A general UMAT for finite-strain viscoelasticity with damage

A UMAT for general finite-strain viscoelastic materials exhibiting strain softening and temperature dependence is presented and shared. The model builds on the thermodynamically consistent formulation of Reese and Govindjee (1998), extended to support a general deviatoric strain energy function depending on the invariants $I_1$ and $I_2$, as well as isotropic damage mechanisms affecting both deviatoric and hydrostatic responses. The paper first outlines the modeling assumptions and describes the numerical implementation, including modifications for the flexible incorporation of general strain energy functions, compatibility with hybrid finite elements, and the structure of the UMAT subroutine. The implementation is validated through a series of uniaxial and shear benchmark tests under various loading conditions. Finally, a structural simulation involving the cyclic torsion of a slender rectangular bar confirms the correct implementation of the consistent tangent modulus. The proposed UMAT is versatile and applicable to a broad class of materials, including quasi-incompressible rubbers exhibiting Mullins softening and solid propellants undergoing volumetric damage due to matrix-filler debonding.