Dear all,
I am modeling a 2D concrete element in abaqus with user subroutine UMAT in which I defined an isotropic damage model. The element is constrained in the bottom edge, and displacement applied in the top edge.
The analysis is completed but there are a lot of error message say that THE SYSTEM MATRIX HAS NEGATIVE EIGENVALUES. Morever, I get a strange curve which does not represent the softening behavior of the concrete.
Can anyone help me ?
Thank you.
What i am undetstanding you are trying to find out buckling modes.....for this u can provide minimun eigen value while defining step.....
I think that kind of error is related to the incompressibility constraint.
A good idea is to include an stability verification into the UMAT . That is, check if the determinant of DFGRD1 is between two predefined values. Case not, set a new value to PNEWDT and return to a new iteration.