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For this material, the specific reaction rate depends on the deviation of the specific strain energy from a threshold value, $$\begin{equation} k\left(\mathbf{F},\rho_{r}^{s}\right)=\frac{B}{\left(J-\varphi_{r}^{s}\right)M^{s}}\left(\frac{\Psi_{r}}{\rho_{r}^{s}}-\psi_{0}\right)\,,,\label{eq531} \end{equation}$$ where $B$ is a constant, $\Psi_{r}$ is the strain energy density of the solid, $\rho_{r}^{s}$ is the referential mass density of the solid, $\psi_{0}$ is the threshold value for the specific strain energy. In this relation, $J=\det\mathbf{F}$ is evaluated from the solid deformation and $\varphi_{r}^{s}$ is evaluated from (2.10.1-2).