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enable / disable shell elements?

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enable / disable shell elements?

Mechainica Independent Mode:

Is there a way to leave shell elements in a model and give them
some property (example: zero thickness) so that they don't
participate in any way in the results?


Example: Given some large frame, with two styles of legs....

1. Mesh the frame.
2. Mesh leg style 1.
3. Mesh leg style 2.

4. Disable leg style 2 and solve the overall model.
5. Enable leg 2, Disable 1 and solve the overall model.

Want to go back and forth between the two leg styles without deleting
and then re-creating all the elements in the two legs... all the
while working in the same large overall model.

One idea I have not tried is to give the "disabled" leg a thickness
near "zero". Remove all loads from this leg, exclude elements,
then view with these groups turned off.

You can't just leave off the material properties and/or thickness..
error check will flag these elements as having problems.

Any better ideas? Anything built in the software to do this?


Bruce Peterson
Great Plains Mfg.
Assaria, Ks 67416

5 REPLIES 5

Re: enable / disable shell elements?

Create a special material (e.g. No_Shell) with Young's Modulus and Density a couple orders of magnitude lower than the rest of the materials in your model. Assign that special material to your shells. Then it is easy to change it back to e.g. Steel.



- wrote:
Mechainica Independent Mode:

Is there a way to leave shell elements in a model and give them
some property (example: zero thickness) so that they don't
participate in any way in the results?


Example: Given some large frame, with two styles of legs....

1. Mesh the frame.
2. Mesh leg style 1.
3. Mesh leg style 2.

4. Disable leg style 2 and solve the overall model.
5. Enable leg 2, Disable 1 and solve the overall model.

Want to go back and forth between the two leg styles without deleting
and then re-creating all the elements in the two legs... all the
while working in the same large overall model.

One idea I have not tried is to give the "disabled" leg a thickness
near "zero". Remove all loads from this leg, exclude elements,
then view with these groups turned off.

You can't just leave off the material properties and/or thickness..
error check will flag these elements as having problems.

Any better ideas? Anything built in the software to do this?


Bruce Peterson
Great Plains Mfg.
Assaria, Ks 67416




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Re: enable / disable shell elements?

You could also use Design Parameters for both density and modulus and run a
Standard Offset Study for the two different cases.

---Jim Holst

re: enable / disable shell elements?

Bruce,

Just be careful when playing with 'almost zero stiffness' elements (no matter whether you create them via almost zero thickness or via material properties) -- before you know it, you may end up with an ill-conditioned assembled stiffness matrix (excessive difference in diagonal elements) that'll cause solution instability or excessive computational errors.

You could avoid the above by also 'grounding' (i.e. constraining all 6 DOF) all the nodes that assemble the 'almost-zero-stiffness' elements, which would effectively eliminate corresponding equations from the system; however, a) if you think of it, it may not be that easy (what to do with the nodes that assemble both 'normal' and 'zero-stiffness' elements?), and b) the whole excercise may become too cumbersome.

Yuri

RE: enable / disable shell elements?

Dear Bruce,

You could always use the simplified rep functionality in Pro/Engineer. Make
a model with both leg styles present and create two reps, one for style1 and
the other for style2. Go into Mechanica with style1 rep and mesh and save
it. Come out of Mechanica and activate style2 rep. Go back into Mechanica
and mesh and save again. If you look in your working directory you will have
two mesh files, <model_name>#style1.mmp and <model_name>#style2.mmp. (The
extension is .mma if it's an assembly.) When you want to run the analysis,
go into Mechanica with the simp rep you want to analyse and before running
the analysis make sure that the run settings are set to 'mesh from file'.
Mechanica will automatically pick up the correct mesh file for the
appropriate simp rep. This technique also works from within Intralink.

We use this method all the time when we want to simplify the model for
analysis purposes. All of the extraneous parts and detail can be removed
from the simp rep, and unlike using suppress, it does not care about
parent/child relationships.


Regards,


Rod



Rod Giles
Elite Consulting Ltd.
The Mechanica Specialists - since 1993
<http: www.elite-consulting.com=">




RE: enable / disable shell elements?

And if you use this technique, copy the analysis to a different name so you
don't overwrite the results.

You will have to keep this straight and know which is which when defining a
result window.

Randy Speed
President and CEO
Speed Consulting, LLC
(972) 938-0490 ph (972) 937-2319 fax
www.speedconsulting.com