Material Law 82 Implicit Solver Matrix Error

John Crowder
John Crowder Altair Community Member
edited September 2021 in Community Q&A

I'm working on converting a FE model from Abaqus to radioss, and throughout this process, verifying the model behavior is matching. I am currently trying to verify that Material Law 82 is performing as expected for the implicit solver. I am getting the error "STIFFNESS MATRIX IS NOT DEFINITE" and "SOLVER IMPLICIT STOPPED DUE TO MODELLING DATA. ISTOP=-1". Could I have some help getting this model to run?

I have expected stress v. strain plot for the Ogden material behavior. To verify this, I am trying to displace an element and then plot the stress/strain to compare to expected results. 

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Answers

  • Rogerio Nakano_21179
    Rogerio Nakano_21179 New Altair Community Member
    edited September 2021

    Hi

    I suggest you use the Explicit for this analysis, at least to see that results are as expected

    Also, check the material information according to the consistent unit system you are considering.

    density is about 2200 (kg/m3) ?

    apha_1 is about 0.9 (Pa) ? It seems too low .

     

    also element dimension is 10x10 (m3) ?  but thickness is 0.00237 (m3)

     

    Regards

     

  • John Crowder
    John Crowder Altair Community Member
    edited September 2021
    Rogerio Nakano_21179 said:

    Hi

    I suggest you use the Explicit for this analysis, at least to see that results are as expected

    Also, check the material information according to the consistent unit system you are considering.

    density is about 2200 (kg/m3) ?

    apha_1 is about 0.9 (Pa) ? It seems too low .

     

    also element dimension is 10x10 (m3) ?  but thickness is 0.00237 (m3)

     

    Regards

     

    Thanks for the reply. I have:

    • Reduced the element dimension to 1m x 1m x 2.37 mm
    • Changed from implicit to explicit analysis
    • Kept the same density
    • kept the same alpha1
      • I would prefer to keep the alpha, mu, and D parameters the same to match the curve I have calculated (shown in attachment). 

    The model still doesn't appear to be running with M82, or at the minimum results aren't showing. I have also included materials in the model for M1_ELAST and MLAW69. Both of these seem will run and output results, but when I switch to M82_OGDEN I get nothing. I am modeling something wrong in my material card, or is there some non-compatibility I'm unaware of?

    Thanks, 
    John

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