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UMAT - Nonlinear Viscoelasticity

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Posts: 7
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Joined: 9 years ago

Hello everybody,

My task is to write a UMAT for nonlinear viscoelastic material based on the attached paper. For now, I have written the UMAT (1D formulation) and I am able to generate the output. My problems are


Validity of the code I have attached the stress vs time graph for a relaxation test on 1D Truss element. The behaviour looks normal (correct me if you have to) to me.



If i change the input parameters, say I play with the initial time increment of Steps or change the displacement from 5 units to 10 units, the job keeps running and does not complete. I have to manually delete .lck file from the folder and again run the job. I am not able to understand what is going wrong. Would be really helpful if somebody could throw some light on it.


Thanks in advance for your time


P.S I have attached the input file and the UMAT for your reference. (

Problem with UMAT? - ResearchGate. Available from: [url] [/url] [accessed Feb 14, 2016].

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Posts: 3998
Joined: 5 years ago

Writing a proper UMAT is not easy and requires massive testing using simple FE models, etc.

Unfortunately I dont have the time to review your files 🙁

Best of luck,

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