Hi,<div><br></div><div>Thank you for your replies. Abaqus UMAT is in fact a material update routine providing an interface to implement the constitutive equations of a specific problem into the FE software Abaqus. In this subroutine, the inputs are strain tensor for the increment and the strain tensor for the beginning of the current increment, the&nbsp;deformation&nbsp;gradient tensor etc. The UMAT routine then updates the stress tensor and the corresponding material Jacobian for the end of the current increment at each integration point. The UMAT routine is normally written in Fortran.&nbsp;</div><div><br></div><div>In my case, I deal with a highly nonlinear elasto viscoplastic material behavior.</div><div><br></div><div>Best,</div><div><br></div><div>Hadi&nbsp;</div>