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## #1 2022-05-04 07:53:41

jonas loenartz
Member
Registered: 2021-10-01
Posts: 51

### strange reaction force at constraint

Hello everyone,

I obtained strange results, when I calculated the reaction-forces at the constraints, which I can't quite explain myself.
I have two gravitational loads in negative z-direction as defined in the following code samples:

``````eigengewicht_gestell = AFFE_CHAR_MECA(
MODELE = model,
PESANTEUR = _F(
GROUP_MA = ('s_doppel_20mm', 's_doppel_30mm','s_profile_10mm', 's_profile_20mm', 'v_aufhaengung'),
GRAVITE = 9810.0,
DIRECTION = (0.0, 0.0, -1.0),
),
)

eigengewicht_m_vert = AFFE_CHAR_MECA(
MODELE = model,
PESANTEUR = _F(
GROUP_MA = ('m_5200kg'),
GRAVITE = 9810.0,
DIRECTION = (0.0, 0.0, -1.0),
),
)``````

The second gravitational load "eigengewicht_m_vert" only affects a point-mass POI1, which is connected via LIAISON_SOLIDE to the rest of the model, while the first load "eigengewicht_gestell" affects the rest of the model. I used two seperate loads, because in further loadsteps I want to change the direction of only the second gravitational force.

Because the gravitation in x- and y-direction is 0.0, I would expect the reaction forces at the constraints to be near 0 in these directions as well, considering numerical imprecisions.

I calculated the reaction forces as usual, using

``````sum_reac=POST_RELEVE_T(
ACTION=_F(
INTITULE='Kraefte',
GROUP_NO=('n_spc'),
RESULTAT = lin_stat,
NOM_CHAM = 'REAC_NODA',
TOUT_ORDRE='OUI',
RESULTANTE=('DX', 'DY', 'DZ',),
OPERATION='EXTRACTION',
),
)``````

and printed it to the resu file.
The mass of the model calculated using POST_ELEM is 6.165t (mm-t-s used here)
So I would expect the following reaction forces: DX = DY = 0, DZ = 60479.
This is what was printed in my .resu-file:

``````INTITULE         * RESU     * NOM_CHAM         *  NUME_ORDRE   *  INST         *  DX           *  DY           *  DZ
Kräfte           * 0000001a * REAC_NODA        *             1 *     0.000E+00 *    -3.947E+02 *    -3.148E+02 *     6.005E+04``````

As can be seen, the reaction-forces are very much not 0. I cant quite explain this, because there should literally be no force at the constraints in any other direction than negative z.
Does the use of LIAISON_ ... introduce inaccuracys into the model perhaps?

Attached are two more verbose command-files, one used for the calculations, and one used for the postprocessing. I can't share the model due to confidentiality and it is way to big.

Thanks and greetings
Jonas

Last edited by jonas loenartz (2022-05-04 13:13:52)

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## #2 2022-05-09 18:18:11

jonas loenartz
Member
Registered: 2021-10-01
Posts: 51

### Re: strange reaction force at constraint

Ok, sadly there have been no answers, but I did check a few things the last few days and I made some progress, even though the false reaction-forces still remain.

I tested virtually everything. I tested if different orientations of the shell-elements, that carry the constrained nodes, could be the reason, but it had no effect.

I think I can pin it down on my use of LIAISON_MAIL (MASSIF_COQUE), which I use to connect 3D-Elements and shell-elements, in combination with the modelisation COQUE_3D. Interestingly, if I switch to MODELISATION = 'DKT' for the shells, the calculated reaction forces are as expected. Only when I use COQUE_3D, I get this discrepancy.
I think there is nothing wrong with the solution in general, only with the calculation of the reactions at the constraints, since the linear and quadratic results are very similar, and I double-checked with our commercial FE-software.

Has someone else experienced the described phenomenon, when using LIAISON_MAIL and COQUE_3D?

Further, I was not able to reproduce this effect. I tried so with a smaller, more basic model, which I could share in this forum, but the problem didn't occur there.

Jonas

Last edited by jonas loenartz (2022-05-09 18:19:24)

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