Rigid Body Modes 3

[Spirit]

Hi pals.
I am having non-zero frequencies for the rotational rigid body modes of a FEM built with solid parabolic elements, while the translational RB modes are OK.
Any of you has an idea of where these frequencies can come from? Can I expect uncertainties in evaluating the stiffness characteristics of this kind of model?
Thanks, bye.

Spirit
'Ability is 10% inspiration and 90% perspiration.'

 

[idly123]

Consider the simple case of the first bending mode of a rigid uniform bar in a planar world. If you constrain the ends with pin joints to suppress the X Y and RZ rigid body modes then do you also affect the frequency of the first bending mode?

Which way does the frequency move?

Given that you have added stiffness (ie additional constraints) to the system does that surprise you?

Do you understand why ? Raj

 

[GregLocock]

Quotation without attribution is not wisdom. Cheers

Greg Locock

 

[Spirit]

'Criticism without suggestion is not helpful.' (Spirit)

Cheers
'Ability is 10% inspiration and 90% perspiration.'

 

[idly123]

Well greglock it was ur resposne to one of my querries that i had cut and pasted here for spirit without acknowledging that it was ur response and not mine....irony is that ur name wasnt copied along with the response.. well the mistake is regretted.....
regds
raj Raj

 

[GregLocock]

Sorry Raj, that was hot tempered of me. Nobody answered those questions the first time round, which struck me as alarming. I must admit I can't actually answer the question at a hand-waving level, even if the answer can be made to fall out the bottom of some algebra. Here's my ////guess//// :

I would have thought that adding constraints would always stiffen the system.

By pinning the bar to the substrate we have actually coupled an infinitely large mass into the system.

So perhaps the slight increase in k is more than offset by the infinite increase in m.

I don't like this solution one little bit. Cheers

Greg Locock

 

[Spirit]

OK, better be more complete.
I have a FEM model constituted by two bodies meshed separately, ant the welded at the interface using MPC relations. Such a model, when free-free NMA checked, exhibits three non-zero frequencies for the rotational rigid body modes. The question is: is this a signal that the stiffness of the complete assembly is misestimated or is this simply a mathematical problem? Thanks, cheers.


Spirit
'Ability is 10% inspiration and 90% perspiration.'

 

[GregLocock]

Yes, we had been ignoring your actual problem. If you run each free body separately do either have non-zero RBMs?

Are you talking about very small frequencies?

Sorry, I'm no node pusher, what is an MPC?

Cheers

Greg Locock

 

[vonlueke]

Spirit: How is your entire assembly constrained? Is the entire assembly a cantilever or simply supported structure? Solid element nodes have no rotational dofs; i.e., they have no bending resistance. If you happen to have a situation in which clamp, pin, or MPC constraints are connected to nodes along a perfectly straight line, then these constraints would constitute a hinge, in which case your model might be a mechanism. Ensure your constraints do not lie along a straight line on solid elements. If unavoidable, then you would need to either (a) spoke the "hinge" constraints to adjacent solid element nodes using rigid bars or RBE3 elements, or (b) shell coat (cover with shell elements) the solid elements where hinge(s) exist.