AC YIELD output and state variable in UMAT

[nownew]

Dear buddies,

As far as I know, the output variable AC YIELD is only available for built-in material models. You need to use a state variable to track this in your UMAT.
But I don't know what state variable I should use to do that.

Do you guys have any idea? Thank you.

NN

[Jorgen]

AC YIELD I believe is a flag indicating if the material is currently yielding or not. You are right that this variable is only available for built-in material models.

You can add the same functionality by adding a state variable to your user-material. I suggest that you simply add this state variable to the end of the state variables that you are already using.

- Jorgen

[nownew]

Dear Jorgen,

Thank you very much for your reply.

But I am still confused that I am not using any state variable about AC yield output. So, I have no idea what I shall do.

Here is the subroutine I am using. I am looking forward to your further instruction. Appreciate it.

NN

[Jorgen]

I suggest that you:
(1) calculate the AC YIELD at the end of your subroutine
(2) add one more state variable and assign the calculated AC YIELD value to that state variable

If you understand the UMAT then this step should be rather easy
- Jorgen

[nownew]

Thanks a lot for your response. But here are the problems.

1, According to my understanding, AC yield is just a yes/no flag. That means it is either '1' or '0', we don't need to calculate it, right?
From: Analysis User's Manual 4.2.1
PE
All plastic strain components. This identifier also provides PEEQ, a yes/no flag telling if the material is currently yielding or not (AC YIELD: “actively yielding”), and PEMAG when PE is requested for the data or results files. When PE is requested for field output to the output database, PEEQ is also provided.

2, If I use one more statev, say, statev(14), what kind of value I should assign to it? Either '1' or '0' ?? Or other values? And, how does ABAQUS know it is for AC yield flag??

If you are so kind to answer my question, I really appreciate it cause it will make me understand abaqus and subroutine much better. Thank you very much.

NN

[Jorgen]

Here are my comments:

(1) If you know if it is yielding or not, then you don't need to calculate it.
(2) The new state variable should be given a value of 0 or 1. Unfortunately there is no way you can tell ABAQUS that your new state variable is the AC yield flag. You can plot and output the state variable any way you want, however.

- Jorgen

[nownew]

Thanks again.

yep, I don't need to calculate AC yield, when stress is greater than the initial yield stress, then it's '1'.

However, I sent an email to ABAQUS for this question, it seems they said it can be done by some way. But they ask for a support ID which I don't think our department paid that money for it. ( I am currently a master student from The university of Toledo.)

I attach their response for our further discussion. It's a great joy to have somebody to talk about abaqus together. Hope you think so too.

Response (Fernando Carranza) 02/20/2008 09:21 AM
The output variable AC YIELD is only available for built-in material models. You need to use a state variable to track this in your UMAT.

Please note that we cannot provide you with additional technical support unless you provide a valid support ID (not to be confused with your site ID, which is something entirely different). The Academic Support Policy is attached below for your reference. Please read it carefully.

Please also note that even with an ATSA, we do not support user subroutines beyond ensuring that the interface works properly (i.e., we do not debug user subroutines).


Best regards,

Fernando Carranza
Manager: Engineering Specialists and Education Products
SIMULIA

NN

[Jorgen]

I think it sounds like your best approach is to implement it through a UMAT.

Best of luck,
Jorgen

[nownew]

Yep, that's what we are talking about. I just don't know how to do that.

And, it's weird that I don't see how other state variables connect with ABAQUS and tell abaqus what they are. They are just statev(1), ... , statev(13), how abaqus distinguishs them???

[Jorgen]

That's the trick - ABAQUS really do not know what the different state variables are for. The state variable are only useful for your user-material to keep track of the internal state.

The one exception is the failure flag that can be used with a VUMAT.

- Jorgen