how to handle infinite tangent for viscous material

[bjturner]

I am coding a new uniaxial material for soil which includes a viscous dashpot component to model radiation damping along with other elastic and plastic components. If the velocity during the previous converged step was nonzero, and then the trial displacement increment for the current step is zero, the dashpot stress will go from nonzero to zero, causing the stress in the element to change even though the total displacement increment was zero. Therefore, the tangent and damping tangent are either positive or negative infinity (depending on direction of velocity during previous step), since a change in force has occurred with zero corresponding displacement. (Note-- internally each component has a nonzero displacement to account for the change in force, but the total displacement sums to zero.) What is the best tangent value for the material to return to the solver in this case? Should I simply use a very large pos./neg. number relative to the material stiffness for dy=nonzero cases?

Thanks in advance for your thoughts.

[fmk]

you can use a large value .. put some default in and allow yourself to specify the number on the command line as well.