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 ?