Viscoplasticity

Viscoplasticity is a model for rate-dependent plasticity. Rate dependent plasticity is important for (high-speed) transient plasticity calculations. It should be used in combination with a plasticity law. Viscoplasticity influences the stresses via the plastic strains.

The group_materi_plasti_visco_exponential model reads

$$\dot{{\epsilon_{kl}}} ^{\rm plas} = \gamma ~ p ~ e^{\alpha f} \;\; \frac{\partial f^{\rm flow}}{\partial \sigma_{kl}}$$

where $\gamma$ and $\alpha$ are material fluidity constants and $p$ is the pressure. In case the ${\alpha f}$ becomes larger than a limit, is is substituted by the limit to prevent the exponent from becoming excessive large. You can set the limit with the group_materi_plasti_visco_exponential_limit record. This model was first developed for visco-plastic soil behavior.

The group_materi_plasti_visco_power model reads

$$\dot{{\epsilon_{kl}}} ^{\rm plas} = \eta ( f ) ^ p \;\; \frac{\partial f^{\rm flow}}{\partial \sigma_{kl}}$$

where $\eta$ (fluidity constant), and $p$ (power) are user specified parameters.