Hi,

DDSDDE is the stiffness matrix derived for a specific constitutive equation under general, three-dimensional loading conditions. It models the stress-strain behavior of the material under consideration. Therefore, defining strain is insufficient to obtain DDSDDE.

1) Decide which constitutive equation adequately models the mechanical behavior of your material.

2) Go through the literature to find mathematical expressions of DDSDDE for that constitutive equation.

3) Implement the constitutive equation in conjunction with DDSDDE into UMAT subroutine.

4) Perform simple test calculations to see whether your UMAT is working correctly. This can be done without Abaqus, just write your own small testing program.

Regards

Whats is you physical problem? If you are dealing with relaxation your constitutive law must regard for viscous effects.

The DDSDDE is the stiffness matrix, but you need to program the correct constitutive law.

Regards