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The Lagrange strain tensor $\mathbf{E}$ is related to $\mathbf{F}$ via Eq.(5.6.3-2). Its maximum principal value is denoted by $E_{1}$ . For this criterion, we let $$\begin{equation} \Xi\left(\mathbf{U}\right)=E_{1}\,.\label{eq:dmg-model-MNLS} \end{equation}$$ Then, the damage surface normal is given by $$\begin{equation} \mathbf{N}=\frac{\partial E_{1}}{\partial\mathbf{U}}=\sqrt{1+2E_{1}}\mathbf{n}_{1}\otimes\mathbf{n}_{1}\,,\label{eq:dmg-MNLS-N} \end{equation}$$ where $\mathbf{n}_{1}$ is a unit vector along the principal direction of normal strain.