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Free-Free Modal Analysis

Creator
Creator

All,

 

Does free-free modal analysis validate the accuracy of every FE analysis? My colleague insists on getting the 6th mode sum to 1 for both translation and rotation (attached). What does it exactly mean if i cannot sum tranlation or rotation to 1 around 6th mode. Could someone please explain in simple terms? Thank you very much.

5 REPLIES

Re: Free-Free Modal Analysis

Legend
Legend

Hi,

 

Free-Free analysis = Free Constraints and Free Loads. A rigid body “in space” should have 3translational and 3rotational degrees of freedom. That’s why you should have the six first rigid body modes frequencies equal or near to zero (if no other mechanism has been intentionnally defined).

If the 7th mode have a frequency equal/or near to zero then you should check the connectivity of the nodes of your model.

In your case the 6th mode frequency is about 2.15. Try

to animate this mode to see how the structure behaves. One of the six rigid body modes is not free.

 

If you are running the free-free modal analysis just to check the FEM then having the first six rigid body modes frequencies near to zero is enough to overview the model connectivity. Note that Free-Free analysis is one of the several methods we generally use to check the FEM.

 

 If possible drop your model and we'll take a look.

Seifeddine Naffoussi

Re: Free-Free Modal Analysis

Creator
Creator
Thank you!

Re: Free-Free Modal Analysis

Experimenter
Experimenter

Hello,

 

thank you for you great answer! I have a similar problem with the 5th-6th modes being near-zero but not zero. From experience would you consider 0.54 Hz ( the 6th mode) as zero?

 

Thank you again for your contribution.

Lina

Re: Free-Free Modal Analysis

Legend
Legend

Sorry for the late answer,
Yes i would consider these two modes as free modes proportionally to the 7th modes frequency.

Regards,

 

Seifeddine Naffoussi

Re: Free-Free Modal Analysis

Legend
Legend

whataboutcve wrote:

Hello,

 

thank you for you great answer! I have a similar problem with the 5th-6th modes being near-zero but not zero. From experience would you consider 0.54 Hz ( the 6th mode) as zero?

 

Thank you again for your contribution.

Lina


0.5Hz could be concerning... You'll rarely have 0Hz (except with seismic mass models), but 0.5Hz seems high...  Did you check the mode shape to really assess if it's a rigid body mode?  Is Nastran "complaining" in the f06 about something else?