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Problem solving Strain Gradient Viscoplasticity based on the framework of Fleck and Hutchinson

I am tyring to solve simple shear problem using a large deformation FE code based on the framework developed by Fleck and Hutchinson for gradient plasticity/viscoplasticity problem (see the attached paper). As the per the formulation, the independent variables are the displacement increments and eqv. plastic strain rate increments. Two sets of equations are solved, one for displacements and the other for plastic strain rates at nodes, following the the above formulation.

The solution for plastic strain rate increments starts to blow up after few steps of entering the plastic regime. I've tried several things:

i) running the problem, reducing the time steps.

ii) refining the mesh.

But the problem persists. 

And how to deal with the negative solutions for plastic strain rate increments ? Physically eqv plastic strain increment can not be negative.

 

Does anyone have any idea?

 

Thanks

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