Material Laws

Altair Forum User

Hello,

 

i have a question of the implemented material laws because i want to know what are the differences. Especially what are the differences between LAW 42, LAW 69 and LAW 82. Because all of them use the strain energy density function with some small differences (this is what i can see in the comments). Now i want to know when i should use which law. Especially if i'm dealing with biological tissue.

 

Thanks

 

Best

Max

Altair Forum User

Hi Max,

In RADIOSS, the available models for modelling  hyperelastic behavior are Mooney-Rivlin and Ogden ((Material Law 42 and Material Law 82). Material Law 42 incorporates the viscoelastic behavior. If the user is having stress-strain curve from uniaxial tension and compression tests, Material Law 69 can be selected to uniquely define the hyperelastic behavior. 

 

Altair Forum User

Hello,

I am using Hypercrash 12.0 110 (RADIOSSV12X), and I want to use the Ogden model with viscosity (LAW82, 'visco-elastic'), but 3 parameters are recquired (alpha, mu and D).
But I do not know what is the parameter 'D'. I check on the 'RADIOSS 10.0 Reference Guide2.pdf' (which is LAW62), but that is not mentionned.
Can you help me to understand how the LAW82 works in Hypercrash 12.0 please?

 

One more question:

Ogden model can be used in different ways (custom formulations... ex: lambda[1]^(alpha[1]-1)...), is it possible to create its 'own' custom formulation with Hypercrash 12.0?

 

Thank you,

Corentin Rabot

Altair Forum User

Hi Corentin,

Alpha, mu and D are the three Ogden material model constants you need to define in the material card.  Di parameters introduce compressibility. And I think you need to find these values experimentally. There are many journals and information available on web regarding the Ogden material formulation. I recommend you to please go through the web regarding these material constants and material formulation.

In /MAT/LAW69 there is no need to input not the material coefficients but stress/strain curve directly. and RADIOSS automatically calculates the material parameters. 

 

Altair Forum User

Hi George,

Thank you, but I unfortunately did not find what is the D parameter for the Ogden formulation through the web.

However, I am particularly interested about the LAW69, I think I will use it. But I have misunderstanding about the diference between:

[FCT_ID] Bulk function identifier, and

[FCT_ID1] Curve identifier for user input data curve

and, what is the aim of the '[N_PAIR] Number of data pairs' parameter? If I want to use severals datas?

Thanks

Best

 

Corentin

Altair Forum User

Hi Corentin,

fct_ID1 is the function identifier for engineering stress-strain curve.

fct_IDblk is an option that provides additional control for the incompressibility.

N_Pair is the number of material parameters and default is 2.  RADIOSS automatically calculates the material parameters from the given stress-strain curve (fct_ID1).

Altair Forum User

Hi george,

 

I did not find what is the function (Ogden formulation) used for the LAW69.

I found this reference 'RADIOSS 10.0 Reference Guide2.pdf ' for now, but this one is not there.

 

Can you help me please?

 

Thanks

Altair Forum User

Hi Corentin,

In /MAT/LAW69, fct_ID1 is the function identifier for engineering stress-strain curve. Please check the material law details attached where a sample material card is also there.

Which HyperWorks version you are using?. Please update with the latest software that is HyperWorks 14.0 Student Version which can be downloaded from http://www.altairuniversity.com/get-the-free-hyperworks-14-0-student-edition/ 

 

Unable to find an attachment - read this blog

Altair Forum User

Hi George,

Ok thank you for this pdf! I am actually performing only uniaxial compresion test on my rubbers... Do you think it's enough for the LAW69?
It seems ok to me.

 

I am using version 12.0 unfortunately (with a school license). And I can not update it by my own...

Thanks,

Best,

 

Corentin

Altair Forum User

And I forgot. With the LAW69: Is it possible to know the paramters that the software computed (mu and alpha)? Or is it only a definition of the material through the stress-strain curve (without the 'output' Ogden parameters, mu and alpha) ?

 

Thanks again,

Corentin

Altair Forum User

Hi Corentin,

/MAT/LAW69 is good for modelling  hyperelastic and incompressible materials like rubbers, foams and I feel its good for your application.

In LAW69 we are not providing the material coefficients but stress/strain curve directly and RADIOSS automatically calculates the material parameters. And I don't think that we have an option to find those parameters. I'll check regarding this and will update you.

Altair Forum User

Hi George,

Ok thx. I will also search computed paramters if they 'exist'...

I performed a compression test on my rubber, but there is a ERROR (attached) about negative stress-strain curve... It seems it is impossible to use compression test then... Or there is an alternative?

 

Thanks,

Best,

 

Corentin

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Altair Forum User

Hi Corentin,

Please recheck the function defined for the material and make sure the curve values are correct.

Please go through our Help Menu on /MAT/LAW69, where an example material card is shown with function defined.

If the issue persist, please share the model file for a workaround.