Rigid Body Modes
Rigid Body Modes
(OP)
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
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.'





RE: Rigid Body Modes
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
RE: Rigid Body Modes
Cheers
Greg Locock
RE: Rigid Body Modes
Cheers
'Ability is 10% inspiration and 90% perspiration.'
RE: Rigid Body Modes
regds
raj
Raj
RE: Rigid Body Modes
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
RE: Rigid Body Modes
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.'
RE: Rigid Body Modes
Are you talking about very small frequencies?
Sorry, I'm no node pusher, what is an MPC?
Cheers
Greg Locock
RE: Rigid Body Modes