VUMAT for finite strain viscoelasticity

Together with my student Damien Ollion, we have converted the finite-strain viscoelastic UMAT originally developed with Florian Gouhier into a VUMAT for Abaqus/Explicit. The implementation reproduces the constitutive model of Reese and Govindjee (1998) for a generalized Maxwell model and includes the following features:

• an arbitrary number of viscoelastic branches;
• general strain energy density functions of the form W(I1,I2)+U(J));
• an optional viscoelastic contribution to the volumetric strain energy U(J);
• time–temperature superposition based on the Williams–Landel–Ferry (WLF) equation.

A comprehensive How-to-use.pdf document is provided. It presents the constitutive equations, illustrates the implementation through several validation examples, and explains how to adapt the VUMAT to different strain energy density functions. Several single-element Abaqus input files are also included to facilitate testing and verification.