Hello all,

I am new to the forum, but I have been reading posts here as a non-user from quite some time.
I have run into a problem with DFGRD0 and I am clueless about what to do.
I have written a UMAT subroutine for modeling of woven fabric materials, based on a rheological model of the fabric mesostructure, as found in a 2006 paper by King, Jearanaisilawong and Socrate, "A continuum constitutive model for the mechanical behavior of woven fabrics".
The model allows to convert the deformations on a macroscopical plane continuum structure (e.g. simple membranes or shells) in forces acting on the yarns in the mesostructural model, and finally from these forces to obtain the stresses back on the continuum structure, as if it were not continuum but a woven fabric.

In the mesostructural model, the two orthogonal yarn families, warp and weft, are conceptually imagined as trusses connected via torsional springs.
The state of deformation is passed in as information, from which the geometry of the trusses, and the axial and bending forces and moments acting on them are estimated.

I modelled the continuum structure in Abaqus (the UMAT is for Abaqus Standard) as a plane stress, 2D, 1 CPS8R finite element, square plate, pinned on the left edge, and with a little horizontal ramp-in-time displacement on the right edge, of 0.1.
The results I obtain from my UMAT are very wild, and blatantly not accurate at all. In particular, I got that the stresses were =0 during all the simulation.
I wrote a few "WRITE(*,*)" statements in the UMAT to track the evolution of the variables in the .log file of the abaqus job.


What I think makes my simulation fail is the fact that the initial value of DFGRD0, DFGRD1 and DROT are:

Drot:
0.0000000E+00 0.0000000E+00 0.0000000E+00
1.875000 0.0000000E+00 0.0000000E+00
0.0000000E+00 0.0000000E+00 0.0000000E+00

DFGRD0:
0.0000000E+00 0.0000000E+00 0.0000000E+00
1.875000 0.0000000E+00 0.0000000E+00
0.0000000E+00 0.0000000E+00 0.0000000E+00

DFGRD1:
0.0000000E+00 0.0000000E+00 0.0000000E+00
1.875000 0.0000000E+00 0.0000000E+00
0.0000000E+00 0.0000000E+00 0.0000000E+00

The same tensors, with a shear deformation value when I give an axial, 1-direction displacement... This tricks the mesostructural model to return some important internal geometrical variables as 0, and the stresses in return come out as 0 too.

Another possible problem that I can think of, from the "WRITE(*,*)" statements, is the dtime variable, which is set at the beginning as dtime: 89128.96 , while my static analysis on Abaqus is scripted to last only 1 second.

I really don't know what to do, I would be grateful to receive help on this!!