RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
(OP)
Hallo guys,
I am not newbie in NASTRAN environment, but I don't seem to crack what on earth is wrong with my model (Model Attached).
I ran a FREE-FREE check on my structure and requested for all the 'SETS', to my surprise, the model fails the 4, 5 and 6 directions while the eigenfrequencies show that the structure is fully a rigid one. Can somebody with more experience help me crack it please!. I will really be grateful for your effort. Thanks in advance.
*** 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 1.232662E-04
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 1.898201E-09 PASS
2 1.770204E-09 PASS
3 1.048686E-09 PASS
4 1.633240E-03 FAIL
5 1.100441E+01 FAIL
6 4.751907E+00 FAIL
MODE EXTRACTION EIGENVALUE
NO. ORDER
1 1 -9.614290E-06
2 2 -3.726920E-06
3 3 -1.669621E-06
4 4 -4.575413E-07
5 5 4.400790E-07
6 6 3.495363E-06
7 7 4.155432E+02
8 8 5.656006E+02
9 9 8.297285E+03
10 10 1.224819E+04
I am not newbie in NASTRAN environment, but I don't seem to crack what on earth is wrong with my model (Model Attached).
I ran a FREE-FREE check on my structure and requested for all the 'SETS', to my surprise, the model fails the 4, 5 and 6 directions while the eigenfrequencies show that the structure is fully a rigid one. Can somebody with more experience help me crack it please!. I will really be grateful for your effort. Thanks in advance.
*** 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 1.232662E-04
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 1.898201E-09 PASS
2 1.770204E-09 PASS
3 1.048686E-09 PASS
4 1.633240E-03 FAIL
5 1.100441E+01 FAIL
6 4.751907E+00 FAIL
MODE EXTRACTION EIGENVALUE
NO. ORDER
1 1 -9.614290E-06
2 2 -3.726920E-06
3 3 -1.669621E-06
4 4 -4.575413E-07
5 5 4.400790E-07
6 6 3.495363E-06
7 7 4.155432E+02
8 8 5.656006E+02
9 9 8.297285E+03
10 10 1.224819E+04





RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
First of all, your unit system is incoherent...you are working in mm, so start by changing:
E = 73000 to 73e+6MPa
Density = 2.7e-9!!? to 2.7e-4 g/mm3
Seif Eddine Naffoussi, Stress Engineer
www.Innovamech.com
33650 Martillac û France
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
he don't need a constraint for a free free analysis nor for ground check.
Seif Eddine Naffoussi, Stress Engineer
www.Innovamech.com
33650 Martillac û France
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
Nothing is wrong with the units,
E= 73100 Nmm-2
Density = 2.78E-9 Tonmm-3
So, it can't be the units.
@ rb1957, as CompositeCurves specified, we don't need constrains for a fee-free check.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
Seif Eddine Naffoussi, Stress Engineer
www.Innovamech.com
33650 Martillac û France
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
If you have time and found out something, just let me know. Thanks for taking your time.
I am also working on it, in case my solution works, I will post it.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
units ... yes, not the problem but are Tonnes the consistent mass unit (or weight unit) for N ?
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
Regarding the geometry and connection between parts I will be curious to see what it gives...
Seif Eddine Naffoussi, Stress Engineer
www.Innovamech.com
33650 Martillac û France
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
@ CompositeCurves, Adding more slaves is what I thought would solve the issue, I just did that, but the results stay the same.... Still FAILS.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
MODAL_FIXED SUBCASE 1
R I G I D - B O D Y E I G E N V A L U E S
MODE EIGENVALUE STRAIN ENERGY
NUMBER EXTRACTED NORMALIZED EXTRACTED NORMALIZED
1 1.588782E-10 3.581042E-13 3.669735E-07 8.271415E-10
2 3.167704E-10 7.139860E-13 8.286201E-07 1.867672E-09
3 7.115370E-10 1.603772E-12 1.778880E-06 4.009514E-09
4 8.235215E-10 1.856180E-12 8.218623E-07 1.852440E-09
5 1.061114E-09 2.391703E-12 2.890820E-06 6.515777E-09
6 2.582696E-09 5.821279E-12 3.670704E-06 8.273599E-09
All I did was remove param bailout, remove autospc and used neinastran.
www.stressebook.com
Stressing Stresslessly!
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
I guess i will never get around it, BUT am still trying......Thanks for taking your time...If you have a shot at it again, just let me know.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
looking at your structure, see attmt, i think you've got a pattern of rivets where i've drawn boxes, but i think you've only got a line of rivets where i've drawn lines ... if so, then the lines will create a mechanism.
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
All in all, coming back to your suggestion, I think you are right, Some mechanism exists in the structure...The positions you indicated are the ones with fasteners (All the RBEs on the model, indicate the positions of the FASTENER). I guess, i will have to give up on it. The rest of the checks are perfectly correct and results generated from GLOADs, are perfectly acceptable.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
I am not the designer, I am the stress guy and of course I will have to make suggestions on how to improve it and that's why am thankful for your "Experienced look". By the way, which single line attachment are you talking about (A screenshot would be great).
