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# Problem: Neo-Hooke and Arruda-Boyce act linear

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(@metan)
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Joined: 9 years ago

Hello,

I am doing a simulation of a PBT tensile test, with Ansys WB14.5. I want to compare various material models between each other. I have got experimantel data of that tensile test, which I use to define the material parameter.

I did so far the Bilinear Plascticity Model and the Moonley-Rivlin. The results of those two models are reasonable.

But meanwhile I did a simulation with the Arruda-Boyce and Neo-Hooke model. The results of both models have got a linear behaviour. Now I am wondering, if that can be right. Or more, where is my error.

First I got the parameters of the Neo-Hooke with curve-fitting: Mu=585Mpa

Incompressibility Parameter D1 = 0

Then I calculated them by my own:

Mu= E/(2(1+n)) n=0,35 (poissons rate)

= 2500Mpa/(2(1+0,35) = 925,93 MPa

Bulk modulus k = E/(3(1-2n)) = 2500/(3(1-2+0,35)=2777,77Mpa

incompressibility Parameter D1 =2/k= 2/2777,77=7,2E-4MPa

For the Arruda-Boyce model, I used the same incompressibilty parameter and fitted the Initial shear modul Mu and Limiting network strength with the help of Ansys. What there is to say, that with the option of normed error I get a negative Limiting network strength, which Ansys does not accept. Only by choosing absoult error it gives me a positive Limiting network strength.

Whatever I do, they are always acting linear. Could somebody give me a feedback, if I have done an error or if it is right. I attached a picture of my results and one picture of the definition of the models in Ansys.

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Posts: 5
Topic starter
(@metan)
Active Member
Joined: 9 years ago

Isnt there anybody who can give me a hint? I would really appreciate it.

thank you

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

The stress-strain response from the Arruda-Boyce and Neo-Hookean models can look linear at small strains. Perhaps that is what you see.

If you are exploring the response of different material models then you may also want to check out the [URL= https://polymerfem.com/content.php?21-Material-Model-Calibration-Abaqus-ANSYS ]MCalibration [/URL]software.

-Jorgen

Posts: 5
Topic starter
(@metan)
Active Member
Joined: 9 years ago

Hello,

I increased the displacement to get a strain of more than 10 percent but the results are still linear :/

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

Go to larger strains.