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UMAT - Nonlinear Viscoelasticity - Excessive distortion error Abaqus

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Hello everybody.

My task is to write 2d UMAT for nonlinear viscoelasticity. I am trying to write a 1d UMAT for non-linear viscoelasticity based on the Schaperys model (paper attached below) and then extend it. I get Excessive distortion error. I tried printing variables in the .dat file. Problems are

Strain increment (DSTRAN) value will be given by Abaqus (I referred documentation).I understand that the value is based on the DDSDDE (Jacobian) that we calculate and return from UMAT. I am not clear as to how Abaqus is giving me the DSTRAN value for the 1st increment of the 1st Step. Besides, I see DSTRAN is printed as zero for the 1st Step. (Which I assume is a reason for excessive distortion) Please correct me if you have to.

I tried working with nlgeom = YES and without that. Nothing improved. If I am not wrong nlgeom is for Finite strain problems and not using nlgeom means we are in the regime of small strain theory. I am not sure as to when I should have the option turned on and what difference will it make to the UMAT code (if any)

I will attach my FORTRAN code for reference. You see I am a newbie to Abaqus. I would like to have your suggestions and ideas.


It looks like I should not use nlgeom for this analysis. Anyway I was able to successfully run the code with a sample 1D truss (single element). But the*result is not as expected. When trying to simulate*relaxation under constant strain, stress looks to increase and then go constant after a certain time period which is not correct. Any suggestions would help.
If somebody could just skim the attached paper, I was not able to find a starting value to initialize a parameter (q_ij^time) for the 1st time step. The result changes with different initialization values and the paper does not provide any info on that. Would be really helpful if someone could help me in this.

Thanks a lot for your time

Shreeraman S

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