I want to model a viscoelastic material using UMAT in Abaqus. I am facing two problems:

1. There is an example in Abaqus manual for viscoelastic umat. It started with 1D linear standard solid model. When it generalized the differential form to 3D it introduced 3 other constant in addition to Lame's constants. Does any body know how to relate those 3 parameters to spring and dashpot coefficients? or just please introduce me a paper or book that explain those.
2. When I was using the regular viscoelastic material part of the abaqus there are options to directly apply the complex modulus in frequency domain or even in time domain but in frequency based kind. Is it possible to do that in umat ?