Umat diverging for constant stress

Dear All:

How to make sure that the mechanical strain change if the stress is maintained constant?

Thank you

If the stress increment is zero, then the strain increment also has to be zero (for an elastic-plastic material).

In your case I would define the DDSDDE (Jacobian) based on the tangent stiffness. Also note that the Jacobian is defined by the partial derivative with respect to strain increments. Hence you will not have a zero Jacobian even if you have zero hardening.

-Jorgen