Support for MPC bulk data entries

[kmokstad]

The Nastran Bulk data File parser lack support for MPC records describing general linear dependencies between internal DOFs in an FE part.

[kmokstad]

The attached zip-file 01_MPC_Test contains a Nastran bulk data file exported from ANSYS with a total of 168 MPC elements. They are converted into 31 WAVGM elements and associated PWAVGM properties in the ftl-file, using the conversion implemented in openfedem/fedem-foundation#15.

@kristsaet if possible, you could run a basic simulation in ANSYS with the Nastran file (or on the model from which the Nastran file was exported), and then something equivalent in FEDEM with the ftl-file, and verify that they behave similarly.

[kristsaet]

Hi. Did you try to run this file in your own Fedem instance?
I get this warning about the part:
fedem_reducer [17808]: Warning : The total mass computed from the reduced substructure matrices
fedem_reducer [17808]: deviates from the mass computed during finite element assembly.
fedem_reducer [17808]: Mass deviation = -3.03448E+01 ( 31.17%)
fedem_reducer [17808]: This could be caused by poor constraint equations from rigid and/or interpolation constraint elements.
fedem_reducer [17808]: Please check the FE model or contact Fedem support with details.

In addition the model is highly unstable, so it will not start to solve even without loads applied (apart from gravity).
I tried to fix it 6DOF at the bottom, and just add two triads at the two other RBE3's.

In general I have seen tendencies that models in general have been very unstable lately, so that they had to be reduced with 0 component modes to work, however in this instance it is not working with 0 component modes either.

BTW. Remembering a bug found some time back, related to that RBE3's with rotation DOFs activated provided completely wrong results. Can this be related to this? I suppose there might be rotation connection in the MPCs.

[kristsaet]

For reference:
I added 10000N upwards force in Ansys model for the upper horizontal beam, and 10000N downward force for the lower horizontal beam.

Deformations Upper Remote Point (RBE3):
X = 3.1041e-003 m
Y = -1.506e-003 m
Z = 1.5948e-002 m

Deformations Lower Remote Point (RBE3):
X = -5.9084e-005 m
Y = -4.8082e-004 m
Z = -8.3467e-003 m

[kmokstad]

I did not try this model myself in FEDEM yet, and your test now indicates it won't work either. There seems to be something wrong with that MPC-conversion then, if you got all those problems. If done correctly, I believe this model should be stable.
Thanks for providing the ANSYS reference solution, we can now use this as a base case to verify the MPC implementation in FEDEM.

The old RBE3 bug you refer to is probably the solver issue openfedem/fedem-solvers#2, it may have effect in this model also but I did not investigate this any further yet. The MPC couplings consider each DOF separately and may or may not involve the rotational DOFs. For the model here, it seems that only translational DOFs (1-3) are used as master DOFs but they are also coupled to rotational DOFs (4-6) at the reference (master) node.