How to calculate deformation gradient for each increment in VUMAT?

Dear all,

As you know, ABAQUS do give deformation gradient tensor and stretch tensor at each increment.

But the reference configuration for both tensor is the initial configuration (steptime=0, undeformed). Is this right?

So my question is how to calculate deformation gradient in each increment, the reference configuration is the configuration at the begining of this increment. I'm lost here. Really appreciate you guys can help me out.

 Best wishes,

shiyao