shape fixity

Hello:

I have written a UMAT subroutine similar to the one shown in abaqus 6.11 subroutine manual 1.1.40 with a slight differences indicative of the material's rheological model.
my expanded isotropic model looks like:
sigma_x - sigmav_x/3 + vbar*sigma_dot_x + kappa*sigma_dot_x =
lambda*eplv + 2*mu*epl_x + lamdabar*eplv_dot_x +2*mubar*epl_x