Introduction to the Element Types
In some engineering structures, the optimal idealization is a mixture of different element types. Some examples are:
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Structures that are thin in large regions but more three-dimensional at certain locations. A mixture of solids and shells can then significantly reduce the model size.
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Plates or shells having beams as stiffeners.
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Truss elements acting as reinforcement bars in a concrete structure.
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A thin layer of one material on top of another material. In this case, an idealization with shells or membranes covering the boundary of a solid can be useful.
When several physics interfaces are added in COMSOL, the default is always that each physics interface has its individual degrees of freedom. In structural mechanics the first physics interface has the displacement variables (u, v, w), then the second physics interface has (u2, v2, w2), and so on. This means that the physics interfaces initially are independent even when defined on the same geometrical part. To get the intended interaction requires that a coupling is established between the physics interfaces.
Various methods to couple different element types are discussed in this section.