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Fibersim 101: (4 of 14) Plies

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Welcome to Fibersim 101, a step-by-step series of articles meant to familiarize new users with the key concepts of the software. We recommend starting with Part 1 and downloading the “” file at the bottom of that page in order to follow along with these exercises. If you need to go back to Fibersim 101: (1 of 14) Model Setup click here


A ply represents a single piece of fabric in a laminate. A building block of a composite part, it comprises a great deal of 2D as well as 3D information. Together, plies and cores represent the actual materials used to manufacture a composite part. Plies must first be fabricated, generally cut out from a sheet of composite material. This cut out is known as a flat pattern. The pattern is then laid up on a 3D tool.

Fibersim stores all 2D, 3D, and non-geometric information describing a ply, including boundaries, fiber orientation, material type, stackup order, and flat pattern. Since plies exist in both 2D and 3D, there is the potential for several pieces of CAD geometry to be associated to a ply. In 3D, plies require an origin and a boundary. Plies may also have holes and markers associated to them in 3D.

Fibersim uses two different ply boundaries: the Net boundary is the as-designed final part boundary, and the extended boundary defines any excess trim necessary for manufacturing.

This integration of ply boundaries allows Fibersim to be used for both the design and manufacture of a composite part. If an Extended boundary is defined for the parent Laminate, Fibersim can auto-extend the ply’s net boundary to generate manufacturing trim for the ply. Once the 3D ply definition has been defined and producibility/flat pattern simulations have run, the result is a 2D flat pattern.

Plies are also defined with Sequence and Step values that are used to specify each ply’s order in the laminate stackup. The Sequence value for a child ply or core object is often a single letter, and it should correspond to the Sequence of the parent laminate. The child’s Step value should be a number larger than its parent to denote that it comes “after” the parent. Further, while the actual numerical values for the plies’ Steps do not matter, their relative value with respect to one another determines the layup order. So, for example, a ply with Step 1 would be laid up before a ply with Step 2, but if a new ply is created with a Step of .7, this new ply would come before the ply with Step 1 in the stackup.


Composite fabrics of a wide variety of architectures can be accurately defined in Fibersim These fabrics are generally described according to their weave type and the number of yarns per inch, first in the warp direction (parallel to the length of the fabric coming off the roll), then in the perpendicular weft direction. The two most common types of composite fabrics used in Fibersim are unidirectional and woven. Unidirectional fabrics comprise fibers lying parallel in the warp direction that are loosely held in position by tie yarns in the weft direction. Uni fabrics tend to be less tolerant of high-curvature tooling, so they are often used to cover large, relatively flat areas in spliced layers. Woven fabrics typically incorporate equivalent tows of warp and weft fibers woven around one other. Woven materials tend to be much more drapable than uni, and can thus be used on a wider variety of tooling geometries.

When defining new objects in Fibersim, the material information is pulled from an XML-based database located by default in the program directory (MaterialsDB.xml). This library of materials can be fully customized, and it allows a large number of parameters to be defined for each material. Among these parameters is material width, which defines the material’s bolt/roll width (and thus, a ply’s maximum width in the weft direction). Also critical for processing in Fibersim are a material’s Warning and Limit Angles. These parameters define how drapable a material is. When running the producibility simulation (covered in a later section), the Warning Angle determines where the simulated fibers transition from blue (low deformation) to yellow (higher, but acceptable, deformation). The Limit Angle defines where the fibers turn red (out-of-tolerance fiber deformation).

In addition to being stored in the MaterialsDB.xml file, any materials used in a CAD model actually get stored in that model. This allows the model to be transferred to other users without manually having to synchronize databases.

1. Please open the example model and start Fibersim. In the Applications tree on the left select Ply. Right click in the Ply list view on the right-hand screen, and select Create New.Plies1.jpg

2. The boundary and origin default to the laminate boundary and rosette origin, but our design intent is not to cover the entire part with this ply. Additional geometry needs to be selected. Click on the Link Geometry button next to Origin.

