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Normal Mode Analysis Problem.....

Hello everyone..

We are doing Normal Mode Analysis of Automobile headlamp. We have joined parts by rigid elements & applied constraint accordingly.

But we are not getting correct Frequency . In other software Frequency is 52.9 to 314 Hz while in FEMAP-NX Frequency is (2.23E-4 HZ to 36.61 Hz).Why this diffrence is for same model ?

Also I have one more query regarding the units of specified material and properties. I am applying

1).Young modulus in Mpa
2).Density in tonne /mm3

My model (means all parts dimensions) is in mm.
What could be the possible cause behind in FEMAP-NX?

Yaseen Khan

Re: Normal Mode Analysis Problem.....

Siemens Builder Siemens Builder
Siemens Builder

Your properties look correct for something like a Nylon/Polystyrene type of material. Is that what you are trying to analyze?

Is this a free-free analysis?
- If so, is the other software automatically filtering out the rigid body modes (the first six modes reported by NX Nastran are E-4 rigid body modes)?

- If not, check that your intended constraints are being applied

Do the mode shapes look the same as the other analysis?

In general, normal mode are simply sqrt(K/M). If the modes are too low, the model is too flexible or the mass is too large. If the modes are high, the model is too stiff or the mass is too small.

Stiffness is affected by Elastic modulus, geometry (thickness) and connectivity (duplicated nodes merged/coupled, constraints, etc). Mass is affected by density and geometry.

Jim Bernard
Advanced Applications Engineer

Siemens PLM Software
2000 Eastman Dr., Milford, OH 45150-2712

Re: Normal Mode Analysis Problem.....

Dear Yaseen,
Your model is not constrained properly, your first six modes are zero, then you have rigid body movements. Also check your total mass of the model, the units of Density in Ton/mm3, EX in MPa and length in mm are correct, the the output mass will be in Tons. Check your F06 file to see is the resulting computed mass is in line with the mass of your model.
Best regards,

Re: Normal Mode Analysis Problem.....

Dear Jim,
we have checked our constraints.Yes it is ppt30 ( Plastics material ). Two parts are connected with rigid elements,& model has been applied with four constraints. Is there any other way to connected two parts ( If it is actually connected by adhesive like silica material )?
We checked & found duplicate node,now these nodes get merged ( Now see the attached results ).

Dear Blas,
plz Check the mass properties of the model, & suggest...