The Shell interface has two fundamental material models, Linear Elastic Material and
Layered Linear Elastic Material. In either case, the dependent variables are the same, and exist only on the reference surface. The fundamental difference is that in the
Linear Elastic Material, the material properties are assumed to be constant through the thickness, so that stiffness and mass matrices can be computed by an analytical integration in the thickness direction.
In the Layered Linear Elastic Material model, there is a numerical integration in the thickness direction. It is also possible to store states, such as inelastic strains, at different through-thickness locations. Thus, the
Layered Linear Elastic Material forms the basis for all nonlinear material models even if the shell is not layered as such. Also, if you want to write your own expressions as function of through-thickness location, you must use this material model.
When the Composite Materials Module is available, the Layered Linear Elastic Material model can be used to model multilayered shells.
As this setting indicates, there is a virtual mesh in the transverse direction (the extra dimension). When there is a significant variation in the thickness direction, as is the case for plastic strains in state of bending, you need a good enough resolution.