The reduced relaxation function for this material type is given by $$g\left(\mathbf{F}^{u}\left(v\right);t-v\right)=e^{-\left(t-v\right)/\tau\left[K_{2}^{u}\left(v\right)\right]}$$ where $$\tau\left[K_{2}^{u}\left(v\right)\right]=\tau_{0}+\tau_{1}\cdot\left(K_{2}^{u}\left(v\right)\right)^{\alpha}$$ In general, $K_{2}=\left|\dev\boldsymbol{\eta}\right|$ where $\boldsymbol{\eta}=\ln\mathbf{V}$ is the spatial natural (Hencky) strain tensor and $\mathbf{V}$ is the left stretch tensor. In this expression, $K_{2}^{u}$ is the second invariant of the natural strain tensor evaluated from the relative deformation gradient $\mathbf{F}^{u}$ . $K_{2}^{u}$ is evaluated at the time $v$ when weak bonds from the $u-$ generation start breaking.