wireframe,surface or solid and finite element analysis.

following is the text from sdrc ideas tutorial

"I-DEAS uses a solid representation of part geometry.Because it's a
complete representation, solid geometry supports any type of design
and analysis application that requires geometry information.
For example, it gives finite element simulation "

Now my question is , suppose I supply a similar representation of a
particular solid in pure wireframe format( say using autocad),can I do
FEM analysis on it.

My friens who is working on Hypermesh told me that hypermesh has very
limited modeling capabilities but whenever need arises he can create a
wireframe model of a solid and does fem analysis on it.
He also told me that many times the CAD models are not complete and
has some incomplete features.So they repair it in hypermesh using
line,spline commands and then it is used for FEM.

He said to me that geometry is required only for reference and meshing
is a totally different issue.

what is correct? I am totally confused. (as usual .

regards,
Yogesh Joshi

 

-----------------------------------------

Finite element analysis always requires the creation of analysis elements
(planar mesh, solid mesh, bar, rod, etc.). Nurbs, bezier, etc. surfaces
used for most present day "solid" reps are of no use to the solver, but most
pre-processors can reference them for mesh generation so you are money ahead
sending a solid (or surface) vs. wireframe rep to it.

" ... he can create a wireframe model of a solid ..." The wireframe is just
a reference for element / mesh creation.

"... many times the CAD models ar not complete..." If the model is supposed
to be a solid rep any "incompleteness" is due to something getting lost in
translation. (Actually, most solid models contain too much detail for FEA
purposes.)

Basically, the I-DEAS statement is marketing psycho-babble. All the
attributes given to the "solid" can also be given to any set of surfaces
that define a volume; e.g. a closed surface or quilt. Once a volume is
defined, all that's necessary to have a "solid" is a database entitiy for
mass.

 

Thanks Jeff!
but still I have some doubts..
Suppose I draw a cube using only 3D lines and I draw the same cube
using solid modeling operations(extrusion).
How will fem solver treat them..?
in similar or in different manner.?


regards,
Yogesh Joshi

 

The solver won't deal with either one. It can't see the wires or surfaces.
In either case the user will have to mesh the objects using whatever
functions are available in the pre-processor. The meshing operation is
quite similar to other surface modeling techniques (exactly like creating an
old Acad polyface mesh, if you are familiar with that) for shell / plate
models. Where surface data is present most pre-processors can auto mesh the
surfaces for a shell / plate model or fill the volume with a 3D mesh for a
solid solution, with at least some degree of success. I don't have the
foggiest how the solver internal math works, but it requires a matrix of
"nodes" to do the computations. These nodes are the corner vertex (at
least) definitions of "elements". Neither wireframe models nor surface /
solid models created by CAD systems have any representation of this node
matrix. (The exception being the previously mentioned polyface mesh models.
These will translate directly to FEA mesh; all that the user has to do is
merge coinicident nodes, define material and properties, etc.)

 

Sure, your friend is technically right, you can do FEA with just
wireframe. But once you need to do a fairly complex part, using a solid
model and a decent modern mesher (not sure SDRC qualifies will be
MUCH faster. Of course that's not to say you shouldn't learn what FEA is
all about and how far the tools will take you. Vince Adams' book on FEA
is a very good thing to read.

I think there was a day when modelers automatically redrew everything,
mainly because meshers used to be pretty poor, as well as some CAD
systems. Now days, if you don't have a setup that can mesh a fairly
complex molded part, solve it, and get the right answer (converged)
rather quickly, then you need to be out looking at a new system.

Might be fun to compare FEA systems too. There ARE some differences...

-meld