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# viscoelasticity in Abaqus using creep data

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Topic starter
(@meta82)
Eminent Member
Joined: 15 years ago

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??

Thanks,

Andreas

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13 Replies Posts: 3993
(@jorgen)
Member
Joined: 4 years ago

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

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13 Replies Posts: 20
Topic starter
(@meta82)
Eminent Member
Joined: 15 years ago

Hi Jorgen,

I have tried to use the test data function. Abaqus asks for for shear test data and volumetric test data (nominal shear and bulk compliance).

So youre saying that I should calculate E(t)=sigma0/epsilon(t) and then devide it by 3 to get G(t)? Where does the 3 come from? Is this only true for uniaxial tests? What about K? Is there also such a formula for the bulk compliance?

I think I also have to devide by G0 (and K0) since Abaqus needs the normalized shear and bulk compliance.

Thanks,

Andreas Posts: 20
Topic starter
(@meta82)
Eminent Member
Joined: 15 years ago

I see

now I see where the 3 is coming from 🙂 G=E/2(1+v) assuming the Poissons ratio is 0.5.

In this case the bulk modulus K will be indefinitely large.

Andreas Posts: 12
(@margonari)
Active Member
Joined: 16 years ago

Sorry to inperupt your thread but I will probably be having the same problem shortly. If you use the viscoelstic model in conjunction with a hyperelastic model (I define a material and give it both hyper and visco elastic material data) will this represent non-linear viscoelasticity? Or is there a specific non-linear visco model to use?? Posts: 20
Topic starter
(@meta82)
Eminent Member
Joined: 15 years ago

Hi Bigal,

Thats a good point. I am not entirely sure if that would be the case. However, according to the Abaqus manual (see below) it seems to work that way. Im not sure if this then is a nonlinear viscoelastic model, though. To my knowledge there are specific nonlinear viscoelastic models, however, these would need to be implemented using UMAT.

[I]Time domain viscoelasticity is available in Abaqus for small-strain applications where the rate-independent elastic response can be defined with a linear elastic material model and for [COLOR=Red]large-strain applications where the rate-independent elastic response must be defined with a hyperelastic or hyperfoam material model.[/I][/COLOR]

Andreas Posts: 3993
(@jorgen)
Member
Joined: 4 years ago

Good points. The combination of a hyperelastic model and a linear viscoelastic model will give a model that is suitable for large deformations. That type of model is non-linear in the elastic response but linear in the viscous response. There are other models that are non-linear in both the elastic and the viscous response.

-Jorgen

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