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Issue in implementation of Viscoelastic behavior in UMAT

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  • Issue in implementation of Viscoelastic behavior in UMAT

    As a part of a project, I have to implement Linear Viscoelastic behavior in ABAQUS via UMAT. Constitutive equations and numerical internal state variable solution scheme were derived from literature. The problem that I encountered was that the solution is highly dependent on the time increment selected by ABAQUS and increment = 0.01 leads to multiple times strain in comparison with the ABAQUS Visco STEP. hence, we have to reduce increment below the 0.0001 sec resulted in many hours of simulating. however, still there is a discrepancy between ABAQUS solution and that in UMAT. The major difference found in stain. Stress was in almost in agreement. The differences was attached to the post. from anyone who had an experience, please comment on the problem. It seems there should be a restrain on the strain increment and when there is no load (relaxing) strain tend to grow.
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