Ogden material model using UHYPER

[ydeshmuk]

Hello,

I am trying to define a new model having the Ogden model form. Can you please tell me how do we define the stretch ratio as?
I mean, Ogden model being in the terms of stretch ratios, my derivatives are also in terms of stretch ratios. Can you please tell me how to represent stretch ratios which can be understood by ABAQUS while executing UHYPER? The invariants are written as BI1, BI2 and so on. How is lambda written in a similar manner.

Thanks,

Yash

[Jorgen]

I just double checked the Abaqus manuals and it appears that you are not given information about the principal stretches. Hence, you cannot write a UHYPER that is based on the Ogden form. I suggest that you instead switch over to a UMAT.

-Jorgen

[ydeshmuk]

Dr Jorgen,

Thank you for the reply. I am now planning to use the expressions for lambdas in terms of invariants in UHYPER. Lets see how it comes along. Can you please tell me why the expression for potential energy is needed inspite of we providing the derivatives. What does ABAQUS compute using that expression?
The question may sound a little stupid but I wish to know.

Thanks,

Yash

[Jorgen]

As far as I know, the energy is only used for output purposed in CAE, and when using the Mullins damage model.
Oh, I is also used for coupled thermo-mechanical simulations.

-Jorgen

[ydeshmuk]

So, incase I am only interesed in the stresses and strains, is it fine if I do not put in an expression for potential energy. I am asking you this as I do not have an explicit expression for the potential energy.As in, I don't have an explicit expression for the compressible part of potential energy dependent on J.

Thank you very much !
I really appreciate your help!

Yash

[Jorgen]

I have never tried that, so I don't know for sure. I recommend that you at least try to imput an approxiate energy expression.

Perhaps you can try a few different energy expressions to see if it makes a different.
Let us know how it goes.

-Jorgen

[ydeshmuk]

Hi,

As mentioned earlier, I have written a subroutine UHYPER using a model having the Ogden form.I am working in ABAQUS CAE. In the material module, when I specify 'User-defined' as the the type under Hyperelastic,I see an option of 'include compressibility'.
What does it exactly mean? I my derivatives have terms which take care of compressibility, do I still have to toggle on that option?Presently, If I toggle on that option, the job gets aborted. i wish to know the physical significance of 'include compressibility' option. Looking forward to your comments.

Thanks,

Yash

[Jorgen]

I recommend that you check the Abaqus manuals for the most precise information. My guess is similar to yours, if your UHYPER is compressible then you should activate that flag. I typically work with UMAT/VUMATs and not UHYPERs so I am not sure.

-Jorgen