How to calculate the stress increment of a single crystal using VUMAT

Dear all,

I am writing a program for crystal plasticity using VUMAT. I met some problems, hope you guys can help me out.

Assume each node represents a single crystal. At each step, VUMAT will give strain increment which is based on curent configuration(right?)

Then I can calculate the shear strain in each slip system and obtain the plastic deformation gradient. So I can get the elastic deformation gradient for current step. Using polar decomposition, rigid body rotation matrix can be obtained.

But how to update the stress increment? should I consider the stress rate objectivity? Can anyone give me a hint?

Thanks a lot,
shiyao