The material laws for plasticity can also be combined with viscosity to achieve visco-plastic material behavior. Three models are available: linear Perzyna, nonlinear Perzyna and Michel-Suquet.

The three models differ in their complexity. The linear Perzyna model is the least complex of the three. It has only one additional parameter (the reference strain rate) compared to time-independent plasticity. In general, one should first try if this model already fits the needs.

The other models should only be considered if the real material shows a behavior which is too complex for the linear Perzyna model. Both have the rate sensitivity parameter, which allows to define how a change in strain rate affects the material behavior. The Michel-Suquet model additionally has the drag stress parameter, which further influences the material behavior and allows to fine-tune the model.

In general, setting up a visco-plastic material law is more complex than setting up time-independent materials. Measuring a stress-strain curve is not enough to explain the behavior of a visco-plastic material. In contrast, more complex experiments are necessary, like e.g. creep tests, relaxation tests or DMA measurements (Dynamic mechanical analysis).