*** ERROR # 735 *** Negative modal damping obtained from TABDMP1

Luca Campello
Luca Campello Altair Community Member
edited August 2023 in Community Q&A

good morning every body 

 

I'm conducting a FRF analysis and i have this failure message.

*** ERROR # 735 *** Negative modal damping obtained from TABDMP1. 

 

what does it means? how could i fix this?

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Answers

  • viniciusgomes
    viniciusgomes
    Altair Employee
    edited August 2023

    Hello Luca, 

    This is probably related to the extrapolated values found in the frequency range not defined by you directly in TABDAMP1. For example, your damping curve goes only up to 200Hz and your modal space up to 600Hz and when OptiStruct extrapolates the damping curve to 600Hz, it finds negative values. Try to add one extra line in the TABDMP1 curve covering higher frequencies and setting a positive damping value and the model should work fine. 

    Thanks,
    Vinicius

     

  • Luca Campello
    Luca Campello Altair Community Member
    edited August 2023

    Thank you so much 

    Could it be related to the amount of eigenvalue i've asked him to compute in relation to the frequency band my tab cover?

     

  • viniciusgomes
    viniciusgomes
    Altair Employee
    edited August 2023

    Yes, try to extend your tab up to the highest frequency in your eigenvalues extraction.

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