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# Understanding nonlinear viscoelastic parameters to write UMAT ?

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(@tenishasc)
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Joined: 9 years ago

I am trying to write a UMAT in Abaqus to simulate non-linear visco-elastic material response based on the paper attached*below..

I have a small problem in understanding how the non-linear parameters (g0,g1,g2,a) of Schaperys equation (Ref pg 7) are calculated.. As I understand, they are functions of octahedral shear stress (sigma_bar) in the paper, which I think varies with applied stress (Correct me if I am wrong). Now here is where I dont understand..

In case I want to simulate relaxation test,*I give a steady heaviside jump stress of say 30Mpa, then stress tensor is constant, meaning octahedral shear stress is constant as well. That essentially makes nonlinear parameters constant for every time step.

I am pretty sure I am overseeing something..*Please correct my understanding..

Regards

Shree

[B]UPDATE 1 : It has been more than 40 Views. Sincerely looking for somebody to help me out or atleast tag a person who can help me out..[/B]

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(@jorgen)
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Joined: 5 years ago

I dont quite understand your question...
Can you formulate your question in a different way?

-Jorgen

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Topic starter
(@tenishasc)
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Joined: 9 years ago

Well, based on the equation in page-7 of the attached paper, (where non-linear parameters are expressed as functions of octahedral shear stress), is it true that if I apply a constant stress to simulate relaxation, then the parameters remain constant for every time step? Or am I missing something?

Do let me know if you need more details