iMechanica - UMAT - Comments

You can convert between different rate forms

Wei Hong — Thu, 27 Nov 2008 15:14:03 -0500

Hi Shunlai,

I think the way abaqus treat stress/strain rate is more or less determined by its corotational local coordinate system. So direct implementation of orther rate forms are not so easy. But if you have a constitutive relation in the form of one type of stress/strain rate, you can easily convert it into the rate form used by Abaqus.

If not so straightforward to convert analytically, you can treat it as following. Expand everything around the current configuration, and neglect all higher order terms in the increment of strain/displacement. By doing that, the constitutive relation is a small-deformation relation, and the rate forms do not really matter, since the rigid-body rotation is already taken care of by abaqus, before passing into UMAT/VUMAT

Hope this helps.

Wei

Re:

alankar — Mon, 06 Oct 2008 16:20:42 -0400

Hi Ramin,

I wonder how come you get my code. It is still in progress mode. I would like to know which code are you talking about.

Regards,

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

Euler angles can be used too

Arun Prakash — Mon, 06 Oct 2008 13:50:28 -0400

Yuanpei:

The vectors that are specified are just two orthonormal vectors to determine the rotation matrix. They can surely be transformed into euler angles. Infact, one could reprogram the UMAT to deal only with euler angles.

I was perhaps hasty in saying that they DO NOT correspond to (hkl)<uvw>. Since it can be any two vectors, one cld also use (hkl)<uvw>also.

What I normally do is the following. I use the euler angles (obtained from experimental texture) to construct a transformation matrix and then rotate the 100 and 001 axes to the global coordinate system.

Arun

A problem regarding crystal plasticity

RaminBabaei — Mon, 06 Oct 2008 05:30:28 -0400

Hi there,

I am working on crystal plasticity and I have started with your UMAT code you written for Single crystal plasticity. I have a problem with this code and I would be appreciate if you can answer it.

When I use your code for modelling the single crystal or polycrystalline materials on ABAQUS, I get the results for current strength and resolved shear stresses and also for normal and slip direction for each plane but I cannot get the non-zero value results for shear strains and total cumulative shear strain.(state variables #13-24 and #109). If I right, the strains should be computed because stresses defined by strain increment but I do not know why ABAQUS does not show the strain state variables.

In addition, The lattice rotation only happen for first step(first time increment) and becomes constant during the time for others step.

Thanks,

A problem regarding Crystal plasticity

RaminBabaei — Sat, 04 Oct 2008 00:58:19 -0400

Dear Alankar

I am working on crystal plasticity and I have started with your UMAT code you written for Single crystal plasticity. I have a problem with this code and I would be appreciate if you can answer it.

In addition, The lattice rotation only happen for first step(first time increment) and becomes constant during the time for others step.

Thanks,

Abaqus Process Automation Portal

Sprunger — Mon, 29 Sep 2008 13:11:49 -0400

Hello, Re: Sharing Abaqus UMAT and VUMAT subroutines.SIMULIA has an “Abaqus Process Automation Portal” http://www.simulia.com/PAPortal/This portal provides a mechanism to the SIMULIA (Abaqus) community for collaboration through discussion forums for GUI customization and scripting techniques. Users can upload, download, and share solutions and process automation components.

The Abaqus Process Automation Portal is approaching its 5 year anniversary and many engineers found it to be a useful resource. Almost 3000 registered members currently exist and this number continues to increase steadily.

Regards, Rene

hi Arun， Thank you for

Yuanpei Duan — Fri, 19 Sep 2008 06:42:51 -0400

hi Arun，

Thank you for your kindly reply!

You have emphasized that "They are
just two vectors. They DO NOT correspond to (hkl)<uvw>. " What’s the real meaning of
the two vectors of the local system? What do they stand for? Can they be transformed
to three Euler angles?

You said
that you normally use the 100 and 001 axes for
specifying these orientations. Here, are the 100 and 001 axes consist of the global system or the local system? When there are a lot of grains, how do you do to deal with their different orientations?

Yuanpei

hi Joe, Thank you

Yuanpei Duan — Fri, 19 Sep 2008 05:51:55 -0400

hi Joe,

Thank you very much for your detailed explanation！

Glad to hear that both of the methods can
be chosen to define the orientation.

But I have never tried the method of *ORIENTATION
in the CAE. I just used to define the orientation in the INP. Maybe I can try and
find the best method.

Yuanpei

Specifying orientation with Huang's crystal UMAT

Joe Kelleher — Thu, 18 Sep 2008 10:34:44 -0400

With Huang's UMAT, you can choose either to define the orientation of the crystal either using values in the input cards (like you describe), or to specify the orientation using CAE. We've tried both methods and can confirm they give the same answers. Using CAE to specify orientation works by creating an *ORIENTATION entry, with the appropriate numbers, in the resulting Abaqus INP file. If you're not using CAE at all, but instead are editing the input file directly, you can of course add the relevant *ORIENTATION command to the INP file manually. If you use the CAE/*ORIENTATION method of specifying orientation, you must make sure that no orientation is specified in the UMAT card values, otherwise you'll end up simulating another orientation that is the combined effect of both. Naturally you can remove orientation from the UMAT card by specifying directions in the local system that are the same as the corresponding directions in the global system, e.g.

