Solid "skinning"
Solid "skinning"
(OP)
Hi all,
I'm currently testing the "skinning" of a 3D-finite elemente model to reduce the amount of data for a fatigue analysis (mentionned at the end of thread727-232206: Stress at integration points or at nodes ?).
I modelled a complex (cast) part with Tet10 (10-nodes tetrahedal) volume elements, then meshed the surfaces with Tria6 (6-nodes surface elements) shell elements and merged the nodes of solid and surface meshes (including midside nodes).
The shell elements have a thickness of 0,001mm and the same material property as the volume elements.
I then compare the nodal output in solid and shell elements (unaveraged maximum value of strain and stress).
The max principal stress/strain distribution is the same in both solid and shell meshes, however the max stress value in the skin is slightly inferior to the max value at the corner of the solid elements (up to 1%, depending on mesh fineness)... Which one should I trust??...
Thanks for your help!
Simon
I'm currently testing the "skinning" of a 3D-finite elemente model to reduce the amount of data for a fatigue analysis (mentionned at the end of thread727-232206: Stress at integration points or at nodes ?).
I modelled a complex (cast) part with Tet10 (10-nodes tetrahedal) volume elements, then meshed the surfaces with Tria6 (6-nodes surface elements) shell elements and merged the nodes of solid and surface meshes (including midside nodes).
The shell elements have a thickness of 0,001mm and the same material property as the volume elements.
I then compare the nodal output in solid and shell elements (unaveraged maximum value of strain and stress).
The max principal stress/strain distribution is the same in both solid and shell meshes, however the max stress value in the skin is slightly inferior to the max value at the corner of the solid elements (up to 1%, depending on mesh fineness)... Which one should I trust??...
Thanks for your help!
Simon





RE: Solid "skinning"
Also, 0.001mm sounds like a membrane, not a shell, but that's just a "gut feel".
RE: Solid "skinning"
corus
RE: Solid "skinning"
If we have gotten to the point where we are concerned about 1% differences in results then someone is kidding themselves!!!!
Ed.R.
RE: Solid "skinning"
GBor, there's a picture of an example with a simple plate, in green there are the volume elements and in orange the "skin".
For this model indeed I got the 1% error, which rose to about 10% with the complex geometry... but maybe I used a mesh that wasn't fine enough...
Still, do you have some advices for this "skinning"?
RE: Solid "skinning"
Solid elements are incompatible with shell or membrane elements, they have different shape and displacement functions and different nodal degrees of freedom. Mixing element types invalidates the theoretical basis for FEA which abhors any form of discontinuity, yet by "skinning" you have introduced discontinuities over the free surfaces.
www.Roshaz.com
RE: Solid "skinning"
I think this is a Nastran workaround. And at that point I'll bow out gracefully.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Solid "skinning"
corus
RE: Solid "skinning"
If that is true then this is pure "magic" !!
www.Roshaz.com
RE: Solid "skinning"
I thought of filtering/extracting the nodes on surfaces, but the fatigue analysis software reads data directly from the f06 files, not from the postprocessor (Femap), and so it could import only the shell elements (I'm not sure how this works yet).
RE: Solid "skinning"
Skinning is a bit old fashioned. You can limit the results output in NASTRAN using the 'SET n' and 'STRESS(PLOT)=n' commands.
RE: Solid "skinning"
I tested my "complex" model with different mesh sizes. With the coarse mesh I got 10% difference between shell and tetra elements, with a finer mesh it fell to 1%, and with an even finer I got 3%.
In practical use though the calculation is non-linear, and I get convergence problems with the skin, whereas it was ok with tetras... I guess it's due to this discontinuities that johnhors mentionned.
Thanks all for your suggestions.