to avoid singularity in shell to solid

[jsboy]

The issue about handling shell to solid connection came up in this forum a quite few times and understand that this is because of stiffness matrix being singular.
Could someone explain what exactly causes matrixt to be singular and how the use of MPC prevent this from happening?

 

[EnglishMuffin]

Solids use three degrees of freedom per node (the translational ones). But because they have no thickness, shells usually have to use five (three translations and two out of plane rotations). So there are inevitably unconnected degrees of freedom (the rotational ones) at the joint. There are various ways of handling this. You can simply discard the unused rotational degrees of freedom, which can be done automatically but is likely to lead to gross errors in many cases. Another method is to use extra shell elements superimposed on the surface of the solids and couple the shells to those. And as you say, you can use multi-point constraints to couple the rotational degrees of freedom to the translational degrees of freedom of the solid nodes. There are various algorithms that people have developed for doing this. See for example :

http://www.mscsoftware.com/support/online_ex/Patran/Pat302/Exercise_14_MPCs.pdf

There are now special coupling elements which do this automatically, and I believe some of the latest FEMAP and/or Nastran programs have this capability. I've always got away with using the extra shell element method, but it's not as accurate as using MPC's in critical cases.

 

[DickButkus]

In ABAQUS you simply click on the edge of the shell part, then the face of the solid part, and use shell to solid coupling. ABAQUS automatically generates a distributed coupling based on the thickness specified in your shell section in order to properly distribute the moments.

You don't want to embed the shell in the solid because you are artificially stiffening the structure. Likewise, you have to be very careful if you are going to try and write multipoint constraints or equations to do this. You need a distributed flexible coupling which transfers the momements.