These days I've been applying oneself to implement the Chaboche viscoplastic model in ABAQUS user-subroutine for simulating the cyclic loading in single crystals.
The procedure can easily get out. I'm not sure about the problem, the deduction of Jacobian, the convergence? 
Btw, I know there is free subroutine for crystal plasticity by Huang YG. I wonder if the UMAT routine for chaboche model is available, then I can generalize it to fit for my case.
If pity there isn't, guide or example for viscoplastic model can be of great help also.
Thanks a lot

maxianfeng@gmail.com

I had prepared this document for a class project, the level is introductory and the selection of models is motivated by solder deformation, but I hope it is of some help. I have examined four models, proposed by Hart, Anand, Krempl and Busso. The document has 16 pages.

Thanks,
Dhruv

PS: This is a wonderful website!!!