Thanks for your contribution.
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
careful designing "covers" ... they have a nasty habit of vibrating and cracking, 'cause "they're only covers".
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
"this is the way we're always done it, everyone likes it"
"this is new for us, but we (think we) know what we're doing"
"the job doesn't have budget to fuss it ... show it good, now get out (i've got real work to do)"
"jimmy down in manufacturing likes it so it's probably good"
...
another day in paradise, or is paradise one day closer ?
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
On your question, I am still young in the industry, we just fast learner, but we are getting there....)))))
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
Such high grounding values in the rotational dof's usually occurs when the c.g of the model is far away from the origin of the basic co-ordinate system.
The c.g of your FEM is at [6.532052E+04,-1.616993E+02,-3.607411E+02]
When the grounding check is recomputed by using:
GROUNDCHECK(SET=ALL,GRID=10000,DATAREC=YES,RTHRESH=0.8)=YES ----> grid 10000 being an arbitrary grid I created at the model c.g, the g-set grounding results are
*** 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 1.232662E-04
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 2.887553E-09 PASS
2 3.719610E-09 PASS
3 2.314525E-09 PASS
4 5.442943E-04 FAIL
5 1.249674E-03 FAIL
6 5.989947E-04 FAIL
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;
On examining the ground forces at these d.o.f's which are failing:
DIRECTION 4
G R O U N D C H E C K F O R C E S ( G - S E T )
POINT ID. TYPE T1 T2 T3 R1 R2 R3
7467 G 0.0 -1.504304E-07 0.0 0.0 0.0 0.0
7666 G 0.0 1.250181E-07 0.0 0.0 0.0 0.0
7670 G 0.0 1.275758E-07 0.0 0.0 0.0 0.0
7683 G 0.0 1.210719E-07 0.0 0.0 0.0 0.0
1 MSC.NASTRAN JOB CREATED ON 26-SEP-14 AT 15:43:46 JANUARY 31, 2015 MD NASTRAN 5/ 9/08 PAGE 13
0
DIRECTION 5
G R O U N D C H E C K F O R C E S ( G - S E T )
POINT ID. TYPE T1 T2 T3 R1 R2 R3
3106 G 0.0 0.0 1.109781E-07 0.0 0.0 0.0
3127 G 0.0 0.0 -1.000477E-07 0.0 0.0 0.0
7645 G 0.0 0.0 -1.005828E-07 0.0 0.0 0.0
7811 G 0.0 0.0 1.154840E-07 0.0 0.0 0.0
7916 G -1.097471E-07 0.0 0.0 0.0 0.0 0.0
7953 G 0.0 0.0 1.028170E-07 0.0 0.0 0.0
7970 G 0.0 0.0 -1.192093E-07 0.0 0.0 0.0
7983 G 0.0 0.0 -1.192093E-07 0.0 0.0 0.0
7987 G -1.211877E-07 0.0 0.0 0.0 0.0 0.0
7990 G -1.066569E-07 0.0 0.0 0.0 0.0 0.0
1 MSC.NASTRAN JOB CREATED ON 26-SEP-14 AT 15:43:46 JANUARY 31, 2015 MD NASTRAN 5/ 9/08 PAGE 14
0
DIRECTION 6
G R O U N D C H E C K F O R C E S ( G - S E T )
POINT ID. TYPE T1 T2 T3 R1 R2 R3
7405 G 0.0 1.962926E-07 0.0 0.0 0.0 0.0
7437 G 0.0 -1.873332E-07 0.0 0.0 0.0 0.0
7586 G 0.0 -1.639128E-07 0.0 0.0 0.0 0.0
7597 G 0.0 1.937151E-07 0.0 0.0 0.0 0.0
7644 G 0.0 1.639128E-07 0.0 0.0 0.0 0.0
7666 G 0.0 -1.937151E-07 0.0 0.0 0.0 0.0
7670 G 0.0 -1.788139E-07 0.0 0.0 0.0 0.0
1 MSC.NASTRAN JOB CREATED ON 26-SEP-14 AT 15:43:46 JANUARY 31, 2015 MD NASTRAN 5/ 9/08 PAGE 15
The groundng forces are practically zero (< 1.e-6), so your modeling is just fine, and its not worth the effort and time to clean your model by chase after grounding forces in the order of 1.e-7 !!
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
Not only did I delete the manually added text for AUTOSPC he/she had, but I also unchecked AUTOSPC option in the analysis settings.
That means PARAM, AUTOSPC, NO is printed in the deck (that is what I meant my removed) but you had no way of knowing that, so you get the benefit of the doubt.
I do not like AUTOSPC myself. I'd rather turn it off, see the fatal, and then diagnose using a normal modes analysis to find my mechanisms.
www.stressebook.com
Stressing Stresslessly!
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
www.stressebook.com
Stressing Stresslessly!
RE: RIGID BODY CHECKS IN 4, 5 and 6 DIRECTIONS FAIL
@ stressebookllc, Thanks buddy for making it clear. You are right, I had no means of knowing that you had removed it. I thank you for your contribution.