If the model you create is symmetric, you have the option of subdividing the model and working with a symmetric section instead of the entire model. By modeling only a portion of the part, you can greatly reduce the number of elements in your model, thus saving significant analysis time and system resources. Depending on the model, you can also save yourself the overhead spent defining repeated versions of a load or constraint or selecting multiple surfaces, edges, or points during load or constraint definition.

For example, if you were trying to determine how a disk reacted to a uniform load applied to the top surface, you might decide that you only wanted to analyze a portion of the disk. Because the part and modeling conditions are symmetrical, the analysis results for a section of the disk would provide information accurate enough to give you an idea of how the model will behave as a whole.

For a model to be symmetric for Mechanica's purposes, it must exhibit the following characteristics:

The geometry must be symmetric.

The loads, constraints, and idealizations must be symmetric.

There are two types of symmetry you can model in Mechanica—mirror symmetry and cyclic symmetry. Mirror symmetry relies on the principle that one segment of a model is the mirror image of other segments. An example of this type of model would be a rectangular plate with a hole at its center. In native mode you can use the mirror symmetry constraint to take advantage of your model's symmetry. To use mirror symmetry in FEM mode you must apply a displacement constraint to fix translation normal to the plane of symmetry and fix rotations in opposition to the plane of symmetry.

Cyclic symmetry relies on the principle that a segment of the geometry is repeated in a cyclic manner throughout the model, but the segment is not a mirror image, either in its geometry or its load scheme. An example of this type of geometry would be a fan blade or turbine. You can only use cyclic symmetry in native mode. FEM mode does not support this type of modeling.

The methods you use to develop these two types of symmetry differ, as does the application of constraints and certain loads. Both types of symmetry can prove efficient for a 3D solid or shell model. The choice of which symmetry type you use depends on the model and the problem you wish to solve.

Note that, in some situations, you can use 2D axisymmetric modeling in place of symmetry. While not strictly a form of symmetry, 2D axisymmetric modeling provides an extremely efficient alternative to treating your model as a symmetric solid. This form of modeling relies on the principle that a 2D slice of your solid model, if rotated around an axis, can accurately depict the whole of your model's geometry, loads, and constraints. For an example of this type of model, see Setting up a Solid Model for a 2D Analysis on an Internal Surface.

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