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Rotational inertia of nodes with 6 DoF ommited in (.full.mat) file

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Posts: 1
Topic starter
(@pedromillan)
Barista
Joined: 2 weeks ago

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

 

-----------------------------------------------------------------------------------

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

-----------------------------------------------------------------------------------

1 Reply




Posts: 13
(@joedoe)
Student
Joined: 6 years ago

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

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