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Subject: Re: RBE3 constraint equation error
Author: Mark Tate
Date: 2001-04-12 12:00:00

You should check that you are using a 3D mass element.
Also make sure that nodal coordinates are not rotated.

Mark C. Tate
Principal Analysis Engineer
Hamilton Sundstrand - A United Technologies Company
mark.tate@h...

-----Original Message-----
Sent: Thursday, April 12, 2001 8:16 AM
To: xansys

Stan,

Uhm... yes, I am applying standard DOF constraints (using the D
command) to the master node, which is in the centre of the pipe end.
That's the whole point of what I am trying to do: place a simple
support (as used in basic beam structures) on a 3D model of a
pipe/beam. The RBE3 should then transfer rotations as well as
translations to the nodes on the edge of the pipe, in the form of
translations only, if you know what I mean.

This is exactly what I did in my test case, which was a solid
cylinder meshed with 20-node bricks. I selected all the nodes at one
end of the cylinder, including a centre node, which I had meshed with
a mass element (my master node).
Then I used the command: RBE3,, ALL, ALL

I did this in both ends of the cylinder. This should then enable me
to place my DOF constraints on the master nodes only, so I can create
a pinned connection or whatever I want, right? The test worked
beautifully with the PCG solver...

But when I do the exact same thing on my real model, I get that weird
error, and ANSYS claims that my 'batch job has finished successfully'
and closes down completely (after an OK prompt). When I list the CEs,
they seem ok, in the sense that they 'convert' my rotational DOFs to
translations along the edge.

If my model was ill-conditioned, wouldn't I 'just' get convergence
problems? I don't even get to the real solution phase here...

Thanks again :)

Uffe Eriksen,
Rambøll, Esbjerg,
Denmark

--- In xansys@y..., "Schwantes, Stan" wrote:
> Uffe
> Are you applying restraints to any master node? I get nervous about
> applying restraints to nodes involved in any type of constraint
equation.
> (Look at help on CE command). If some DOF's at the master node are
> eliminated by the restraint, I'm not sure that ANSYS and the PCG
solver
> handle that correctly. Did you restrain a master node of RBE3 in
your
> successful small model test case?

> In your non-midside node version the RBE3 worked correctly so that
may not
> be the whole problem. It may be more of an ill-conditioned matrix
thing,
> and the PCG solver does not handle ill-conditioned problems as well
as the
> sparse solver does.

> To avoid restraints on the master node, try a very short elastic
beam tied
> to the master node. The other end of the short beam is on the pipe
> centerline and has the restraints applied to it. This may give you
a better
> conditioned system.
> Hope this helps.

> Stan Schwantes 952/931-5544
> Alliant Techsystems, Inc
> Defense Systems Group E-mail: stan_schwantes@a...

> !!!!!!!!!!!!!!!!!!!!!!!!

> -----Original Message-----
> From: udeansys@y... [mailto:udeansys@y...]
> Sent: Thursday, April 12, 2001 9:25 AM
> To: xansys@y...
> Subject: [xansys] Re: RBE3 constraint equation error

> Thanks Stan!

> 1. I am using ANSYS 5.6.2. We haven't had the opportunity to
install
> ANSYS 5.7 yet.

> 2. I am doing a standard linear-elastic analysis, in order to find
> stress concentration factors in the joint, for later use in our in-
> house frame software.

> 3. I will try the CECHECK command as soon as possible!

> 4. You are right, I do not want that. We have specifically been
asked
> to simulate a simple support for the chord ends, i.e. just fixed
for
> translations in the centre of the pipe end, and with a 'roller' in
> one end.

> 4.2. Actually, my RBE3 web at the brace end seems to work fine.
It's
> the RBE3's in the chord ends that are acting up, as far as I can
tell.

> I ran the analysis with the brace RBE3 only, and all the nodes in
the
> chord end fixed for translations, just as a test. That worked fine.

> But when I make RBE3's at the chord ends, the fatal error comes
right
> after 'preparing elements for solver'. Very weird...

> 5. I would rather not have to use a beam web, but in the end I
might
> have to try it after all...

> The funny thing is, I have done this type of analysis in all shell
> models heaps of times, with stiff endplates for all pipes. In
theory,
> I can't see why a shell endplate wouldn't work with solid element
> pipes as well, but the results I get (with the PCG solver at least)
> are plain crazy! The whole thing rotating and expanding(!), even
> though I have fixed it for axial rotations... I suppose it's a
quirk
> of the iterative PCG solver.

> Thanks anyway, Stan, and keep'em coming! :)

> Best regards,

> Uffe Eriksen,
> Rambøll, Esbjerg,
> Denmark

Posts possibly associated with message #22666AuthorDateScore
22646RBE3 constraint equation errorUffe Dal Eriksen2001/04/12 
22651Re: RBE3 constraint equation errorStan Schwantes2001/04/12 
22653Re: RBE3 constraint equation errorUffe Dal Eriksen2001/04/12 
22655Re: RBE3 constraint equation errorStan Schwantes2001/04/12 
22662Re: RBE3 constraint equation errorUffe Dal Eriksen2001/04/12 
22666Re: RBE3 constraint equation errorMark Tate2001/04/120
22668Re: RBE3 constraint equation errorUffe Dal Eriksen2001/04/12 
22670Re: RBE3 constraint equation errorMark Flanagan2001/04/12 
22695Re: RBE3 constraint equation errorUffe Dal Eriksen2001/04/13