Introduction
Shells and plates are structures which are significantly thinner in one direction than in the other two directions. The mathematical description can then be simplified by using the following assumptions:
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The deformation field is described by the displacements of the midsurface plus the rotation of the midsurface normal.
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The in-plane total strains vary linearly through the thickness.
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The stress in the transverse direction is negligible.
These assumptions make it possible to reduce the number of degrees of freedom significantly from what would be needed for a corresponding 3D solid model. In addition, there are advantages in terms of numerical conditioning and meshing.
In shell theory, all quantities refer to the midsurface. In the Shell interface, you can however use any boundary that is parallel to the midsurface for modeling. If the reference surface where the mesh is located is not the midsurface, you can use an offset distance to describe the true location. If you have a full 3D geometry, there is thus no need to create the midsurface. You can place the mesh on either the top or bottom surface of the actual geometry.
Plates are similar to shells but act in a single plane and usually with only out-of-plane loads. The plate and shell elements in COMSOL Multiphysics are based on the same formulation. The Plate interface for 2D models is a specialization of the Shell interface. In the following, the text fully describes the Shell interface, and the Plate interface is mentioned only where there are nontrivial differences.
A Shell interface can be active on:
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Free surfaces embedded in 3D.
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The boundary of a solid 3D object. In this case, it can be used to model a reinforcement on the surface of a 3D solid.
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Free boundaries in 2D axisymmetry.
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The boundary of a solid object modeled in 2D axisymmetry. In this case, it can be used to model a reinforcement on the surface of a 3D solid.
A Plate interface can only be active on domains in 2D.