Home » forums » Computational Mechanics Forum

Abaqus UMAT implementation

Submitted by Mandar Kulkarni on Mon, 2013-02-18 04:20.

Hi,

I have a thermo-mechanical constitutive model for polymer and want to implement it in ABAQUS through two user subroutines namely UMAT and UEXPAN but I dont know how to use them together. Also, in UEXPAN, I need to pass in the strain tensor as information but I dont see 'STRAN' variable in the list of variables passed in to UEXPAN in Abaqus documentation. Does anyone know answer to this?

Thanks,

Mandar


‹ Query regarding St. Venant Kirchoff material in ANSYS using the USERHYPER routine Abaqus UMAT Problem (Fick's second law in Abaqus) ›
»
Frank Richter's picture

subroutine manual

Submitted by Frank Richter on Mon, 2013-02-18 11:57.

To start with get the file
http://imechanica.org/files/Writing User Subroutines with ABAQUS.pdf
it may show up as
http://imechanica.org/files/Writing%20User%20Subroutines%20with%20ABAQUS.pdf

------------------------------------------
Ruhr-University
Bochum
Germany


»

Hi Frank, Thanks for your

Submitted by Mandar Kulkarni on Tue, 2013-02-26 23:31.

Hi Frank,

Thanks for your help. I have a very specific question now. I was studying the .pdf file mentioned in your reply, particularly UMAT for Neo-Hookean material. On slide number L6.46 they define material jacobian for this hyperelastic material C_ijkl. I read through ABAQUS Theory manual section 4.6.1 but I could not understand where the last two terms in the expression for C_ijkl come from. I see that they are related to the DDSDDE definition on slide number L6.50 by terms TRBBAR (which is trace of left Cauchy deformation tensor) and EK which corresponds to bulk modulus of the material. But the theory manual defines material jacobian by deviding the strain into deviatoric and volumetric parts (Eq. 4.6.1-12 in ABAQUS Theory manual). So my question is how both deviatoric and volumetric behaviors can be included in one entry of DDSDDE array by just adding the terms. This might be a silly question and the anwser might just be the rearrangement of the terms. But I am finding it difficult to see it. I hope you can help me on this issue.

Thanks for your help and time in advance.

Mandar


»

What is your result?

Submitted by zqctate on Thu, 2013-05-16 01:53.

Hi, Mandar

I have the same question. According to  ABAQUS Theory manual section 4.6.1,  the material jacobian C_ijkl are consisted of deviatoric part C-s and volumetric part K, see the equation (4.6.1-12). However, based on these, i can not figure out the second to last term in the expression for C_ijkl of Neo-Hookean hyperelastic model on slide number L6.46.

So could you kindly tell me how you solve your problem?

Thank you.

zqctate


»
Frank Richter's picture

code is used in an example

Submitted by Frank Richter on Wed, 2013-02-27 00:24.

Hello

I am not familiar with this constitutive equation.

The code is used in the example manual, example
1.1.14 ANALYSIS OF AN AUTOMOTIVE BOOT SEAL

For a VUMAT implementation get
http://polymerfem.com/polymer_files/vumat_nh/vumat_nh.f

Frank

------------------------------------------
Ruhr-University
Bochum
Germany


»

integrate them into one UMAT subroutine

Submitted by chen1164 on Thu, 2013-02-28 19:24.

I used to think that UEXPAN is a good way to implement thermal expasion cases. But after i tried. I would recommend you to integrate them into one UMAT subroutine. It could make your life easier.

You just need to modify your constitutive model by replacing your elactic strain with the total strain deducted by thermal strain.

 


»

I just finished coding UMAT

Submitted by Mandar Kulkarni on Sun, 2013-03-10 01:52.

I just finished coding UMAT for hyperelastic material and tested it on one element for uniaxial and simple shear boundary conditions. It works fine for one element. But, if I try to run a simulation for more than one element, ABAQUS exits with an error. The message says that the error can be found in message file if the file exists and obviously the .msg file does not exist. Does any one have solution to this problem? or where should I look for the kind of error I am encountering with?

Thanks in advance.

Mandar


»

Hi all, Just a follow up

Submitted by Mandar Kulkarni on Fri, 2013-04-05 20:50.

Hi all,

Just a follow up question on the leading thread. My UMAT for hyperelastic material works fine if I use automatic time stepping provided by ABAQUS but if I use fixed time stepping it exits with an error message which says that FIXED TIME STEP IS TOO LARGE. I tried using fixed time stepping with time increment even smaller than the minimum increment size in autimatic time stepping but without any luck. Does anyone know why this occurs? I am just curious why fixed time increment wouldn't work.

Thanks for your help.

Mandar


»