A ply is an area of LM_Material which is stored and manipulated as a single entity. A ply represents a piece of reinforcing fabric which is cut from sheet stock and placed on a mould during the manufacturing process. A ply is fully characterized by the LM_Material it is made of, the area it covers, and the way in which it is applied to the surface. The latter is particularly important for non-developable surfaces where there are many different ways of placing the fabric on a surface.

Plies allow easy manipulation of complex data when you assemble and/or modify the layers to form the complete layup. The physical representation of a ply is a piece of fabric.

This defines the starting point of the drape simulation process. It is analogous to the point at which a ply is first attached to a mould surface during manufacture. As the distortion usually increases away from the starting point, it is best to begin draping near the center of a region to minimize shear distortion. If the start point coordinates do not lie on the selected surface, the coordinates are projected onto the surface along the application direction vector. The start point must lie on the selected area.

The application direction defines the side of the surface area on which a ply is subsequently added when the final layup is defined. The “Top” of the surface covered by the ply is defined as the side on which the ply is originally applied when created. When defining a layup, the ply can be added to the “Top” or “Bottom” side of the mesh. It follows that the “Top” side is the same side as the application direction used to define the ply, whereas the “Bottom” side is the side opposite the application direction.

The concept of side is very important as composite structures are often built using molds or forms, limiting the side of application to a single direction. The plies of reinforcing fabric can be added to either the outside of a male mould or the inside of a female mould. When defining plies and a layup, it is useful to consider the manufacturing process.

The application direction is also used to project the start point and reference direction onto the surface.

The reference direction is used to specify the initial direction of the fabric. The input vector is projected onto the surface along the application direction to define the principal warp axis of the material at the start point. Note that the direction of the material will usually change away from the starting point if the surface is curved.

The principal warp axis of the material on the surface can be rotated from the reference direction by inputting a non-zero reference angle. This rotation is counterclockwise when viewed along the application direction.

Note that the application direction is used to project the start point and reference direction vector onto the selected surface. This means that the same start point and reference direction vector results in different values when projected onto the surface along different application directions, as shown in Figure 3‑4. It follows that the start point and reference direction should be defined as close to the surface as practical, while the application direction should be defined as perpendicular to the surface as possible.

This example shows the view as it would appear in the viewport in addition to the input that would appear on the Create LM_Ply Add form.

The principal warp and weft axes are the paths the warp and weft fibers follow along the surface away from the start point. By defining the paths of the principal axes, it is possible to constrain the ply uniquely in the region bounded by the principal axes.

The principal axes can be defined in different ways:

The extension type controls the draping process if no axis type is defined, or the draping extends beyond the region uniquely defined by the principal axes. In this case, the material cells on each edge are kinematically unconstrained, and so some extension type must be specified to control the extension of the fabric.

The extension mechanism can be defined in different ways:

Optimised Energy and Optimised Maxshear modes use a "circular" extension strategy to model the hand layup of woven fabrics. The Tape modes use a "strip" extension strategy to simulate the application of tape in a way which accurately reflects the real-world application of butted tapes of material onto a surface by manual means. This is also likely to reflect butted tape laying via automated means, subject to the fact that the detailed characteristics of the tape laying machine are not known by AFM.

In the "circular" extension modes (Energy and Maxshear), free edges in the warp and weft directions are extended alternately so that the ply extends uniformly in all directions away from the Seed Point. This models the operator progressively smoothing the fabric onto the mould away from the Seed Point.

In the "strip" extension mode (Tape), the first tape is extended from the Seed Point in the warp direction all the way to the ply boundary in both the positive and negative warp directions. Only once this first tape is laid are additional tapes abutted against the free weft edges of the preceding tapes. For each new tape, the material strain is set to zero at the point of first application, which is defined as the point in the most negative warp direction. This reflects the fact that the material is unsheared at the point of first application on a surface. The rate of increase of shear along the tape depends on the width of the tape and the Gaussian curvature of the surface.