Stiffened plate strange modes.

[Alexkoguar]

Hi everyone , i have created a stiffened plate(with ribs and spars)clamped at one end(a wing model for flutter analysis), the plate is meshed with bar2 and quad4 element, When i run the modal analysis in nastran i found strange in plane modes that i cannot completely eliminate. Can anyone help me please? The right sequence of frequencies in Hz is: 4/8.9/25/33/70/76/134/144.

 

[WARose]

I can't open a bdf file. (And I expect there are others.) You may want to try a pdf (to get more responses).

 

[Alexkoguar]

You are right. Here is the file

 

[MIStructE_IRE]

I’m sorry, but that page of nonsense means about as much to me as the original un-openable bdf file.

 

[Alexkoguar]

I am sorry, non sense in what way? Its a nastran input file.

 

[JStephen]

I can offer no advice on the specifics, but trying to visualize from your one-line description- I would check that masses and stiffnesses in all directions were appropriate- specifically watching for something like m versus mm, ft vs inch, E vs Fy, etc.

 

[Alexkoguar]

Thanks, i dont know how to block the membrane action efficiently(strange because impose the bending element). When i constrained two free ends it works fine for the first frequencies and if i block all the in plane dof the frequencies are wrong.

 

[SWComposites]

Post pictures of your model and pictures of the mode shapes.

 

[Alexkoguar]

This ia my model(obviously i offset the beams even if in the picture the plate is in the middle) , It should only bend and twist out of is plane like a normal wing,but in my case i find even the in plane modes. Maybe there is a way to limit my analysis in only 2 perpendicular planes.

 

[WARose]

The nastran input file/code doesn't mean a lot to me....I don't work with that program. (And I cannot tell what the boundary/support conditions are from it.)

The pic is helpful.....but I cannot tell what the boundary/support conditions are there either.

Just guessing based on what you posted: you likely (in order to make the model stable) have some sort of in & out-of-plane restraint/supports at some points along the perimeter. With that set up, you will get a lot of lower modes participating for the out-of-plane modes.....but the in-plane stuff will mainly be with higher modes (and likely low mass participation). That is, assuming you are talking infinitely stiff restraint in the in-plane direction.

You likely aren't interested in the in-plane stuff.....so you can just ignore it if you want, or come up with a in-plane stiffness for the supports that is accurate.

 

[SWComposites]

You have out of plane plate elements?
Post pictures of the mode shapes in question.

 

[Alexkoguar]

Thanks WARose, but when i constrained the nodes for the in plane dof, the natural frequencies of bending and twisting out of plane are wrong. I used shell elements and bar elements.