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Viscoelasticity in Abaqus

Hi All,

 I would like to use Abaqus to model the viscoelastic material  behaviour of a polymer.


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 I have to specify the normalized bulk and normalized shear compliance

  

where is the shear compliance and the volumetric compliance

 

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

       where

    then is the inverse of a "time-dependent elastic modulus":

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

       

where would be the shear compliance and

the Poisson's ratio
      

where would be the volumetric compliance .

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 I'm 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

 

 

 

 

Comments

I've never used the built-in visco-elastic model in Abaqus, so I cannot be much of a help. I looked up that portion of the manual, but obviously I don't see anything you do not.

I think what you need to do really is to divide by the initial values. That gives you normalized values, but scaling them so that a certain value (in this case, the initial value) is 1. This sort of normalization is employed in many engineering problems.

Abaqus does not have built-in, nonlinear viscoelastic models (to the best of my knowledge). To perform a simulation with nonlinear viscoelasticity, you would need to use a UMAT user subroutine to define the material behaviour for Abaqus.

 

Good luck!

Aaron Goh's picture

Andreas,

This has been discussed before:check out this thread:

http://imechanica.org/node/1784

 

ABAQUS does have a user-friendly way of modelling non-linear viscoelasticity.

 

thanks for the link Aaron

 Andreas

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