Hi, there

I am writting a UMAT modeling Drucker-Prager constitutive model (non-associated flow rule)  in Abaqus. When I using one element to test the code (two load steps, the first is isotropic compression, the second is loading by giving one of the element edges a certain displacement) , it looks well when the element is loaded by compression (a negative displacement, so that negative stresses according to the convention in Abaqus). However, if the element is under extension, I received a warning message of "NEGATIVE EIGENVALUE", then the force or displacement cannot converge finally.

 I am wondering where is a way to solve the problem. Anybody can give me a hand?

 Best regards,