Working with Section Stiffness
In some cases, shell models are used as a homogenized approximation of a thin inhomogeneous structure. Examples of this are perforated sheets and corrugated sheets, but also laminates. For such cases, a single material model together with a thickness does not contain enough information to set up the model.
The Section Stiffness material model provides an option to describe the shell properties directly in terms of quantities such as membrane stiffness, bending stiffness, and so on.
Actually, the Layered Linear Elastic Material is a special case of such a homogenized model, where the homogenization is performed in the thickness direction. The ABD matrices computed as side effect by this material model provides exactly the same form of data that is used as input in the Section Stiffness node.
In order to use the section stiffness approach, you must obtain all terms in the stiffness matrix. In the case of dynamic analysis, also inertial matrices are needed. For some common cases, like perforation and corrugation, expressions for these quantities may be found in the literature. In general, however, you have to set up a finite element model of a representative part of the structure, and compute the homogenized data based on the results from various load cases.
Given only the homogenized stiffness and computed deformations, there is not enough information to compute stresses. The stress state will depend on the local geometry. In the Section Stiffness node, you have a possibility to define expressions for the relation between section forces and stresses. Such expressions must then be based on your knowledge of the peak stresses in the original, not homogenized structure. As an example, for a perforated structure, the stress concentrations around the holes should be taken into account.