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# Plasticity

Fri, 2007-11-30 19:35 - Ray_Lau

Dear all,

Do you have any reference about the theory of plasticity? Especially about flow theory and von Mises yield criterion?

For example if I have an effective stress - plastic strain relationship, and I after yielding, I increase my strain again, how can I obtain the increment of the plastic strain and the stress deviator tensor?

Thanks a lot.

Best,

Ray

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## On a Plasticity theories

Dear Ray lau,

I have read your blog and I want to tell you about my some comments:

What kind of plasticity theory you want to use?

Well known that available the deformation plasticity theory and yielding theory. In case of Ilushin plasticity theory you must investigate the loading process and unloading process differently. But also well known that yielding theory have privelegous that defroamtion theory. How I understand your problem on yielding theory. In this theory the strains velocities tensor and stress tensor relationships defined from associate yielding law. And there are not problem depended from strains increasement define the stress increasement. But well known that the mentionet relationships are nonlinear. And the mentioned increasements definition is problem. Also well known that the any nonlinear problem is the different problem. Also you have not signify you investigated material is istrop or not? If you have isotrop material than you can use the isotropy conditions (The general ways of strains velocities and stress are coincides).

I think you know the physical interpretation of Mises condition. By the Mises criterion we have the region where we have the plastic defroamtions, too. I want to note that the investigation of the elastic and plastic problems are hard problem of mechanical engineering. Do you know about full plasticity or no full plasticity?

Any problem with plasticity statement have the single character and it is necessary definite approach.

My personel web page vailable in: http://www.mechins.sci.am/Staff_en.html and you can send me your comments by e-mail.

I can note some literature but in Russian. If you have interests inform me.

Sincerely

Sedrak Vardanyan

## Book Recommendation

Ray,

Try the book of Simo and Hughes, Computational Inelasticity, 1988. It has exactly what you have asked about.

Prof. Dr. Sanjay Govindjee

University of California, Berkeley

## Convergence problem in UMAT

Hi,

I am writing a UMAT for crystal plasticity. I am having convergence

problem in my UMAT. As far as FEM is concerned, I have tried many

options e.g. minimum time increment, maximum time increment, and

different options with *STEP but I could never reach the complete

solution.

Could anyone suggest any numerical method or procedure in UMAT

itself which I could use to get a convergent solution. I would like to

know how to go to the next step once specified number of iterations

have been completed (even if the solution for earlier time increment is

not complete..but sufficiently close and can be decided by me).

Thanks,

Alankar

Ph.D. Student

School of Mechanical and Materials Engineering

Washington State University, Pullman

## single element test first

Hi Alankar,

UMAT sometimes can be very difficult to deal with.

According to my expereince, when you saw a solution was not converging, most of time it was't because of (1) minimum time increment, (2) maximum time increment, (3) different options with *STEP, nor (4) number of iterations...

Try to do a single element test first before you implement to your model.

Best,

HY Shadow Huang

Postdoctoral Associate

Materials Science and Engineering

Massachusetts Institute of Technology

## Thanks HY Shadow, Yes I

Thanks HY Shadow,

Yes I did try my UMAT for single element, and it works fine but when I try using it for bigger geometries, like 50 grains, sometimes it does not converge. It does converge sometimes, when I tune of my parameters in equations.

I wanted to fix this problem permanently, and wanted to set up some limit on number of iterations or convergence so that I runs completely.

Alankar Ph.D. Student School of Mechanical and Materials Engineering Washington State University, Pullman

## Dear Ray Lau: I am not

Dear Ray Lau:

I am not aware on your mathematical background and so, I would like to warn that an excellent book on plasticity like Computational Inelasticity by Simo and Hughes could rapidly go beyond your current possibilities, making you to feel prematurely dissapointed and discouraged.

In that case, you could try by start reading the first two parts (about 50 pages) of a classical book: The Mathematical Theory of Plasticity, by R. Hill and then move forward to more advanced and modern books, like the one mentioned above, among many others.

Hope this help you in overcoming your problems.

Regards,

Carlos