Here, the domain boundary conditions for the strain (and stress respectively) may be set to Periodic, Symmetric, or Mixed. Note that displacements do not need to hold these boundary conditions.

In general, periodic boundary conditions should be used for periodic structures, whereas otherwise symmetric or mixed boundary conditions may be applied.

Mixed boundary conditions mean that symmetric boundary conditions apply in the load direction, whereas periodic boundary conditions apply in the tangential directions. Mixed boundary conditions can only be used for tensile, or compression loads along the coordinate directions. In the figure below, periodic and symmetric boundary conditions are illustrated for a non-periodic structure.

The structure below was created as an example and does not have a real counterpart.

When periodic boundary conditions are applied to non-periodic structures, it might lead to stress peaks at the boundaries as illustrated below for a uniaxial load.

However, simulations with periodic boundary conditions are much faster and need less memory than simulations with symmetric boundary conditions.

In many cases, for example for composite structures with low fiber percentage, the results for periodic boundary conditions are comparable with the results for symmetric boundary conditions even if the analyzed structure is not periodic. Therefore, it is recommended to work with periodic boundary conditions until the best simulation settings are found and only use symmetric boundary conditions in the final simulation.