Plies2.jpg3. Select the point shown for the origin for this ply.


Plies3.jpg4. Click OK. Back in the “create Plies” form, click the Link Geometry button next to Boundary and select the curve highlighted below.


Plies4.jpg5. Change the Step value from the default of 2 to 10. Since the parent laminate, “DUCT”, has a Step value of 1 (and both objects have a Sequence of A), this arrangement appropriately denotes that P001 comes “after” its parent, DUCT.

6. To assign a material to the ply, please click the Link with Database Link Dialog next to Material.Plies6.jpg7. This will open up the link material dialog box; PPG-PL-3K will be automatically selected as it is set as the laminate default material.

Plies7.jpg8. Next the orientation of the fibers with respect to the rosette needs to be defined by the Specified Orientation value. This Ply will have an orientation of 0 degrees, Fibersim’s default value, so no change is necessary. 

PLies8.jpg9. The “create Plies” form should now look like the one shown. 

Plies9.jpg10. Click OK to save the ply. Right-click in the ply list view and select Create New again to create another ply.

11. For this second ply, the origin and boundaries that need to be selected are shown in the following images. Please select PPG-PL-3K as the material, and assign the ply a Specified Orientation of 45 degrees and a Step of 20.




Plies11.1.jpg12. The completed “create Plies” form should appear as shown below.

Plies12.jpg13. Click OK to save the ply.

14. Create the final Ply by selecting the Origin and Boundary shown in the following images. The material will again be PPG-PL-3K, the Specified Orientation will be 0 degrees, and the Step value will be 30.





Plies14.1.jpg15. The “create Plies” form for P003 should appear as shown. 


Producibility simulation

Now that your plies have been created, producibility simulations can be run on them.  As the producibility simulation drapes material over a 3D surface with any sort of compound curvature, it will require the material to deform to maintain contact with the surface. As composite fabrics incorporate fibers that do not stretch, this deformation applies only to the angles at which the fibers meet. For example, the typically orthogonal warp and weft fibers of a woven fabric will “trellis”, distorting the angle between the warp and weft fibers in a given region. If the deformation exceeds the user-specified Limit angle of the material, it is considered to be either wrinkling or bridging out of the material’s tolerance, and Fibersim displays all fiber paths in this region in the color red:

  • WrinklingThis wrinkling is common in apparel (for example, it occurs behind the knee in a pair of pants when the knee is bent). It is caused by an excess of material in a given region of the surface. The material, having draped all available surfaces, deforms off the surface in a plane that is normal to the surface. This phenomenon is sometimes referred to as “out-of-plane” deformation.
  • Bridging – Bridging or tearing is caused by a lack of material in a given region of the surface. The material is stretched to the point where the angles between the fibers lock, and it becomes no longer physically able to drape over the entire surface. The material can either pull away from the tool surface and “bridge” (or span) across concave regions, or the material can potentially tear in convex regions. The material deforms in a plane that is parallel to the surface. This is sometimes referred to as “in-plane” deformation.

To determine the type of deformation occurring in the simulation, we look at the simulated fiber “cells”. These are the individual parallelogram-shaped areas bounded by two warp and two weft fibers in the simulated mesh. One must observe the direction the red fiber cells are deforming relative to the ply origin (see below). If red fiber cells are elongating along a line drawn through the ply origin, the material is wrinkling. If red fiber cells are contracting along a line drawn through the ply origin, the material is bridging.

With that knowledge, we can now run the Producibility simulation on our Plies to see how each one will deform when laid on the 3D Tool Surface.

1. First select P001 in the Ply list and click the Net Producibility button in the tool bar as shown below:

Plies16.jpg2. Click OK when the message “Material Width Exceeded” appears, and observe the model in NX to see the results of the simulation.


Plies17.jpg3. Run the Net Producibility simulation on the other two Plies, P002 and P003. The results are shown below. Again, dismiss any “Material Width Exceeded” warnings that appear.





