As per the LS-Dyna manual, when using the NULL material model with dynamic viscosity, a deviatoric stress term is added using the following:

S_ij = 2*mu*e'_ij

where S_ij is the stress tensor, mu is the dynamic viscosity, and e'ij is the deviatoric strain rate.

I have implemented the UMAT as follows:

davg=(eps(1)+eps(2)+eps(3))/3.

savg=(sig(1)+sig(2)+sig(3))/3.

sig(1) = sig(1) - savg + 2.*mu*(eps(1)-davg)/(dt1)

sig(2) = sig(2) - savg + 2.*mu*(eps(2)-davg)/(dt1)

sig(3) = sig(3) - savg + 2.*mu*(eps(3)-davg)/(dt1)

sig(4) = sig(4) + 2.*mu*eps(4)/(dt1)

sig(5) = sig(5) + 2.*mu*eps(5)/(dt1)

sig(6) = sig(6) + 2.*mu*eps(6)/(dt1)

When I compare the results against the built in NULL material model, the results match fine when the dynamic viscosity is 0, but I can't get the results to match at all when I assign a value to the dynamic viscosity.

Does anyone know where I am going wrong?