Dear All,
I want to using Abaqus UMAT to model stress relaxation due to the viscoelastic behavior of the material.
I created a 3D RVE contains fiber(elastic) and matrix(viscoelastic) and applied periodic boundary conditions. I already have Prony series of matrix, so I know the properties of matrix in each time step. I know abaqus has viscoelastic function for isotropic materials, but I need the umat work so that I can analyze viscoelastic behavior of anisotropic material later. The umat codes are attached. I don't know how to apply constant displacement loading on my model so that the STRAIN in umat is constant in the time loop. Any comments and suggestions are highly appreciated.
Thanks
Hi Ahmad,
Thank you for your answer. I have used the equation you suggested in my model, but the strain will always increase from 0 to the values you defined in a step. So the question now becomes how to define "step" in Abaqus and how to make it converge. Now I am defining two steps. The first step is very short and the strain will be linearly increased to the constant value. Then, the second step keeps this constant strain. However, the attached updated umat has some convergence issues. Please let me know if you have any suggestions. Thanks a lot.
the the problem is in your umat...
one note: try to use time(2) instead of time(1) in your umat... hope this works
also I found several mistakes in your umat.. for example stran() is not updated stran..so you need to add stran + dstran when calculating the stress..
see umat in this link http://barbero.cadec-online.com/feacm-abaqus/Examples/Chapter_7/Ex_7.7/CAE_files/UMAT3DVISCO.FOR
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