The Shell interface has two fundamental types of material models. The first is represented by Linear Elastic Material, Rigid Domain, and Section Stiffness. The other type consists of Layered Linear Elastic Material and Layered Hyperelastic Material. In either case, the dependent variables are the same, and exist only on the reference surface. The fundamental difference is that in the first group, 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 and Layered Hyperelastic Material models, 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 Nonlinear Structural Materials Module is available, the Layered Linear Elastic Material can be used to model Plasticity, Creep and other nonlinear materials; and when the Composite Materials Module is available it can be used to model multilayered shells.

The accuracy of the results depends on the resolution in the thickness direction. For each layer, you have the option to set the resolution. In a layered material, this is the Mesh elements property in the layer definitions. When working with a single layer material, then it is the Mesh elements property in the Shell property group.

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.

The virtual mesh depends on the in-plane discretization, and so does the number of integration points in the thickness direction. In Table 5-1, the number of integration points that are used in the thickness direction are summarized.