The wrinkling and bridging conditions resulting from the ply geometries we created will be addressed later in the section on darting.

Fiber Spacing Factor

When running Net Producibility to determine material deformation, it may seem like the simulated fiber cells are a bit large and don’t accurately conform to finer areas of curvature on the tool surface. If you look at the simulation results for P002, the cells seem large compared to the results of the other two simulations. Fibersim has the ability to change the cell size for the producibility simulation of a ply. This is called the Fiber Spacing Factor.

1. Please double click on P002 to modify it. Then click the Simulation Options button on the Standard tab.


This will bring up the Simulation Options window. The Fiber Spacing Factor is highlighted in red below: 

Plies21.jpg2. Change the Fiber Spacing Factor from 1 to .5 and then run Net Producibility again. The new factor will alter the results to look like this:


The mesh of cells is clearly much finer. The smaller the Fiber Spacing Factor used, the finer the mesh of cells, and the more accurate the simulation results will be. The finer mesh will take longer to calculate, however, and may only result in a trivial change in the resulting flat pattern shape (which will be discussed later). Assigning an appropriate Fiber Spacing Factor depends on the geometry of the tool surface and the ply’s boundary and fiber orientation. A Fiber Spacing Factor of 1 (the default value) is fine for most composite parts with gentle curvature, but if more complex tooling geometry is being laid up, trial-and-error may be used to determine the most appropriate value to use. With experience (seeing how Fiber Spacing Factor affects flat pattern shape over a variety of geometries), this process becomes much more intuitive.

Propagation Method

There are two different modes by which simulated fibers can propagate in Fibersim: Standard and Geodesic. Standard simulation starts at Origin and works progressively outward in a radial fashion, thus simulating a hand layup process, where the material is spread uniformly outward from the point at which the ply first contacts the tool surface (denoted by the Origin point).

Geodesic simulation begins at the Origin point and smooths material along a straight geodesic path relative to the fiber direction. This method is typically used to simulate when the ply is smoothed by hand along a straight line (such as along the spine of an aircraft stringer or C-channel) first, then smoothed uniformly outward from that path.

Simulation Surface

Holes selected for plies will be discussed later this chapter, but one important thing to understand about Fibersim’s producibility simulation is that even with Holes defined for a ply, the simulated fibers will appear to propagate through the holes as though they were not there. Such a contiguous, uninterrupted simulation is necessary because there would otherwise be no way for the simulated fibers to propagate from the Origin point, around a Hole, then somehow line back up on the opposite side of the Hole. The Hole boundary will highlight when selecting the Ply, but the full simulation (through the Hole) is necessary when computing the shape of the flat pattern.

This behavior may not be desirable if the ply’s Hole is meant to cut out a region of the tool surface that has high curvature: a boss or other protrusion, for example. The Hole is meant to eliminate this feature, so that the ply’s material drapes smoothly around it like a picture frame. The problem here is that this ply’s producibility simulation will take into account the boss, resulting in a highly-distorted flat pattern which would not be accurate to the real-world tooling.

The Simulation Surface option is used to avoid such situations. This Simulation Option allows the user to select an alternate surface on which to run the ply’s producibility simulation. The Simulation Surface is typically created as a copy of the parent laminate surface, but with the boss or other extreme-curvature geometry smoothed over. This way, when the producibility simulation is run, the simulated fibers will propagate through the Hole without accumulating additional deformation caused by the feature.


As mentioned above in the Simulation Surface section, Holes can be defined in a ply. The hole will highlight with a dashed line, and it will flatten much like the ply boundary and appear in the ply’s flat pattern. Again, as mentioned previously in the Simulation Surface section, Fibersim will still calculate the producibility simulation over the area of the hole.

1. To create a Hole, double-click on ply, “P001”, in the Ply list view to modify it.
Plies23.jpg2. Click on the Link Geometry button next to the Holes field on the Standard tab. 

