I am now using ABAQUS UMAT to carry out a constitutive model.
As the model is considering the large deformation, firstly I have to caculate the time rate of change of the deformation gradient to obtain celocity gradient. The following is my confusion:
The ABAQUS gives the deformation gradient at the beginning of the increment in the array of DFGRD0(3,3), as well as that at the end of the increment, i.e. DFGRD1(3,3). What I want to make sure is: could I then caculate the time rate of change of the deformation gradient by the equation (DFGRD1(3,3)-DFGRD0(3,3))/DTIME?
If negative, how should I get the variable?
Thank you very much~