Small pivot terms in calculation

[sev]

Hello,

I try to make an analysis on a small flat thing (extremely rigid compared to the glue) glued on a glass, which is represented as follows.

I apply a force F at a node N, which is connected (by BEAM4 elements) to each node of one surface of the glue. This surface is represented by SHELL43 elements. The glue depth is made of SOLID45 elements. All DOF's of the opposite surface of the glue are fixed.

When I launch the solver, I obtain 1834 small equation pivot terms.

What is wrong in my model ? May I combine BEAM4, SHELL43 and SOLID45 elements ? Aren't the constraints transmitted from one element type to the other ?

Thanks a lot for your answer,

Sev
CAD Engineer in Automotive

 

[Drej]

Small pivot problems are usually associated with under- or poorly-constrained models i.e. model boundary conditions, indicating a problem of rigid body motion. If you have beams meshed to your solids then you will get DOF conflicts which will probably cause your error (one element has 3 DOF, the other has 6, and it will be the lack of rotational DOF in the solids which causes the error.) Try Tying the DOFs together with constraint equations using the CEINTF command. (Beams to shells is ok in this respect.)

-- drej --

 

[sev]

Thanks Drej for your answer. I tried the CEINTF command, but it didn't work, I don't know why (even if nodes were on elements, it said "node doesn't lie on the element").

Because I need rotation DOFs, I changed my element types, removing shells and replacing solid45 elements by solid73, which also support rotation. I am now trying to check my model, compared to physical reality, because results seem strange.

Anyway, thanks again for your answer and hope to see you soon in this forum.

Sev

 

[Drej]

Try using the general CE command for this. If you need to connect element types with different DOF sets, you can write CE’s to transfer loads from one to the other. The CE command can be found within the GUI at:

Preprocessor > Coupling/Ceqn > Constraint Eqn

Best of luck,

-- drej --