Plies24.jpg3. Then select the geometry shown below:


Plies25.jpg4. Click OK to return to the “modify Plies” form and the NX curves comprising the Hole boundary will appear in the Hole field.



Again, since the producibility simulation runs through Holes, their presence does not affect the simulation. When the flat pattern for this ply is created, however, the hole will be present.


Similar to Holes, Markers are miscellaneous 3D curves or points that are mapped onto the flat pattern. Unlike the Ply or Hole boundaries, Markers present in the flat pattern will not be cut; instead, they are typically marked out with a pen. The geometry selected for a Marker thus does not need to be a closed contour, or even a contiguous set of curves. 


As mentioned in the previous section on Producibility Simulation, darting techniques attempt to eliminate wrinkling or bridging/tearing conditions in a ply. As illustrated by the simulations for our three plies, fiber deformation is cumulative. When the fiber deformation exceeds the material’s Limit angle, the red out-of-tolerance region generally propagates outward in the direction of the layup, even over simple curvature in the layup surface. Darting attempts to cut the initial fibers that begin deforming out-of-tolerance, and prevent that deformation from propagating through the ply.

There are two types of darting techniques, one that produces a slit dart and another that produces a “V”-shaped dart. Slit darts are used to relieve wrinkling while

“V”-shaped darts are used to relieve bridging/tearing.



Slit darts are used to eliminate wrinkling in a ply without splicing it. The first step in creating a valid slit dart is to determine which initial fibers must be cut in order to prevent subsequent wrinkling. These fibers are determined from the producibility simulation: 

First, the area of red or wrinkled fibers is identified. Next, the two fibers that border the red fiber region must be located. These are the fibers that point back toward the Ply origin. The location where these two curves intersect indicates the location of the fibers that must be cut. Often, these fibers are not red fibers, and can be some distance from the area of red or wrinkled fibers.

A slit dart is composed of two roughly parallel curves on the layup surface, which are at least 0.030 inches apart at every location, and a third curve to adjoin them. The free ends of the two parallel curves must terminate on or be incorporated into an existing ply boundary.



The next step is to determine the path of the dart over the layup surface. The optimal path starts at the initial fibers that must be cut, and passes through the center of the red wrinkled region, to the edge of the Ply boundary. Sometimes the optimal dart path may pass through regions of the part where splicing or darting is not allowed. A slit dart will still likely relieve the wrinkling condition even if its path is not ideal, however: as a general rule, a valid slit dart path is any that cuts the initial red fibers and continues to the edge of the ply boundary. This image shows the optimal dart path.

Please open the example model and create a slit dart.

1. Select Ply in the application tree, double-click P001 to modify it, and go to the Net Geometry tab of the “modify Plies” form.

2. Run Net Producibility and observe the simulated fiber mesh in NX. The goal is to analyze how the cells are deforming in order to determine whether a slit dart or a v-shaped dart is appropriate.

The origin is circled in red, and from there, we can see that the red cells are lengthening along a line through the origin. This is an indication of wrinkling. A slit dart is therefore required in this case.

3. In Fibersim, click the link dialog button next to the Darts field and select Slit Darts. 

Plies29.jpg4. This brings up the Slit Darts dialog box. Right-click and select Create New to bring up the “create Slit Dart” form:Plies30.jpg5. The slit dart we are trying to create will be based on Points, so leave that radio button selected under “Slit Curve Definition”. Click the Link Geometry button next to Base Curve Points (as seen above).

6. A slit dart is created by picking two or more points on the tool surface. The first point will be the “top” of the dart (the point furthest from the edge of the ply). The last point should be within the ply boundary, but close to the edge, as this point will represent the “bottom” of the dart (Fibersim will automatically trim the dart curves to the laminate net boundary or any “Trim Curves” you have selected).

