Dear all,
I am extracting the mass matrix of a structure, in hypermesh OptiStruct, modeled with shell elements (CQUAD4) using OUTPUT,MATRIX,FULL,SPARSE. However, the rotational inertia corresponding to the nodes (with 6 DoF) is omitted in the .full.mat file.
Below is an example of the results I am getting, for a lumped mass matrix, where the rotational inertia (Ixx, Iyy, Izz) for node 1 are missing (which would correspond to missing the headers: '4 4 4', '5 5 5', and '6 6 6', that are omitted).
Please let me know if you any idea how to solve or go arround this problem.
Best regards,
Pedro Millan
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MASS 0 2 3 408 408 1p,8e16.9
1 1 1
1.524147950E-07
2 2 2
1.524147950E-07
3 3 3
1.524147950E-07
7 7 7
3.051891042E-07
8 8 8
3.051891042E-07
9 9 9
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This is a common issue in OptiStruct when dealing with shell elements and mass matrices. Let me explain the situation and provide some potential solutions:
1. Default Behavior:
- Shell elements in OptiStruct typically don't generate rotational mass terms by default
- The missing rotary inertia terms (Ixx, Iyy, Izz) are related to how shell elements handle mass distribution
2. Possible Solutions:
A. Using PARAM,WTMASS:
PARAM,WTMASS,value
This parameter can help include rotational mass terms, though results may still be incomplete.
B. Add Concentrated Masses:
CONM2,EID,G,CID,M,X1,X2,X3,I11,I21,I22,I31,I32,I33
You can explicitly define rotational inertia terms using CONM2 elements at key nodes.
C. Modify Element Properties:
- Use PSHELL with non-zero bending inertia
- Add appropriate thickness values that consider rotational effects
D. Alternative Approach:
1. Extract the translational mass matrix
2. Calculate rotational terms separately using:
- Element thickness
- Material density
- Element area
3. Combine both matrices in post-processing