how to define the tangent modulus related to the Green-Naghdi Stress Rate
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?
School of Engineering