Im testing a umat for a shell element using a simple shear of a plane sheet as an example.
The imposed deformation gradient in the input file is
F=(1 0.3 0
0 1 0
0 0 1)
I did a check of the dfgrd1 variable coming into the umat at the end of the step and it is about:
( 0.992 0.156 0.00
0.156 1.0166 0.00
0.00 0.00 1.0160)
For comparison, I did an equivalent 3D simple shear test on a cube with the same imposed deformation gradient. This time inside the UMAT at the end of the step, the dfgrd1 is about
(0.959 0.295 0.0
0.0 1.0588 0.0
0.0 0.0 0.972)
Why is there such a difference between the shell element and solid 3D element case? Is there some rotation occurring prior to the umat for the shell element case and not for the 3d element?
Thanks for any help.