[QUESTION] Understanding warp.fem.deformation_gradient behaviour in deformed configurations

Hi,

I'm implementing corotational FEM structural dynamics using warp.fem, and I'd like to clarify how fem.deformation_gradient works in the context of a deformed configuration.

My base geometry is a 3D grid, and at each simulation step, I create a deformed geometry using:

deformed_geo = u_field.make_deformed_geometry() # u_field is the displacement field (vec3d)
deformed_domain = fem.Cells(deformed_geo)

Since u_field.dof_values stores relative displacements from the base configuration, I call make_deformed_geometry using the default relative=True parameter.

I then compute per-element rotation matrices using fem.interpolate:

# in each step:
fem.interpolate(
  element_rotation_matrix_integrand,
  domain=deformed_domain,
  quadrature=fem.RegularQuadrature(deformed_domain, 0),
  values={'Re': Re} # output array of wp.mat33 with one slot per element
)

Inside element_rotation_matrix_integrand, I call:

F = fem.deformation_gradient(domain, s)

I expected F to represent the deformation gradient relative to the reference configuration, but in my case, F is always a diagonal matrix with the diagonal entries corresponding to element dimensions.

I want to make sure that I correctly compute the deformation gradient relative to the base configuration while working with deformed geometries in warp.fem.

Would you be able to clarify how this should be handled? Am I using deformation_gradient wrong? Thanks in advance!

Regards,
Chris