viscoelasticity in Abaqus using creep data

Hi All,

I would like to use Abaqus to model the viscoelastic material behaviour of a polymer (test run to see how it works).
I have material data from a simple uniaxial creep experiment (nominal strain vs time). I tried to use the viscoelastic material model in Abaqus. I am a bit confused as to how I need to enter my data (what format). The manual says Ihave to specify the normalized bulk jk(t) and normalized shear compliance js(t).

js(t)=G0Js(t), jk(t)=K0Jk(t)
where Js is the shear compliance and Jk the volumetric compliance

Is it correct if I use my strain epsilon(t) to calculate the creep compliance, which is defined as:

J(t)=epsilon(t)/sigma0 where sigma0=F/A0

J(t) then is the inverse of a time-dependent elastic modulus: 1/E(t)

And then to get the shear and volumetric compliance, is it valid to use the following equations:

1/G(t)=2(1+v)*1/E(t) where 1/G would be the shear compliance Js and v the Poissons ratio
1/K(t)=3(1-2v)*1/E(t) where 1/K would be the volumetric compliance Jk.

Now according to the manual I have to calculate the normalized shear and bulk compliance by multiplying the values by G0 and K0, respectively. That part Im not really understanding... I guess G0 and K0 are the initial shear and bulk modulus, respectively. To get them I need to again use the above equations (G=E/2(1+v) and K=E/3(1-2v)) and use the initial elastic modulus, which I can get from the initial elastic deformation.

What confuses me is that the shear and bulk compliance are supposed to be 1 at t=0.

All of the above of course assumes linear viscoelasticity... What if it is not linear??

Does anyone have any hints or comments about my approach?

Thanks,

Andreas

The easiest way to imput creep data is to use the Test Data option in CAE. Using this approach you can import the shear creep modulus as a function of time, and CAE will determine the Prony series for you.

In your case I would assume that all creep is due to shear, and use G = E/3.

You are right that the linear viscoelasticity model in Abaqus assumes a linear viscoelastic response, and there are many material and loading conditions where that is not a good approach. To handle there more general conditions you need a more advanced materil model.

-Jorgen