Cohesive (zero thickness): NODE NUMBERING MIDE NOT BE CORRECT

[StevenJS]

Hi all,

I'm trying to model a 2D cohesive layer with a zero-thickness in Abaqus (6.11.2)
The shape of the layer is a quarter circle.

When modelling this i first create a cohesive layer with a certain thickness.
I then convert the part of this cohesive layer to a orphan mesh, then edit the mesh, such that a zero thickness layer is created.

However when i want to submit the job i get the following error message:

Error in job XX: THE NODE NUMBERING MIGHT NOT BE CORRECT FOR ELEMENT YY INSTANCE ZZ

This error is given only for a certain number of elements, the other elements seem to work just fine.
How can i resolve this error? When chechking the elements with the error message, they seem to be numbered just fine and exactly the same as the other element that do not generate this error message.

Also when deleting the elements where the error is occuring for, the simulation runs just fine.

I added an .inp file where this error occurs for me

 

[compositesFEAguru]

Steven - I do this type of modeling quite a bit but have never converted the part to an orphan mesh. You can modify the nodes on a meshed part that is associated with a part without converting to an orphan mesh (might complicate things?). Also, check that stack direction is consistent across the entire mesh.

Regards

 

[StevenJS]

compositesFEAguru,
Thank you for the reply!

The stack direction is indeed not consistent for the mesh.
The elements that cause the error message, have an inconsistent mesh stack orientation.
However since these are cohesive elements with a zero-thickness i can't adjust the mesh stack orientation of these elements
Do you know of any other way to change the orientation of elements?

The reason i use an orphan mesh is actually for 2 reasons;
- First i found it more easy in Abaqus CAE to connect the different (original) parts, since i can then easily merge the coincident nodes.
I also created several models where i use tie-constraints to connect the parts. However i found that i need to connect each coincident node manually with a single tie-constraint. When i use one tie-constraint for a complete surface incoorect results where found. Since this is quite some work to do for a fine mesh i thought it was easier to just merge the nodes when creating an orphan mesh. But i also did it without the orphan mesh.
- For the cohesive layer with a zero thickness however, i always use an orpahn mesh, since i'm not aware of any other way to model elements without a thickness. What i do is first create cohesive parts with a thickness and then convert this to an orphan mesh, so i can move the nodes of one side and place them on the other side.
Is there any other way to create layers without a thickness in Abaqus? I was not able to find this in de documentation or any other source.


One other note, i found that it can occur that the standard node numbering in Abaqus for cohesive elements with a zero thickness is incorrect.
In the image below, when the cohesive element has a thickness of zero (thus node 3 has the same coordinates as node 8, and node 2 has the same coordinates as node 5) Abaqus sometimes defines the cohesive elements as [2,5,8,3]. When running the job for this kind of element definition, an error occurs wich says the surface of the element is negative. When adjusting the element definition of the cohesive element to [3,2,5,8] or [5,8,3,2] Abaqus now calculates a zero or positive surface for this element and the calculation runs just fine.

Regards,
Steven

 

[compositesFEAguru]

For your cohesive layer, first create the part with a thickness and mesh using cohesive elements (COH3D8). Second, assign the stack direction to the part with thickness so you can pick surface easily. Third, without converting to an orphan mesh, use the edit mesh tool to offset nodes from one surface to the other. The limitation here is that this only works with simply geometries like flat plates, but it works well for that case.

 

[StevenJS]

Hi,

Thanks again for your reply.
On this way i can indeed easy adjust the stack orientation.

However to connect the parts correctly i need to tie al coincidents nodes manualy.
Using the tie-constraints for the complete surface at once gives inaccurate results.
This beceause the connecting surface has the shape of a quarter circle (it's a 2D-model)

Therefor i think i will still need to convert the model to an orphan mesh.

But now i can indeed get a correct mesh orientation.
So thanks a lot!


Steven