Frequency Disappearing for half model 2

[Jayeshjhe]

I was doing modal analysis of PCB board in ANSYS. I have done for full model, half model and quarter model. But some of the natural frequencies in the full model is disappearing in the half model and some more in quarter model.

Is there any article regarding this subject?

Any reasons available?
Will this affect for my analysis?

 

[Elabbasi]

Even though your structure is symmetric, it has both symmetric and non-symmetric natural frequencies and mode shapes. When you model half or a quarter your model cannot capture these non-symmetric modes. There is a workaround however. You can solve the half model twice, once with symmetry and once with anti-symmetry boundary conditions. The combined solutions should give you all the natural frequencies of the full system.

Nagi Elabbasi
Veryst Engineering

Nagi Elabbasi

http://www.veryst.com

 

[rb1957]

yes, you'd lose the symmetric modes ... yes?

Quando Omni Flunkus Moritati

 

[boyze]

if the PCB is square and populated "uniformly" you'll have be careful not to miss the modes about the diagonals if you happen to be using the other symmetry lines. My experience has been to avoid symmetry for modal analyses. Unless you are looking for very high modes I don't see the need to be stingy with the elements.

Have Fun!

James A. Pike

http://www.xl4sim.com

http://www.erieztechnologies.com

 

[Jayeshjhe]

@rb1957

i lost the frequencies in between while doing half model

 

[Jayeshjhe]

Is there any problems if I do analysis on half/quarter model? Do i need to take any more steps?