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help seeking on VUMAT for Neo-hooke model in ABAQUS

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Jorgen
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Very interesting comments. I have not been able to look at your images, for some reason I dont have permission to reach the jpg-files.

Here are a few more questions/comments:

(1) What density did you use?

(2) You said that you got perfect agreement for the 1 element case. What loading conditons did you try? Did you try both uniaxial and shear?

(3) I am surprised that you did not get the same answer for the 4-element case. Did you try uniaxial loading for the 4-element case? Do you get the same answer as for the 1-element case?

It sounds like you pretty close to getting it right 🙂

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ADIRAJU
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The density I am using is 1104.43 kg/m3

I run the 1 element case again, and found consistent results for uniaxial tension. For some reason I thought Ive got fit results for simple shear, but this run shows there still is problem with simple shear of 1 element.

So I should not conclude 1 element test is good. Sorry for that.

I am sending bunch of plots to your email box with the cae file. NH.for is the NH model VUMAT subroutine downloaded from this website.

Thank you again for taking a look!

Jun

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Jorgen
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vumat validation

Based on your comments I wanted to make sure that the vumat worked as I though that it should. To verify that this was the case I ran a number of validation experiments with the VUMAT and I got the following results:

➡ For uniaxial tension, I get very similar results using these methods:

* Closed-form analytical solution,

* ABAQUS/Explicit with one 3D element using the vumat,

* ABAQUS/Explicit with one 3D element using the built-in NH model,

* ABAQUS/Explicit with one axisymmetric element using the vumat,

* ABAQUS/Explicit with one axisymmetric element using the built-in NH model,

* ABAQUS/Explicit with 4x4x4 3D elements using the vumat

See the following figure

[img] [/img]

➡ For simple shear, I get very similar results using these methods:

* Closed-form analytical solution,

* ABAQUS/Explicit with one 3D element using the vumat,

* ABAQUS/Explicit with one 3D element using the built-in NH model,

Due to the deformation state, I get a slightly different stress response for the following two methods:

* ABAQUS/Explicit with 4x4x4 3D elements using the vumat,

* ABAQUS/Explicit with 4x4x4 3D elements using the built-in NH model

Although these two methods are different than the one element case, they are both consistent.

See the following figure

[img] [/img]

These results seem to indicate to me that the vumat is working correctly 😀

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Anonymous
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Three questions here:

1) What are the results for plane strain elements e.g. CPE4R as I used?

2) How are the results compared if strain goes up to, say 500%? Rubber normally has very high stretch ratio before breaking.

3) Do you need to add on a pure elastic code just for the first time increment as suggested for VUMAT in ABAQUS manual?

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Jorgen
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answers to 3 questions

Answers:

1) The results should be qualitatively similar for plane strain elements. The call structure for axisymmetric and plane strain elements are the same with regard to the vumat (ndi=3, nshr=1).

2) There should be no problem to go to 500% strain. The vumat will behave the same.

3) As far as I know, it it not necessary to make the vumat behave in a certain way for the first time increment. The given vumat works as it is.

If you want, you can try out the response at larger deformations, etc.

Best of luck.

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ADIRAJU
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Thank you, Dr. Bergstrom, for helping me on this issue these days.

Btw, could you send me the cae file or inp file for the testing you did on this VUMAT?

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Jorgen
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input files

You can download the input files [url= https://www.polymerfem.com/modules.php?name=Downloads&d_op=viewdownloaddetails&lid=34&ttitle=vumat_nh_inputfiles.zip ][u]here[/u][/url].

Note that the input files are simple examples aimed at quickly testing some aspects of the accuracy and validity of the VUMAT subroutine.

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