Symmetry regions can be defined in Explicit Dynamics analyses. Symmetry objects should be scoped to faces of flexible bodies defined in the model. All nodes lying on the plane defined by the selected face are constrained to give a symmetrical response of the structure.
Note:
Anti-symmetry, periodicity, and anti-periodicity symmetry regions are not supported in Explicit Dynamics systems.
Symmetry cannot be applied to rigid bodies.
Only the General Symmetry interpretation is used by the solver in 2D Explicit Dynamics analyses.
Symmetry conditions can be interpreted by the solver in two ways:
In general, a symmetry condition will result in degree of freedom constraints being applied to the nodes on the symmetry plane. For volume elements, the translational degree of freedom normal to the symmetry plane will be constrained. For shell and beam elements, the rotational degrees of freedom in the plane of symmetry will be additionally constrained.
For nodes that have multiple symmetry regions assigned to them (for example, along the edge between two adjacent faces), the combined constraints associated with the two symmetry planes will be enforced.
Note:
Symmetry regions defined with different local coordinate systems may not be combined, unless they are orthogonal with the global coordinate system.
General symmetry does not constrain eroded nodes. Thus, if after a group of elements erodes, a "free" eroded node remains, the eroded node will not be constrained by the symmetry condition. This can be resolved in certain situations via the special case of Global symmetry, described in the next section.
If a symmetry object is aligned with the Cartesian planes at x=0, y=0 or z=0, and all nodes in the model are on the positive side of x=0, y=0, or z=0, the symmetry condition is interpreted as a special case termed Global symmetry plane. In addition to general symmetry constraints:
If a symmetry plane is coincident with the YZ plane of the global coordinate system (X=0), and no parts of the geometry lie on the negative side of the plane, then a symmetry plane is activated at X=0. This will prevent any nodes (including eroded nodes) from moving through the plane X=0 during the analysis.
If a symmetry plane is coincident with the ZX plane of the global coordinate system (Y=0), and no parts of the geometry lie on the negative side of the plane, then a symmetry plane is activated at Y=0. This will prevent any nodes (including eroded nodes) from moving through the plane Y=0 during the analysis.
If a symmetry plane is coincident with the XY plane of the global coordinate system (Z=0), and no parts of the geometry lie on the negative side of the plane, then a symmetry plane is activated at Z=0. This will prevent any nodes (including eroded nodes) from moving through the plane Z=0 during the analysis.
Note: Global symmetry planes are only applicable to 3D Explicit Dynamics analyses.