FEM anaylysis giving object acceleration

[Anonymous]

Hi

I do not know how to do the folowing:

I have an object that has to be accelerated by an external force. It can freely move in space.

But when I apply the forces,the analysis complains that there are no constraints.

I don't want to constraint the object. Because that would result in stresses that will not be there in practice.

I just want to see the stresses when the forces are applied to accelerate the object.

So how to deal with this?

[JDMather]

Sounds like you might want to use the Input Grapher in Dynamic Simulation and then a Motion Loads FEA analysis.

[henderh]

Hi melomania,

  If there aren't balanced loads (or sufficient restraints) then by by definition the object isn't in static equilibrium and Stress Analysis will complain about it.

  I understand what you're saying though.  Take for example and aircraft in flight, where the propulsion force is equal to the opposing drag force.  There are balanced loads, but no where to add a restraint since it needs to remain free to move in space.  Another example could be a pressure vessel 'floating' underwater...there aren't any structural restraints to apply.

  FEM can account for case1 by using either the 'inertial relief method' or case2 by "Detect and Eliminate Rigid Body Modes".  It is an option you can check in the Simulation Properties dialog.

  JD makes an excellent point to use Dynamic Simulation (in conjunction with FEA) to do the export to FEA / motion loads workflow.  This will automatically include the inertial relief method during solving and may be a feasable approach depending on the system you're simulating.

  I'd like to mention that Detect and Eliminate RGM should be used primarily for investigating where restraints / constraints should be applied.  In many cases, by constraining certain vertices only in certain directions (via vector components checkboxes) you won't constrain object from deforming freely yet it can still constrain rigid body movement.

Hope this helped!  Please let us know if you have any additional questions, comments or suggestions.

Best regards, -Hugh