1, 0, 0, 1, 0, 0

0, 1, 0, 0, 1, 0

It sounds like you already know the orientation you want the crystal to have, in the form of (hkl)<uvw> or Euler angles. The (hkl)<uvw> system is most commonly used for specifying the preferred crystal orientation in rolled sheet metal, where the (hkl) crystal plane is parallel to the surface of the sheet, and the <uvw> crystal direction lies in the direction of the rolling. According to http://www.materialsknowledge.org/index.php?option=com_docman&task=doc_download&gid=30 , if people use this system when they're not talking specifically about rolled sheet, then the (hkl) plane is the plane perpendicular to the global Z direction and the <uvw> direction is the global X direction. So to use the UMAT cards to specify orientation, you would need to know which direction was perpendicular to the (hkl) plane. If your crystal is any kind of cubic crystal, this is easy: the [hkl] direction is normal to the (hkl) plane for any h, k, l. For other crystal systems, there is most likely a formula that involves the unit cell dimensions. For cubic crystals only, the required input cards for a (hkl)<uvw> orientation would therefore be:

h, k, l, 0, 0, 1

u, v, w, 1, 0, 0

For non-cubic systems, you would replace h, k, l with the direction that is perpendicular to the (hkl) plane.

If you know the orientation in terms of Euler angles, you need some trigonometry to get the input card values. Probably the easiest way is to derive (or look up on Wikipedia!) the rotation matrix or similar equations for Euler angle rotation, and use these to calculate the direction in the global system that the 1, 0, 0 direction in the local system corresponds to, and again for another direction like 0, 1, 0 or 0, 0, 1. Alternatively, using CAE might be some use here: you could start by creating an orientation that's the same as the global orientation, and then rotate it by each Euler angle in turn around the appropriate axis for that angle.

Joe

Re: Different local systems for different grains？

Arun Prakash — Thu, 18 Sep 2008 03:57:28 -0400

Hi Yuanpei,

To clarify your query, the orientation of a crytal can be expressed in a lot of ways - Miller Indicies and Euler angles are just a couple of them. What is however expected in the UMAT of Huang is the following. You need to input the orientation of the crystal through two non-coplanar (preferebly perpendicular) vectors. These vectors do not have to correspond to any slip plane normal or direction. They are just two vectors. I normally use the 100 and 001 axes for specifying these orientations. Again I repeat - they DO NOT correspond to (hkl)<uvw>. The Miller Indicies of slip families are given prior to specifying the orientation - for more details refer to information on input cards in the report of the UMAT.

Given the fact that you specify the orientation through the props array, the *orientation parameter in the abaqus input file is not required.

Hope this clarifies your query

Arun

Different local systems for different grains？

Yuanpei Duan — Wed, 17 Sep 2008 22:08:41 -0400

In Prof. Huang's single crystal umat, the orientation
is described as follows:

-1. , 0. , 1. , 0. , 0. , 1. ,

direction in local system , global system ,
of the 1st vector

-- , -- , -- , -- , -- , -- ,

(the first vector to determine crystal
orientation in global system)

0. , 1. , 0. , 0. , 1. , 0. ,

direction in local system , global system ,
of the 2nd vector

-- , -- , -- , -- , -- , -- ,

(the second vector to determine crystal
orientation in global system)

As we know, the orientation can be
expressed by both (hkl)<uvw> and three Euler angles.

So, I think that the first vector (-1. , 0. , 1. )and the second vector(0. , 1. , 0. ) here, which represent grain's orientation, consist of the local system and correspond to (hkl) and <uvw>. We can easily change the two vectors by modifying the data cards. When the two vectors are changed, the orientation of grains is also changed and the different local systems is founded.

But someone said that the grain’s
orientation is defined by assigning different Material Orientations in ABAQUS
CAE. Should different local systems be set by using Material Orientation of
property in CAE for different grains?

Now, I am so confused with the definition
of the orientation of polycrystal model. Which is the right way?

Could anybody help me?

Thank you!

Yuanpei

About the initial orientation

Yuanpei Duan — Tue, 16 Sep 2008 11:07:36 -0400

Hi Arun,

Thank you for your suggestions!

I am a beginner of UMAT and have a lot of
questions. I want to program a single crystal based polycrystal model. I have
read the UMAT of Prof. Huang. In his UMAT, the initial orientation is expressed
by two vectors of local and global systems, while not the Euler angles. How to modify
the umat of Prof. Huang so that the Euler angles can be used as the props
input?

Ps: What’s the SX plasticity model?

Looking forward to your kind reply. :)

Yuanpei

It's an old topic.

Shunlai.Zang — Sun, 24 Aug 2008 09:05:30 -0400

Just as open source vs. closed source

But I think sharing knowledges is a good thing.

Shunlai

UMAT for Shape Memory Alloys

msimsek — Wed, 20 Aug 2008 12:40:21 -0400

Hi,

I am an M.Sc. mechanical engineering student and I am trying to model super elastic property of shape memory alloys in ABAQUS. I am new to ABAQUS and trying to learn it by reading the tutorials. It seems I have to use UMAT but I dont know how to use it or how to create a subroutine for my case. If anybody helps, makes contribution it would be appreciated.

Look forward to getting some responses.

Thanks

Miray

searching Umat Vumat

Tosche96 — Sun, 20 Jul 2008 15:59:56 -0400

Hello,

I'am looking for a vumat or umat file for thixotropic material! Has somebody already written such kind of file for abaqus! I want to simulate thixoforming / semi-solid process!

thx