I am interested in running a simulation using a linear viscoelastic material model. I created a new material and the parameters I have to put in are instantaneous shear modulus and the viscoelastic decay constant. Since I do not have these parameters for the type of material I am simulating (nylon) I am thinking about doing a tensile stress relaxation test, however I have a few questions. 

Since I only have access to a tensile testing machine I am planning to use the relationship between poisson's ratio and elastic modulus to get the shear modulus.

1. Shear Modulus

G =E/(2(v+1)) where v = poissons ratio, G = shear modulus, and E = youngs modulus

- Is this an appropriate assumption to make?

2. Viscoelastic decay constant:

My plan is to measure tensile stress over time with an initial displacement. I should end up with something similar to graph 'b' from https://en.wikipedia.org/wiki/Stress_relaxation#/media/File:StressRelaxation.svg

- Can the decay constant be calculated from a graph like this? For example I would have an equation like e^(-bt) where 'b' is the decay constant and t is time. 

Lastly, is there a database where these parameters can be found for common materials?