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3D fractional viscoelastic springpot

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Hello everyone, 

I am trying to develop a model for soft tissue using fractional viscoelasticity in abaqus. I am using a springpot model defined using a VUMAT subroutine. 

I am testing with a single solid element (C3D8) in a displacement controlled simulation. I am currently running a tension test with displacement control along x varying linearly from 0 to 0.1 at the end of the dynamic step. With VUMAT, the strain at the end of the time increment is already known and the corresponding stress has to be evaluated and updated.

However, when I run the model, within a few increments my simulation errors out with excessive element distortion. Upon printing the strains, I notice that the strains in the y and z directions (lateral strains) quickly blow up causing excessive distortion.

Has anyone encountered similar issues? if so, can you please suggest how to debug the problem.