont

it’s clearly shown on my inp files.

??¿¿

@xyont

this is something odd behavior

Hi Xyont,

Darker lines when looking just at the model edges representation is a warning of possible edges overlaping or unnexpected line elements in the mesh in Megidd model.

[xyont]

Thanks for mebtioning about, could you post analysis tesult of your mesh model? an example unrestrained frequency

[Disla]

They tend to show as slightly darker isolated or nonclosed lines on the surface

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297,291,316,321,296
38,42,48,49,43,90,95,96,91,320,324,172,299,323,100,101,
302,296,321,99,301
39,36,43,44,37,83,91,97,92,300,102,103,304,303,127,128,
307,263,301,104,306
40,43,49,3,44,91,96,98,97,172,130,133,102,101,176,180,
127,301,99,173,104
*ELEMENT,TYPE=C3D15
3,19,27,20,51,56,52,125,126,106,131,132,114,110,129,111
9,19,26,27,51,64,56,160,171,125,165,174,175,110,162,129
10,20,27,28,52,56,66,126,177,178,132,181,182,111,129,179
11,20,28,21,52,66,57,178,183,134,182,184,139,111,179,137
17,26,34,27,64,74,56,216,217,171,219,220,174,162,218,129
18,27,35,28,56,75,66,221,222,177,224,225,181,129,223,179
24,26,33,34,64,73,74,210,257,216,215,258,219,162,212,218
25,27,34,35,56,74,75,217,259,221,220,260,224,129,218,223
*ELSET,ELSET=Default
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
*MATERIAL,NAME=Steel
*ELASTIC,TYPE=ISOTROPIC
210000000000,0.3
*DENSITY
7850
*SOLID SECTION,ELSET=Default,MATERIAL=Steel
*STEP,PERTURBATION
*FREQUENCY
8
*NODE FILE,GLOBAL=YES
U
*EL FILE
S,NOE
*END STEP

[JuanP74]

I think that as the hexahedrons do not have a “face centered node” not all nodes of the tetrahedrons are connected to all nodes of the hexahedrons, so the meshes are connected but not using all nodes, hence those faces are detected as free surfaces by prepomax and cgx
(use Viewing/Toggle Move-Z/Zoom to create a clipping plane after plotting all faces)

[xyont]

Where’s the tetrahedral element conecting to hex? The solver logs or screenshot of plot results.

Ok, it’s interesting me to discuss directly with the developer of GMSH instead.

[xyont]

Resonable to visualiznation, however this odd behavior also happen when displaying linear element in which it should not.

[Disla]

The inp show users that slightly darker or isolated lines should be inspected carefully.
Even if one checks the nodes on the surface and they are merged. The overlapping sides might be on the nodes inside the volume. In this case on the other side of the plate.

[vicmw]

@xyont Even linear elements can’t have hex and tet connected properly. The edge of the tet has linear displacement but the diagonal of the hex face can deform to be curved in a kind of saddle shape. So it makes sense to show them as potential problems.

[xyont]

thanks @vicmw for remembering. right, the formulation of mixed linear element have a problem, so an improvement of linear tetrahedral i discuss previously is required.

however, nodes continuity of a mesh probably more important than element formulation itself e.g quadratic mixed element (as figures before). another FE mesher and solver has pyramid or 27node hex element to fill this gap.

[vicmw]

Is that improved 4-node tet really generally useful without weird problems in odd cases? I don’t like to read random papers on new elements because there are so many and they tend to hype up the advantages but if you don’t know what to look for, you might not notice a showstopper of a disadvantage. I know one of Bathe’s improved shells turned out to have surprise zero-energy modes for certain geometries that he didn’t notice in his first paper.

I wonder if you could get away with quadratic tets connected to half-sized linear hex mesh, so the each node on the tet connects to a corner node on a hex. People do this sort of thing, perhaps because it sort of look good enough. It wouldn’t be as bad as midside nodes flapping around in the breeze.

[Megidd]

Custom code to count parts

I implemented a custom code to count the number of separate components/parts. Every two elements that share a node are considered inside the same component/part. I used the breadth-first search (BFS) algorithm.

The custom code shows the component/part count as 3:

PrePoMax assumption

PrePoMax reports the parts count as 4 the same as before. So, I guess PrePoMax assumes different the hex and tet element types as different parts/components. Even though they share nodes.

[xyont]

@vicmw

I don’t like to read random papers on new elements because there are so many and they tend to hype up the advantages but if you don’t know what to look for, you might not notice a showstopper of a disadvantage.

indeed, i’m only interest and searching about it’s crucial element even not expertise in these field. questioning why many mainstream FE solver does not have such as element, only Ansys does by Roxana C. (2006) reported here.

not really sure and know. it seems still have limitation and not a general purpose element, but this improved linear tetrahedral element can be a big deal when it used to fill gap in hex-dominant meshing.

[vicmw]

@Megidd My guess is it’s probably counting them as separate parts because they don’t share all the nodes of a face, so they don’t form a properly connected FEA mesh. If you had transition elements (wedge, pyramid), it should count it as one part.

[Megidd]

@vicmw

… probably counting them as separate parts because they don’t share all the nodes of a face, so they don’t form a properly connected FEA mesh. If you had transition elements (wedge, pyramid), it should count it as one part.

Makes sense

[xyont]

please make clear discussion instead of doubting, only makes another discussion follower lead to confuse.

why not to take simple test? below pictures clearly shown there’s visualization artifact. some extra face (internal) and edge (bounding) are generates even it;s actually does not exist.