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Usermat: Linear viscoelastic model with geometric nonlinearity


I have implemented a linear viscoelastic model using the user subroutine Usermat in Ansys.
I have used the classical paper: "Formulation and implementation of three-dimensional viscoelasticity at small and finite strains" of Kaliske & Rothert (Simo's approach) and it works fine for small deformations.

It is written in the documentation that in the case of a simulation with large strain (geometric nonlinearity), one must rotate the state variables.

Export Mass Matrices in Abaqus



I have to export stiffness and mass matrices from Abaqus (dynamical simulation)

and I have been using the commands *Matrix Generate and *Matrix Output


I need to export the matrices for different  time points (time intervals) and I have noticed that the entries

of the MASS matrices are not equal at different  time points.


Is that correct or should the MASS matrices be constant for a dynamical simulation?


many thanks

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