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Correlation between test and analysis results

We have a plate of 120mm X 90mm X 1.6mm aluminum plate with four holes at the corner. We have performed a modal analysis and conducted a test. But, we are not able to correlate with the test result.

The frequency in test is 420Hz whereas in analysis we are getting 300Hz. what modifications can we do in analysis?.Appreciate your help and suggestions.

Thank you in advance.

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Altair Forum User

Hi Shashi,

check the constraints, loads, loading points, etc... (entire boundary conditions)

Check if the material used (density of the material used, etc...) is same as the test material, thickness if any is same as the test coupon.

If you have RBE2 elements, replace them with bar elements or beam elements and check if you are able to see any change in results as rigids induce artificial stiffness.

try to increase the mesh density and see if you can see good results.

Altair Forum User

Hi Prakash,

We have checked all the parameters but we are not getting any change.

What is the element quality criteria for modal analysis? We are maintaining jacobian as 0.7 , aspect ratio less than 5. Do we need to consider anything other than this which may have large impact on the result?

Altair Forum User

Hi Shashi,

Values 0.7 and above are acceptable for jacobian. Aspect ratio you have provided is acceptable as anything above 5:1 is very rare. Again drawing a conclusion on element quality is very hard. Few OEMs go through a trail and error process comparing the FEA results with test results for different element quality values.

Can you share the files and results via FTP link https://ftam1.altair.com/filedrop/~Ce4god

Please note: if you are a commercial customer, you can contact your local support @ http://www.altairhyp...ortProduct.aspx

Altair Forum User

Hi Prakash,

Can you please tell me how to apply bolt pretension? I will apply pretension and see if it increases the natural frequency. We have fixed the plate to the vibration fixture at four corners. So,I need to apply pretension to these four bolts and see the results. I have seen the tutorial but cannot understand.

Altair Forum User

Hi Shashi,

Please refer to the attached tutorial on pretension.

Unable to find an attachment - read this blog

Altair Forum User

Shashi,

Instead you can apply Preload and use the same in normal modes subcase.

Altair Forum User

How to apply the preload?

Altair Forum User

Hi Prakash,

U der?

Altair Forum User

Hi Shashi,

Create a linear static subcase with load applied on bolts and refer the same subcase in Normal modes loadstep.

Maybe this Tip and trick may help you: http://www.altairuniversity.com/wp-content/uploads/2014/02/Gavity+normal-modes.pdf

I dont know if pretension an help in your case as Pretension STATSUB(PRETENS) is supported for non-linear static subcase