How to use shell elements? 1

[ladykin]

Hi there,

I am a new user of Abaqus. I have model a zebrafish embryo penetration process. I used solid elements for both the inner matter and the outer layer. I have been told that shell elements is preferable for the outer layer of the embryo since the outer layer is very thin, it can easily have large bending deformation and generally, solid elements are not a good choice
for a thin structure subjected to bending deformation.
My model is an axisymmetric model. I do not know exactly where I should define the outer layer as a shell. I have shell option in different steps like creating part, creating sections and Element Type in mesh module. in case of axisymmetric, I can choose shell option in the creating part, creating sections but in Element Type I have no option for shell.
I appreciate if anyone help me please with shell elements.

Regards,
Ladykin

 

[ThisIsMe86]

Hi Ladykin,

I approximately think that I have understood the problem and I am trying to answer it. If its not the right answer, please pardon me (especially because I am a fresh grad and have loads to learn).

I am not a Abaqus GUI user (I use Hypermesh). But if I wanted a thin walled shell elements over solid elements, I would simply extract faces from the solid elements and move the required faces (in your case, the outside of the fish) into proper collector. Assigning properties to the components should give the shell elements the required thickness and mechanical properties.

It would be interesting to see the proceedings.


-This is me

 

[ladykin]

Dear ThisIsMe86,

Many thanks for your reply. What do you mean by extracting faces? Do you mean a different part for the outer layer or by partioning the embryo? I tried both of them, but Abaqus gives me error and says :
95 elements have missing property definitions. The elements have been identified in element set ErrElemMissingSection.
This is while I assing appropriate materials to the sections.
BTW as I told you before I can not assign Shell elements in the mesh module for the axisymmetric model, there is no option of shell elment for Axisymmetric in the mesh module.

 

[rb1957]

i'd've thought you could extract the surface mesh from the solid mesh (but i don't know abaqus so i could be wrong). it seems like a fairly common thing one would want to do.

maybe you have to define a surface on the outside of your solid (in geometry terms) then mesh that using existing nodes ?

have you tried the abaqus help desk ?

the Very last thing you want to do is define shell elements by hand, knowing the surface nodes. that sounds Very Olde school ...

 

[ThisIsMe86]

Hi Ladykin,

What I said in my previous post is inline with what rb1957 said here.

By "extracting faces", I meant you could automatically duplicate the "SIDE" of the solid mesh that you need to generate the shell mesh on. Once again, I am not an abaqus user, but, Abaqus being such a solid piece of technology, I assume that abaqus can do it. May be refer to the help file??

Sorry, I couldnt understand the axisymmetric system that you mentioned. Could you please elaborate on it? I know what is axissymetric, but I couldn't understand how this could possibly stop you from generating a shell mesh.

Coming to "95 elements have missing property definitions", it simply means you need to assign properties to these elements (assignning property to the whole component is my preferred choice. but it depends on what you are trying to achieve too).


- This is me

 

[ladykin]

Dear rb1957 and ThisIsMe86,

I appreciate your attempt to help me, but I tried doing what you said, and was unsuccessful.
Abaqus CAE (which has GUI) does not let me to choose shell elements for axisymmetric model. I am not sure if it is the limitation of CAE or not. I tried to change the input file manualy and assign shell elements to the extracted boundary of the cell, it could run and complete the job but mix up my model settings a little bit!

Cheers,
Ladykin

 

[rb1957]

i'm confused ... "assign shell elements to extracted boundary" tells me you have a surface extracted from your solid, yes?

you should be able to mesh the surface and the solid together, it is quite a common thing to do. then you want to assign 2D shell elements to the surface and 3D solid elements to the volume.

have you tried the abaqus help desk ?

i'll see if i can ask an abaqus user here ...

 

[ladykin]

Ofcourse I am able to mesh surface and solid together but when I want to choose type of element Abaqus does not show any option for shell element.
I really appreciate if you please link me to a professional abaqus user.

Regards,
Ladykin

 

[ESPcomposites]

- Shell elements would be appropriate if your model was actually in 3D. That is, you would have a 3D model when you rotated it in ABAQUS.

- An axisymmetric model is in 2D and axisymmetric elements are used to simulate the effect of a 3D model.

You were correctly told that shell elements might be a good choice (as opposed to solids), but that would only be applicable if you had a 3D model in space. Since you essentially have a 2D model and the 3D is simulated, just use the axisymmetric elements (and don't worry about a shell in 3D space).

Brian

http://www.espcomposites.com

 

[crisb]

Somebody here seems to have a similar problem. I don't use Abaqus but in my software one has to define the element group as isobeam and then axisymmetic shell as the sub type.

http://comments.gmane.org/gmane.comp.mathematics.abaqus.user/16965

 

[rb1957]

it sounds like you have the surface meshed and the volume meshed (with a consistent mesh). abaqus should be smart enough to realise that the volume needs 3D elements (10 node tets, 8 node or 20 node bricks, ...) but the surface can only accept 2D elements (6 node tri, 4 or 8 node quads) whether they are shells or plates.

not understanding why abaqus would limit element choices based on axisymmetric modelling ... axisymmetric modelling "only" imposes constraints on the model ... yes ?

call the abaqus help desk.

post on the dedicated abaqus forum.

 

[ladykin]

Many thanks crisb, it was very helpfull, actuly the problem was resolved!
Thanks Brian and rb1957 for your attempt to help me.

Cheers,
Ladykin

 

[rb1957]

what was the fix ?

 

[ladykin]

The point was to select "Wire" for your base feature. The resulting part can be meshed using axisymmetric shell elements.

 

[Kwan]

RB,

Here is an example of axisymmetric modeling. Essentially you model in 2D and the software evaluates this in 3D.

http://cbjg.jpkc.cc/uploads/cbjg/swf/7/38.pdf