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How can I compute MAAX and KAAX matrix , extracted from Nastran, to obtain the eigenvalues and eigenvector from a not constrained model ?

kenny.fernando.conto
How can I compute MAAX and KAAX matrix , extracted from Nastran, to obtain the eigenvalues and eigenvector from a not constrained model ?
  • 0
    Kenny,
     
    I am uncertain of your question- do you already have KAAX and MAAX matrices? Or are you asking how to generate them?
     
    Don
  • 0
    Hi Don
    I already have obtained the KAAX and MAAX matrices using PARAM,EXTOUT,DMIGPCH inside begins bulk section (but apparently the order of the KAAX and MAAX matrix are different each other, always the MAAX matrix order is lower than KAAX). I would like to know if with these matrices I can obtain the eigenvalues and eigenvector from a not constrained model.
    Thank you for your help
  • 0
    Yes, you can do this... assuming there were no constraints in the 'creation' run, then as long as you do not add constraints in the 'assembly' run, then it will be free-free. Here is a sample input file. Please let me know if there are other questions...
     
    SOL 103
    CEND
    K2GG=KAAX
    M2GG=MAAX
    ECHO =NONE
    METHOD = 1
    DISP(plot)=ALL
    BEGIN BULK
    PARAM   POST   0
    EIGRL   1                      20     0                      MASS
     
    <include the grid definitions here>
    <include the KAAX/MAAX DMIG here>
     
    ENDDATA
  • 0
    Thank you a lot. I will work on it.
     
    By another hand. I have to extract the global stiffness and mass matrices from different models (I am using PARAM,EXTOUT,DMIGPCH again). I'm planning to use these matrices to do various mathematical operations on them in Matlab. For that, I did a simple beam example with 4 nodes. When I extracted the MAAX and KAAX matrices in .pch file I could notice that the MAAX matrix has a lower order than KAAX matrix furthermore and the stiffness matrix is always singular.
    I would like to know:
     
    If those steps that I did are correct to obtain the global stiffness and mass matrices (I mean, using PARAM,EXTOUT,DMIGPCH).
     
    Why the mass and stiffness matrices have a different order in the .pch file? (in my example, Kaax is a sym matrix with order 24, and the Maax is a diag matrix with order 16)
     
    Thank you for your help
  • 0
    Kenny, I think it is best to work this issue offline as a technical support question... I'll contact you via our support system for more details.
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