This model implements the rate dependent viscoplasticity (Lemaitre & Chaboche, 1990) with multiple internal variables. First the viscoplastic potential is represented as,
$$\begin{eqnarray} \Omega = \int_V \tilde{\Omega} \left[J_2(\sigma-X)-\sigma_y + \left(\frac{D}{2C}J_2^2(X)-\frac{2DC}{9}J_2^2(\alpha)\right)\right] dV, \end{eqnarray}$$

where, $$J_2(\sigma)=\sqrt{\frac{3}{2}(\sigma':\sigma')}$$ with $$\sigma'$$ being the deviatoric stress. $$X$$ is the back stress, calculated as,
$$\begin{eqnarray} X=\frac{2}{3}C\alpha. \end{eqnarray}$$

Here, $$\alpha$$ is the internal variable corresponding to kinematic hardening.

The flow rule is defined as,

$$\begin{eqnarray} \dot{\epsilon}^p=\frac{\partial \Omega}{\partial \sigma}=\frac{3}{2}\dot{p}\frac{\sigma'-X'}{J_2(\sigma-X)} \end{eqnarray}$$

Plastic strain rate $$\dot{\epsilon}^p$$ is a RankTwoTensor and $$\dot{p}$$ is the rate of change in internal variable due to isotropic hardening.

## Isotropic Hardening

Internal parameter for isotropic hardening represents the cumulative plastic strain and is defined as,

$$\begin{eqnarray} \dot{p}=\bigg\langle {\frac{J_2(\sigma-X)-R_0-R}{K}}\bigg\rangle^N \end{eqnarray}$$

Here, $$R_0$$ is the yield stress and $$R$$ is the isotropic hardening represented by increase in the size of the elasticity domain and $$K$$ is the drag force. This has been further simplified as,

$$\begin{eqnarray} \dot{p}=\bigg\langle {\frac{J_2(\sigma-X)-\sigma_y}{\sigma_y}}\bigg\rangle^N \end{eqnarray}$$

## Kinematic Hardening

Internal parameter for kinematic hardening is defined as,

$$\begin{eqnarray} \dot{\alpha}=\dot{\epsilon}^p-D\alpha\dot{p} \end{eqnarray}$$

Internal variable $$\alpha$$ is a RankTwoTensor.

$$C,D$$ are temperature dependent material parameters

**Note:**
This model is currently being implemented in TensorMechanics. Further details about the model will be added once the implementation is complete.