FINITE ELEMENT MODEL VALIDITY CHECK 2.
Free-Free Dynamics with a Stiffness Equilibrium Check
Purpose
The Free-Free Dynamics with a Stiffness Equilibrium check verifies that the model will act as a rigid body when it is unconstrained. It also checks the stiffness matrix to verify that it doesn't contain any grounding effects, such as illegally specified SPC or rigid elements. To perform this check, remove all external constraints (usually SPCs) and perform a standard normal modes run recovering at least the first six modes.
Output Results
The model should obtain a minimum of six rigid body modes. These modes should have frequencies less than or equal to 1.0E-04 Hz. The GROUNDCHECK case control card should be used as a stiffness equilibrium. Strain energy filters are set by the THRESH parameter in the card. By default, the threshold is calculated by dividing the largest term in the stiffness matrix by 1.0e+10. The output will indicate whether the check is passed or failed.
The equilibrium check gives possible grounding in the model. In the output file look G-SET, N-SET, F-SET, and A-SET. The maximum strain energy for each DOF will be shown in these tables as well as whether or not it passes or fails. The following descriptions give possible problems from a particular table. Note that an error in a given set will cause errors to appear in subsequent sets, regardless if any exist.
- G-set
- CELAS problems. Non-coincident grids or misaligned output coordinate systems (most common).
- DMIG or DMAP inserted matrices associated with GRIDs. For the latter they should obviously be inserted before the check.
- Bad NASTRAN elements. This is rare but possible. Also small errors from a lot of elements will eventually add up, so very large models often have relatively poor equilibrium.
- N-set
- Incorrect or imprecise MPC equations. Usually similar to CELAS problems. NASTRAN runs with improperly inserted Craig-Bampton models will show errors in this table if the boundary, as well as modes, were placed at SPOINTs. This is because this is the first set where geometry is associated with the boundary.
- F-set
- Incorrect SPC's. Note that a constrained model will obviously show grounding.
- A-set
- The model is constrained to ground (and not intended to be Free-Free).
- The reference GRID point in the GROUNDCHECK command is located too far from the model CG. The GRID parameter is optional.
- PARAM,AUTOSPC,YES constrains near-singular DOF.
Input Required
EXECUTIVE DECK must include the following information:
SOL 103 for MSC
CASE CONTROL must include the following information:
GROUNDCHECK(SET=ALL)=YES (this will print out all the set info - sample output below) SPCFORCES=ALL DISP=ALL SPC=100 (only for normal plate rotations, structure must free at base) METHOD= 20
BULK DATA must include the following information:
EIGR, 20, (Use any method you want to obtain all rigid bodies modes and the first flexible mode. We recommend LANCZOS.)
Sample GROUNDCHECK Output
*** USER INFORMATION MESSAGE 7570 (GPWG1D)
RESULTS OF RIGID BODY CHECKS OF MATRIX KGG (G-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 9.216514E+00
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 2.098354E-05 PASS
2 6.123831E-06 PASS
3 3.367564E-08 PASS
4 1.326509E-03 PASS
5 9.882150E-04 PASS
6 4.239914E-04 PASS
SOME POSSIBLE REASONS MAY LEAD TO THE FAILURE:
1. CELASI ELEMENTS CONNECTING TO ONLY ONE GRID POINT;
2. CELASI ELEMENTS CONNECTING TO NON-COINCIDENT POINTS;
3. CELASI ELEMENTS CONNECTING TO NON-COLINEAR DOF;
4. IMPROPERLY DEFINED DMIG MATRICES;
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*** USER INFORMATION MESSAGE 7570 (GPWG1D)
RESULTS OF RIGID BODY CHECKS OF MATRIX KNN (N-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 9.216514E+00
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 2.098167E-05 PASS
2 6.120336E-06 PASS
3 3.724166E-08 PASS
4 1.326376E-03 PASS
5 9.887038E-04 PASS
6 4.241481E-04 PASS
SOME POSSIBLE REASONS MAY LEAD TO THE FAILURE:
1. MULTIPOINT CONSTRAINT EQUATIONS WHICH DO NOT SATISFY RIGID-BODY MOTION;
2. RBE3 ELEMENTS FOR WHICH THE INDEPENDENT DEGREE-OF-FREEDOM CANNOT DESCRIBE
ALL POSSIBLE RIGID-BODY MOTIONS.
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*** USER INFORMATION MESSAGE 7570 (GPWG1D)
RESULTS OF RIGID BODY CHECKS OF MATRIX KFF (F-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 9.216514E+00
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 2.098167E-05 PASS
2 6.120336E-06 PASS
3 3.724166E-08 PASS
4 1.326376E-03 PASS
5 9.887038E-04 PASS
6 4.241481E-04 PASS
SOME POSSIBLE REASONS MAY LEAD TO THE FAILURE:
1. CONSTRAINTS WHICH PREVENT RIGID-BODY MOTION.
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*** USER INFORMATION MESSAGE 7570 (GPWG1D)
RESULTS OF RIGID BODY CHECKS OF MATRIX KAA1 (A-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 9.216514E+00
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 2.098167E-05 PASS
2 6.120336E-06 PASS
3 3.724166E-08 PASS
4 1.326376E-03 PASS
5 9.887038E-04 PASS
6 4.241481E-04 PASS
IF THE MODEL HAS PASSED THE PREVIOUS CHECKS FOR THE G-SET AND N-SET,
THEN SOME POSSIBLE CAUSES ARE:
1. THE MODEL IS NOT INTENDED TO BE FREE-FREE WHICH INDICATES THAT THE MODEL IS
PROPERLY CONSTRAINED TO GROUND;
2. THE REFERENCE GRID POINT (GRID=GID ON THE GROUNDCHECK COMMAND) IS LOCATED
TOO FAR FROM THE MODEL'S CENTER OF GRAVITY. IT IS RECOMMENDED THAT THE
REFERENCE GRID POINT BE LOCATED AS CLOSE AS POSSIBLE TO THE MODEL'S CENTER
OF GRAVITY OF THE MODEL (SEE THE GRID POINT WEIGHT GENERATOR OUTPUT);
3. PARAM,AUTOSPC,YES CONSTRAINS NEAR-SINGULAR DEGREES-OF-FREEDOM. WHEN A
FINITE ELEMENT MODEL WITH AUTOSPC FAILS THE A-SET CHECK, IT IS NOT EVIDENT
THAT GROUNDING HAS OCCURRED. THE USE OF PARAM,SNORM WILL NOT ELIMINATE THE
SPURIOUS FAILURE.
