Ply entities define a FEA ply which is the FEA correlation to a physical ply.

Physical plies manufacture laminates which make up composite structures. A physical ply has attributes of material, shape (area), thickness, and fiber orientation; where its shape is any complex flat pattern that can be cut from a roll of material. Similarly, a FEA ply is composed of the same data attributes as a physical ply (material, shape/area, thickness, and fiber orientation). The shape of a FEA ply can either be defined by closed lines or approximated from the elements which most closely represent its actual complex shape. In the case where plies are defined on lines, perform a realization to convert this information into a definition by elements. Ply data defined on lines is imported from Catia Composites Parts Design (CPD) data.
Plies defined by elements
The shape (area) of an FEA ply is defined by selecting elements which most closely represent the complex shape of a physical ply. In Figure 1, an elliptical physical ply shape is defined by the brown line. The corresponding FEA ply shape is defined by the gray shaded elements of the associated FEA mesh. Typically, if an element's centroid exists within the bounds of the physical ply shape, that element is considered part of the FEA ply shape.

Figure 1.
Plies defined by lines
Plies can be defined by selecting lines which build a closed area. If CPD data is imported from Catia files, plies are defined on lines.
Once a mesh is available, plies defined by lines can be converted into plies defined by elements by performing a realization. Realization/conversion methods include:
Project normal to target mesh
If the element centroid projected along its normal lies within the geometrical ply definition, it is associated with this ply.
Normal by ply contour
Projection along a normal on a surface derived from the lines of a ply contour.
Specified direction
Manually define a projection direction.
The ply thickness is typically defined as the final cured thickness of a single ply of material. In addition, the ply can be made of any material: isotropic, orthotropic, anisotropic, or any other material law.

Figure 2.
The fiber orientation of a ply defines the direction fibers lay within that ply. The ply fiber orientation is typically an integer value between -90 and 90. The fiber orientation of a ply is always defined relative to each elements material direction using right hand rule around the elements normal, or thru-thickness direction, to define positive angles. Even though a ply's fiber orientation is a constant integer, element material directions can vary from element to element, and this allows varying fiber directions within a ply to be modeled. Element material directions are defined differently from solver interface to solver interface.

Figure 3.

Once all of the plies which make up a composite structure are defined, just as in the actual hand-layup manufacturing process, plies are stacked in a specific given order within the laminate entity to define a laminate of the structure. It is possible to stack plies, whether they are defined based on geometry or elements.

Turn the display of plies on/off or change the way plies appear in the modeling window with the Ply layers display setting, which can be accessed from the Mesh Display settings on the View Controls toolbar.