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QLV vs Linear Viscoelasticity

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(@koerma)
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Joined: 16 years ago

Can anyone explain what is main difference between Linear Viscoelasticity vs qlv (Quasi linear Viscoelasticity) proposed by Fung. As far as I understand stress relaxation function and elastic response functions separated and given in convolution form which provides strain rate independent results in qlv application.

Is linear viscoelastic description is strain dependent?

To understand difference, I have generated simple stress relaxation experiment using Abaqus where different strain rates are applied into same model (kept constant a while) which has material property of viscoelastic (Prony ) + hyperelastic (Mooney-Rivlin).

I have used same model but only changed material property hyperelastic to elastic + viscoelastic with different strain rates applied again.

In overall I have see no difference (characteristics of the stress relaxation curve) between two models (visco+hyper VS visco+elastic) .

How can I design a such a experimental model to be able to understand difference between linear viscoelasticity vs quasi linear viscoelasticity in Abaqus?

I hope,I have expressed myself clear enough...

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(@jorgen)
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Joined: 5 years ago

I am not particularly familiar with Fungs quasi linear viscoelasticity, so I dont want to make too many comments about that theory. But I quite familiar with linear viscoelasticity and it limitations. First, linear viscoelasticity is not strain amplitude dependent.

Can you provide a reason you are interested in QLV? As opposed to other non-linear viscoelatic models.
-Jorgen

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Posts: 6
Topic starter
(@koerma)
Active Member
Joined: 16 years ago

Linear Viscoelasticity versus Nonlinear Viscoelasticity

Dr. Jorgen thank you for the response.

I will provide more comments on QLV later on, First of all its calibration is easier then fully nonlinear models. Time dependent relaxation function and strain dependent instantaneous elastic response function seperated into two parts, nonlinearity is provided from elastic strain energy density function. Ofcourse selection of linear time dependent stress relaxation function results all strain levels relax into same relaxation curve.

I would like you to ask simple question to make subject more clear. I will be happy if you can answer in a detail.

What are the differences between Linear Viscoelasticity and Nonlinear Viscoelasticity then?

Can I design simple experiment in Abaqus to see difference between them ?

I mean if I create simple 3d cube and apply step strain and held constant (ramp and hold test) to see stress relaxation, what will be the differences between 2 different 3d cubes one with linear viscoelastic material property and other with nonlinear. As far as I know Abaqus only supports linear viscoelasticity, am I right ? So how can I add nonlinear viscoelasticity into Abaqus ?

What will be the differences if i create two different materials one with visco+elastic (young and poisson) and other with visco+hyper in Abaqus ? Do you have any paper or graphics which explains differences between linear vs nonlinear viscoelasticity ?

I you previous post, You have mantioned that linear viscoelastic material is not strain amplitude dependent. How can I test that in abaqus with simple setup ? I mean , if step strain let say %5 is applied in 1 sec and hold constant and increased to %10 in second test. Do you means that stress relaxation curves will be the same (coincident to each other) ?

Kind Regards.

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Posts: 3998
(@jorgen)
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Joined: 5 years ago

Oh, I see. What you call Quasi Linear Viscoelaticity, is what I call linear viscoelasticity with a hyperelastic strain energy function. I now know what you are talking about 😎

➡ There is a huge difference between linear viscoelasticity and non-linear viscoelasticity. It is like a the difference between linear eleasticity and plasticity. Linear elasticity is simple and can be useful on occastion, plasticity is more advanced and accurate.

➡ Non linear viscoelasticity is an area that Abaqus does not support very well yet. Abaqus has some support for the Bergstro-Boyce (BB) model, which they call the *Hysteresis model. This is a non-linear viscoelastic model that I developed a few years ago. Another alternative is to use a user-material subroutine, that I what i do with my [URL= https://polymerfem.com/index.php?pageid=umats ]advanced material models[/URL].

➡ Yes, you can try the stress relaxation test that you mentioned. It is an interested test.

-Jorgen

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Topic starter
(@koerma)
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Joined: 16 years ago

is you umat library free to try for scifentific work? And if I look for Hysteresis in Abaqus nothing results. can you guide me for a specific document in Abaqus ?

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Posts: 3998
(@jorgen)
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Joined: 5 years ago

I am afraid that my UMAT library is only commercially available (just like Abaqus, ANSYS, etc).

You can find information about the Bergstrom-Boyce non-linear viscoelastic model by looking up the command *Hysteresis in the Abaqus keyword manual.

- Jorgen

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