how to define the tangent modulus related to the Green-Naghdi Stress Rate

[cardiffben]

Dear all

I am a new comer here. I am a Ph.D student with the project named 'face movement simulation using ABAQUS'. However, I am wrapped by several questions for a long time. I hope I can seek help here.
Now, I am trying to develop a ABAQUS UMAT program for my facial skin model.
The face is meshed with membrane elements now. I got some trouble in developing the corresponding UMAT program. As it says in ABAQUS, the Green-Naghdi stress rate is used for shell and membrane elements. I dont know how to update the stress and the corresponding tangent modulus in the UMAT. How I change the membrane element to shell element, shell transverse stiffness must be defined, how to define it?
I cannt find any related umat program from the website. Is there anyone who got this kind of umat program? Could you give me some advice or send me one simple program example?


many thanks,
Ben
School of Engineering
Cardiff University

[Jorgen]

Hello Ben,

Sounds like you have a challenging project! Here are a few comments:

• You don't have to worry too much about the Green-Naghdi stress rate. I instead recommend that you develop your own material model, a procedure for solving the material model equations, and then derive the tangent modulus for the UMAT. This, of course, can be a tall order.
• There are no particular changes that need to be implemented in the UMAT in order to switch from membrane elements to shell elements. You can typically guess a value for the transverse shear stiffness. It often does not have much influence on the results.
• As you have noticed, there are not many exemplar UMAT codes available. And the examples that are available typically do not support shell elements
  However, I have done a lot of UMAT work for membrane and shell elements and would be happy to provide helpful suggestions.

- Jorgen

[cardiffben]

Hi Jorgen

Thanks a lot for your quick reply.
However, I still got no idea.
Could you send me one of your successful UMAT for membrane element?

Many thanks,
ben
Ph.D student
Cardiff University
bennett2001@gmail.com

[Jorgen]

Hi Ben,

I am afraid that my user-material models are only commercially available. I recommend that you carefully read the Abaqus manuals, and feel free to ask any specific questions that you might have.

Best of luck,
Jorgen