viscoelastic material short term shear modulus

[Anonymous]

Hi

I am a doctoral student. I study with biomechanic materials. My analysis type is static analysis. I have to input viscoelastic material properties as "short term shear modulus, long term shear modulus and decay factor" for a tissue of the brain. But in the material library in viscoelastic materials i could not find any of that parameters. I looked every material but failed to find.

Can someone suggest me an idea about this problem? Is there a way to input the short term shear modulus, long term shear modulus and decay factor parameters?

Thank you.

Note: I work with Algor simulation professional 2011

[S.LI]

This is a part in Help to describe linear viscoelastic material model.

Hope it's helpful on your problem.

Linear viscoelastic material model accounts for rate-dependent material behavior due to dissipative losses caused by internal damping or viscous effects. Polymers, rubbers and metals at high temperatures undergo viscoelastic properties. The linear viscoelastic material properties are listed below. In addition to these properties, it may be necessary to define some Isotropic Material Properties or Orthotropic Material Properties.

Elastic Moduli

This drop-down box is used to describe how the linear viscoelastic material is being loaded. If the material will be loaded slowly so that time-dependent behavior is negligible, select the Long-Term option. If the material will be loaded instantaneously, or as a step load, select the Instantaneous option.

Prony Series

The Prony series that will be used to calculate the viscoelastic effects will be defined by pressing the Prony Series Definition button. To apply a temperature shift, select the Williams-Landell-Ferry option in the Shift Function drop-down box. You must then define the reference and analysis temperatures and the constants used to define the shift in the appropriate fields.

If the volume and shape changes are not coupled, activate the Independent Volumetric/Deviatoric Relaxation check box. You can then define the relaxation moduli and relaxation times that will define the Prony series.

[Anonymous]

Thanks for your reply. But i read them all and didn't help me. I investigated all viscoelastic materials in the material library but none of the materials have short term shear modulus and long term shear modulus inputs. I have to use that type of variables.

First parameter is G(0) and the second one is G(~). Two shear modulus have to be added. And also the B = beta named decay has to be added.

[S.LI]

The linear viscoelastic material model in Algor does NOT support both material properties in the same formulation.

You should choose either "short" or "long" as described in Help.

viscoelastic is a complicated material model to simulate material behavior. The material response is not only relied on material itself, but also the loading and other environment.

I'm not an expert on biomech.

For your specific problem, you have to pick up short or long according to your case if you want to use linear viscoelastic in Algor.

[Anonymous]

Hi.

I still have the same problem. I tried to input the short-term shear modulus and the long-term shear modulus to the new Autodesk simulation mechanical and multiphysics but still the programs do not accept those material properties.

Why do the autodesk sput those viscoelastic material options to its material library. Is it too hard for Autodesk???

[S.LI]

Can you share more information here?

For examples, what's your material properties? How does the program reject your input?

It happened during model setting or solver running?

Thanks.

[Anonymous]

The program does not reject my input. There is no choice to input the short term shear modulus and the long term shear modulus of a viscoelastic material. In ANSYS it has been done easily. But in Algor you can only input young modulus, poisson constant and the bulk modulus. But in viscoelastic materials there is 2 shear modulus as the short term shear modulus and the long term shear modulus. And Algor does not accept this material properties or i could'nt find it.

The other question is in viscoelastic material properties there is a choice of the elastic modulus as long-term. I have to change it to instantaneous but it is passive. I can't change it. Can someone suggest me an idea?

[S.LI]

The attached is the dialog for linear viscoelastic.

There is a dropdown list called "elastic moduli". By this input, code will know the input (shear modulus etc) is long or short term.

As I mentioned before, formulation here only takes either long or short, and it could NOT take both at the same time.

Please let me know if I got anything wrong here.

[Anonymous]

Thanks for your interest.

I attached a picture that shows you my material properties. You are right for standart materials. But some of nonlineer materials as viscoelastic materials need two shear modulus varying by time. And they are called short-term and long-term shear modulus.  It has to be put into the program. In ANSYS it is so easy. But in Algor i don't know that is there away or is it impossible.

[Anonymous]

Hi. I still have the same problem and could not solve it yet. Is there anyone that has an idea?

[PipePakPat]

I think this issue can be addressed using the Linear Viscoelastic Isotropic material model, although the input data format may be different than what you may be familiar with in ANSYS.

Your input data provides bulk modulus and shear modulus data.  The Autodesk Simulation (Algor) data entry screen require the Young's and Shear moduli and provide the opportunity to define a relaxation parameter(s).  I would suggest to determine the initial Young's and shear moduli by using the correlation described from the following web address:

http://en.wikipedia.org/wiki/Bulk_modulus

Where E = (9*K*G)/(3*K + G)

From your data, the bulk modulus may remain constant, but two values of the shear modulus (G) are provided.  Use the initial value of G to determine the initial value of Young's modulus (E).  One would then select "Instantaneous" or "Long-Term" from the "Elastic Moduli" pull-down combination box to determine how the initial modulus was obtained.  It would seem that this field should be set to "Instantaneous" in your case.  One would then choose the "Prony-Series Definition" button, leave the "Shift Function" to "None" and activate the "Independent Volumetric/Deviatoric Relation" checkbox.  This option will provide separate input for volumetric change compared to deviatoric (or shape) change.  The deviatoric change is characterized by changes in shear modulus.  For volumes that have no change, enter zero for the relaxation parameter.  One will then be required to enter several rows of data for the deviatoric relaxation parameters.

Unfortunately, the image you provided does not provide enough data to properly curve fit the expected exponential decay function used to characterize this type of relaxation.  We have a beginning point (G-initial) and a final point (G-infinity) but we don't know what the half life of these decaying function that eventually reaches an asymptote.  The half life of this function would needs to describe the slope of this decay and at what time this function reaches this half life (I may not be using the correct terms, but the idea is correct).  Even if one just used the two moduli you provided, we still don't know the time period for which this now linear decay occurs.  Nonetheless, each relaxation parameter (or modulus), which is unitless, would incrementally indicate the fraction of the previously used moduli.  So if one row was used, then the fraction would equal (G-infinity/G-initial).  You would still need to determine and define the relaxation time.

The data Algor uses reflects industry standards, so any research you perform in the regard should be applicable to these data entry screens given the description I have provided.  Please let me know if you have further questions.