Constitutive Matrix for Viscoelasticity

Hi,

Can anyone please tell me the constitutive or the 'C' matrix  for a linear viscoelastic material model (with no relaxation in bulk)? I have been using the one outlined in the Computaional Mechanics book by Simo and Taylor, but I feel that ANSYS is using a different matrix as my results are different from ANSYS. I could not find details about the 'C' matrix for a viscoelastic material anywhere else. Any links too might be helpful.

Thanks,

Shriram

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Research Assistant

Structural and Multidisciplinary Optimization Lab.

University of Florida