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About the VUMAT in the books by Dr. Jorgen Bergstrom

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Topic starter
(@PATCH)
New Member
Joined: 9 years ago

Hello.

Did someone use the Neo-Hookean model in the book of Mechanics of solid polymers in Page 450? It seems that the fortran code t3=kappa*log(J)/J was different from the formula in Page 448. Welcome to discuss on that.

Thank you.

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

You might be right. I sometimes switch between the expressions:

kappa*(J-1)

and
kappa*log(J)/J

-Jorgen

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Posts: 3
Topic starter
(@PATCH)
New Member
Joined: 9 years ago

Thanks for your answer Jorgen. Are there any differences between these two expressions? In fact, when I use the first expression for Neo-Hookean model in VUMAT, the computation process seems to be divergent more early than the same ABAQUS model.

-Peter

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Posts: 2
(@thompsot)
New Member
Joined: 8 years ago

Quick question for everybody !

Why a high Bulk modulus over shear modulus ratio leads to high kinetic energy in an Explicit analysis and unstable results ?

Im using Dr. Bergstroms subroutine (NeoHookean) to model indentation. I works and Im really thankful as this subroutine helped in understanding the basic process of using a typical VUMAT.

However the elements under the indenter which suffer high compression start distorting and the analysis stops. The built-in NeoHookean doesnt show this problem.

The higher the Bulk modulus over shear modulus ratio, the smaller the strains at which the distorsion occurs. However the kinetic energy becomes significant even for little mass scaling.

I would be glad if someone can explain the mechanism that triggers that and also discuss about what the example subroutine provided may be missing.

Thanks

Christos

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

I did not examine that issue - interesting.
To me it sounds like an hourglass issue. Have you investigated that?

-Jorgen

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