Shell Elements With CCX Are Solid In Results

Nekativ

As the title states, when I run an analysis with shell elements with CCX, the components in the solution become solids. Is it possible to view the results as a shell without the thickness? This makes it difficult to see details on corners.

Sergio

You should remove the card NODE FILE and EL FILE writted by Mecway (using the "don't generate keyword" option on the CCX branch) and then write your own ("Custom step content") using the option OUTPUT=2D in the NODE FILE and EL FILE cards to avoid the element expansion on results.

Nekativ

This works, thank you, however there is one caveat.

The peak stresses with OUTPUT=3D vs OUTPUT=2D have a 15% difference. The shells in the non-expanded form from CCX are underestimating peak stresses.

I wonder if the reason is because of the excerpt shown below from the CCX manual or if CCX is solving in a different manner when OUTPUT=2D.

"The parameter OUTPUT can take the value 2D or 3D. This has only effect
for 1d and 2d elements such as beams, shells, plane stress, plane strain and axisymmetric elements AND provided it is used in the first step. If OUTPUT=3D,
the 1d and 2d elements are stored in their expanded three-dimensional form.
In particular, the user has the advantage to see his/her 1d/2d elements with
their real thickness dimensions. However, the node numbers are new and do not
relate to the node numbers in the input deck. Once selected, this parameter is
7.46 *EL FILE 375
active in the complete calculation. If OUTPUT=2D the fields in the expanded elements are averaged to obtain the values in the nodes of the original 1d and 2d elements. In particular, averaging removes the bending stresses in beams and shells."



My bigger question is how is Mecway Internal handling this to where I can view the 2D version of the elements in the results and get stresses that are still accurate/not underestimated.

Victor

Yes, that's the problem and it's a big limitation of CCX's 2D shell output. The internal solver keeps those 3 values (top, middle, and bottom stress) separate in its 2D element solution so you can view each layer's stress individually whereas CCX averages them.

That corner in the picture might be too sharp for shell elements, as you can see by them intersecting each other. So the stress there might not be much use anyway. It would be safer to model those details as solids if they're important.

prop_design

I have encountered this as well. it's a very unfortunate formulation. all the penalties of solid elements with none of the benefits.