Shell and Plate
The Shell interface is useful when the object is thin in one direction. Structures built from welded or bolted flat plates are archetypal shell structures and so are pressure vessels. The Shell interface is available in 3D and 2D axisymmetry.
The Plate interface is a specialization of the Shell interface, used for 2D modeling in the XY-plane. A plate model has its main action in bending out of the plane, but can optionally also treat in-plane forces. If the loads act only in the plane, using Solid Mechanics with the Plane Stress option is a better choice.
Shells are modeled on boundaries, and the transverse direction is represented only by the mathematical model. The degrees of freedom consist of displacements and rotations at the modeled boundary. This results in an assumption where the in-plane strains vary linearly through the thickness, and the stress in the thickness direction is zero. The thickness of a shell does not have to be constant, although this is by far the most common case.
When the Composite Materials Module is available, it is possible to model also multilayered shells.
For nonlinear material models, a layered approach with a single layer is used. There is a virtual mesh in the thickness direction, in order to accommodate the potential variation of the material properties in the thickness direction.
Rather than computing the shell stiffness from material properties and thickness, you can also directly enter that stiffness properties in tension, bending, and shear.
The Shell and Plate interfaces can be used both for “thin” and “thick” shells. Shear deformations are taken into account; this is usually called Mindlin theory. The material model is linear elastic.
When modeling with shells, it is important to keep track of “top” and “bottom” side when applying loads and interpreting the results.
The in-plane stiffness of an elastic shell is proportional to the thickness h, while the bending stiffness is proportional to h3. The difference in stiffness along different directions can thus become very large. When an object is very thin, a shell model may be numerically ill-posed due to the negligible bending stiffness. It is then better to use the Membrane interface.