The Spring Foundation and Thin Elastic Layer nodes supply elastic and damping boundary conditions for domains, boundaries, edges, and points.

The features are completely analogous, with the difference that a Spring Foundation connects the structural part on which it is acting to a fixed “ground,” while the Thin Elastic Layer acts between two parts, either on an interior boundary or on a pair.

A Spring Foundation is most commonly used for simulating boundary conditions with a certain flexibility, such as the soil surrounding a construction. Another important use is for stabilizing parts that would otherwise have a rigid-body singularity. This is a common problem in contact modeling before an assembly has actually settled. In this case a Spring Foundation acting on the entire domain is useful because it avoids the introduction of local forces.

A Thin Elastic Layer used as a pair condition can simulate thin layers with material properties that differ significantly from the surrounding domains. Common applications are gaskets and adhesives.

When a Thin Elastic Layer is applied on an interior boundary, it usually models a local flexibility, such as a fracture zone in a geological model.

The elastic properties can be defined either by a spring stiffness or by a force as function of displacement. The force as a function of displacement can be more convenient for nonlinear springs. Each spring node has three displacement variables defined, which can be used to describe the deformation dependency. These variables are named <interface_name>.uspring1_<tag>, <interface_name>.uspring2_<tag>, and <interface_name>.uspring3_<tag> for the three directions given by the local coordinate system. In the variable names, <tag> represents the tag of the feature defining the variable. The tag could, for example, be spf1 or tel1 for a Spring Foundation or a Thin Elastic Layer, respectively. These variables measure the relative extension of the spring after subtraction of any predeformation.

where fs is the elastic spring force, and η is the loss factor.

It is also possible to add viscous damping to the Spring Foundation and Thin Elastic Layer features. The viscous damping adds a force proportional to the velocity (or in the case of Thin Elastic Layer: the relative velocity between the two boundaries). The viscosity constant of the feature can be made dependent on the velocity by using the variables named <interface_name>.vdamper1_<tag>, <interface_name>.vdamper2_<tag>, and <interface_name>.vdamper3_<tag>, which contain the velocities in the three local directions.