negative eigenvalues and zero pivots

[andrewcm]

I've written a UINTER subroutine and I was having problems with defining the stiffness matrix. I think I've fixed that (kind of inelegantly by just rerunning the code with a small change in displacement to get the change in stress) but I'm still getting convergence problems. The .msg file is warning me about lots of negative eigenvalues and I'm also getting zero pivot warnings. I don't see a pattern to the nodes that are called out in the zero pivot warnings, but they all seem to be on the interior of my model, away from the boundaries or interface. I've run it with *constraint controls, print=yes but it doesn't output anything for these nodes which it says "suggests that the node may be underconstrained."

Does anyone have any ideas on what could be wrong?