I have uniaxial stress relaxation data at several temperatures for a mold compound material (epoxy-based thermosetting polymer). A master curve was constructed by shifting these relaxation data. Now what I am trying to do is expressing the relaxation modulus as a Prony series.

Since ABAQUS/CAE (I believe ANSYS as well) requires shear and bulk relaxation modulus, I needed to convert uniaxial relaxation modulus to shear and bulk modulus. Simply I used the relationship G=E/2(1+nu) and K=E/3(1-2*nu) with assuming nu=0.3 (I dont have data for Poissons ratio). My questions at this point are:

1. Is it still valid to use those formula to convert E(t) to G(t) and K(t) in case of viscoelastic materials?

2. Is it safe to use a fixed Poissons ratio? If so, is the assumption of nu=0.3 appropriate for thermosetting materials? I know that nu=0.5 is typical for elastomers.

In order to input relaxation test data in ABAQUS, viscoelastic material input requires normalized shear/bulk modulus (i.e. G(t)/G0 and K(t)/K0). However, since they are normalized values, G(t)/G0 and K(t)/K0 become identical and essentially the same as E(t)/E0 when the above relationship about E, G, and K is used. So I was wondering whether I followed correct steps.:confused: Is it necessary to have both shear and bulk relaxation modulus for FEA? (maybe a silly question)

Thanks a lot for your comments!