Loads and Boundary Conditions
Several loads and boundary conditions can be applied to a thin layer. Exactly what boundary conditions are available and how they are treated depends on the thin layer approximation. Whether the thin layer is an exterior or interior boundary of the physics interface also affects the available boundary conditions.
On exterior boundaries, loads and boundary conditions can be applied to the exterior displacement field of the nonlayered stratum, or on the top side of a layered material for a layered stratum. For example, applying a fixed constraint to a thin layer on an exterior boundary makes is equivalent to a Spring Foundation. It is also possible to apply loads and boundary conditions to edges and points of the thin layer. For a nonlayered stratum, loads and other boundary conditions apply to the exterior displacement field. However, for a layered stratum, all extra DOFs attached to the edge or point are constrained.
When a Roller is applied on a boundary perpendicular to a thin layer, a Roller should in most cases also be added to the edge of the thin layer. This is especially important if the roller represents a symmetry condition.
On interior boundaries, only loads and boundary conditions on edges and points are available. Moreover, these are only relevant for a layered stratum. For a nonlayered stratum, there are no free DOFs on which to apply the boundary conditions.