UMAT_viscoelastic material

[lanfeng]

Hi, everyone,
I have to implement a UMAT of a finite viscoelastic model based on internal variable.
I would like to begin with a very simple model:
Holzapfel "nonlinear solid mechanics" page 286
I have only experience of implementation the UMAT of hyperelastic model based
on the Jaumann rate. But for rate-dependent internal variable, I have no idear how to
do it?
Can you give me some tips how can I do it?

Thank you so much.

Lanfeng

[Jorgen]

I don't have my copy of Holzapfel's book available right now, so I cannot comment on the text on page 286.
It is not trivial to code a UMAT that is using internal state variables. You need to formulate a suitable integration scheme, code it, and then debug it...

-Jorgen

[lanfeng]

Thank you Jorgen for you in time response.
I have coded a UMAT for a simple 3D viscoelastic model, but there is a error when I run this UMAT.
In the attachment are the UMAT .f, .inp and the model.
I guess the error is due to that I use two new stress and stress increment variables in the UMAT.
But I have no idea how I can correct this.

Can any one help me to find the errors. I appreciate to any response.

Thank you.

Lanfeng

[lanfeng]

Hi,
thank you for the helps.
The problems have been solved. The STATEV in abaqus can be used to store
the user-defined intermediate variable, but we have to note that:
1. in the input file of the abaqus model, DEPVAR has to be used to give STATEV space to store the array.
2. AT the beginning of the stress update, the old value of the intermediate variable has to be
recovered in STATEV
3. At the end of the UMAT, the present value of the intermediate variable has to be stored into STATEV

I write down the key solution just for those who have the same problem as me.

Lanfeng