Hi,

I plan to model the Stress Relaxation phenomenon in FEA software ABAQUS/Standard using the User Material (UMAT) Subroutine through implicit analysis.

During Relaxation, the strain is held constant, and a time-dependent stress description is provided. While writing UMAT, we must define the tangent stiffness matrix/Consistent Jacobian DDSDDE (dΔσ/dΔε) for every step increment.

Since the increment in strain (Δε) is zero during a relaxation step, how do we define the DDSDDE matrix during relaxation?