I realized that there are two approaches to generate global stiffness matrix.
1- to add the *MATRIX GENERATE, STIFFNESS in the input file and,
2- defining a UMAT subroutine and printing DDSDDE
I have not tried the second approach, but in the first approach, ABAQUS outputs the same stiffness matrix (mtx file) for different cases of loading and deformation. while I am expecting to get different tangent stiffness matrix for a hyperelastic material with geometric nonlinearity. I have some questions:
1- does the second approach provides a stiffness matrix as a function of the deformation gradient?
2- Is there any other approaches that we can output the global stiffness matrix as a function of the deformation? (e.g. through input file)
3- Basically what is the meaning of the global stiffness matrix that abaqus outputs in the first approach? How is it different from DDSDDE?