modeling symmetry vs whole model

[tr6]

I have a 24" dia tubesheet plate 1" thick with 2" dia holes on a 60^ pitch on 2-5/16" centers welded on the OD. The tubesheet has a surface pressure on one side and zero on the backside. Also, there are surface loads around the hold diameters. I first modeled the plate as a quarter plate using Z & Y symmetry boundary conditions. This resulted in an extremely high stress point (twice the ultimate)at the intersection of the two symmetry planes; the center of the plate. I then modeled the entire plate. Same loads. The stress results were about half.

My question is: why do the two modeling methods produce such different results?

 

[GBor]

I didn't see where either of the load sets were disabled in what you sent me. When you developed the full model, did you just mirror what you have (because I don't think the mesh would like up that way), or did you have a different mesh size? If your full model has a more or less defined mesh, results may vary. If they vary as much as they seem to, you may need to perform a mesh sensitivity study (always recommended) to make sure you are reaching a proper convergence. I haven't had sufficient time to "play" with this, but the displacement dither looked clean and the stress contours didn't surprise me.

 

[Zapster]

So, I am not sure where to go from here.
Stating the obvious, did you contact the technical support folks at ALGOR. Usually when I see questions regarding software applications that would seem to be best answered by the vendor, I think about two things: 1. The technical support folks for the software package must be awful. 2. The software is bootlegged and the operator does not have a legal license.

 

[GBor]

I did notice that you are not using the latest version...v.23 had to translate whatever version you are using (v.21? v.22?). The tech folks at Algor are going to ask you to send archives of each model so they can check for differences. You may want to be ready for that...

 

[GBor]

Just for kicks, I ran a 10" radius, 1" thick 1/4 symmetry disk, pinned (fixed) rim with a 20 psi load and received the following:

VM Stress = 1318.8 psi
Deflection = 0.0012196

Mirroring the 1/4 model about an axis and then the 1/2 model about the other appropriate, perpendicular plane to complete the circle, I get the following:

VM Stress = 1318.8 psi
Deflection = 0.0012152

Your 1/4 model isn't on the global axes, and I didn't take the time to mirror and then move, but the comparison that I ran suggests that the software is reasonably consistent.

 

[4Pipes]

Just to qualify Zapster's rhetorical comment #1 regarding possible quality of support. I own Algor. My personal experience so far has been that support has been very good, patient with my questions and has always been very quick.

I am expecting that the high standard of support will continue under Autodesk ownership.