Non homogenous strain in uniaxial Testcase in ABAQUS

Hello Everyone,

I am trying to verify my implementation of a viscoelastic Materialmodel in ABAQUS via modeling an uniaxial loading/unloading test.

The specimen is a 100x10x1 mm thick solid material (3D Model).

The Top surface is fixed and the load is applied as a displacement boundary condition on the bottom surface.

When I do the caluclations with a purely hyperelastic material, the strain is homogenous in the middle of the specimen, as expected. However, after I add the PRF Model with two nonlinear viscoelastic Elements using the BB-Model, the strains become inhomogenous. The model becomes kinda spotty, meaning there are periodic regions with higher strains (as can be seen in the Figure). This happens after an elongation of about 50%.

I tried playing around with stabilization in the step-controlls, but unfortunatly that did not really help.

I am greatful for any advice.

Thanks in advance

Martin