Figure 5.

Discretization of the bulk with finite-volume elements ${\mathcal{B}}_0^{\text{e}}$ and ${\mathcal{B}}_t^{\text{e}}$, shown in green, and of its boundary with finite surface elements ${\mathcal{S}}_0^{\text{e}}$ and ${\mathcal{S}}_t^{\text{e}}$, shown in red. Surface elements share their nodes with the corresponding volume elements, however, they are equipped with their own energies $\widehat{\psi }$, stresses $\widehat{\mathit{P}}=\partial \widehat{\psi }/\partial \widehat{\mathit{F}},$ , and tangent operators $\widehat{\mathbb{A}}=\text{d}\widehat{\mathit{P}}/\text{d}\widehat{\mathit{F}}.$