To determine the most appropriate placement for the points of a slit dart, follow the yellow fiber paths outlining the red area (these paths are highlighted in red dashed lines in the image below). The node where these two yellow fibers meet is the ideal place to put the top point. Since this is a simple example, only two points are necessary. The second point can be placed close to the net boundary, through the longest parts of the deformed cells. The placement of our points is highlighted in red circles below:


Plies31.jpg7. Click OK once the points are placed correctly, and the “create Slit Dart” form will reappear. Finally, create the dart geometry by clicking Generate Dart Feature:

Plies32.jpg8. Once the Slit Dart has been generated, click OK twice to return to the “modify Plies” form. The dart should now highlight as part of the ply’s net boundary. Net Producibility will be out-of-date due to the change, and will need to be re-run. Click the Net Producibility button again to see how the dart effects the simulation of the ply.Plies33.jpg



As shown above, the Slit Dart has greatly reduced the deformation of the fibers and has eliminated the red cells in the affected area. Use the above steps on the opposite side of the model to eliminate the other red cells. The result should look like the image below:Plies34.jpg

V-Shaped Darts

V-shaped darts are used when a stretching ply’s fabric is pulling away from a concave tool surface, and thus “bridges” or spans over the surface features. In regions where the tool is convex, the material won’t pull away from the surface, but it can run the risk of tearing due to stretching. To avoid these situations, the ply must be cut in such a way that removes the stretching region of the fabric entirely.

V-shaped darts define two curves that start at a single point (the “top” of the dart), then spread out and ultimately terminate at different locations along the ply boundary. The region of the ply that is deforming out-of-tolerance for that material (the red fiber cells) is typically bounded by these two v-dart curves.plies35.png

Although the same slit darting techniques described in the previous section to relieve wrinkling can be used in a region of bridging/tearing, they will produce an invalid flat pattern where the flaps of material will actually overlap. The nature of bridging causes a lack of material in a given region of the surface. Placing a slit dart in a region of bridging or tearing allows more material to enter that region. When the 3D draped material is relaxed and unfolded into a 2D flat pattern, this extra material forms an overlap that would be impossible to cut. An example of a slit dart placed in a region of bridging is shown below, followed by the resultant 2D flat pattern with overlap. These results indicate that slit-darting will not work in regions of bridging or tearing.Plies36.jpg



To relieve the material starvation caused by bridging or tearing and still maintain the same coverage of the original ply, two plies must ultimately be created: the original ply with a V-shaped dart and a patch ply to fill in the V-shaped gap on the tool. An important thing to note is that when working with V-darts, there may not be an “ideal” location for the two dart curves. To relieve the bridging condition adequately, a large enough area needs to be cut out of the ply, but often splicing requirements will dictate a minimum area for the patch ply or drive its location if there are no-splice regions. If there are no such requirements, making a V-dart as small as possible is highly dependent on the tool geometry in the region, and thus, trial-and-error may be required to optimize the flat pattern shape.

1. In the Application tree select Ply and double-click P002 to modify it. Proceed to the Net Geometry tab. Update the ply’s net producibility if it is out-of-date. Observe the simulation results in the NX graphics window:


Plies38.jpg2. The red circle denotes the origin of the Ply. The red cells at the top are shortening along a line through the origin, denoting that there is unacceptable stretching of the fibers taking place. The correct type of dart to use is therefore a V-shaped dart. Return to Fibersim, select the drop-down Link dialog next to Darts, and then select V-Shape Dart.Plies39.jpg

3. In the Link dialog, right-click and select Create New as shown below: 


Plies40.jpg4. A V-Shape Dart is created by specifying two curves. The first curve consists of at least two points, and the second must consist of at least one. To ensure that the two curves meet, Fibersim will use the first point of the first curve as the first point in the second curve (and to denote the “top” of the dart), so only the bottom point of the second curve needs to be selected. As with slit darts, of course, more points can be selected for either V-dart curve if a non-linear shape is required. Under First Curve Definition, click the Link Geometry button:


Plies41.jpg5. To know where to place a V-dart’s curves, you can use similar techniques as a slit dart. Place the first point of curve 1 at the intersection of the yellow fibers that bound the red cells. The second point should be along the same fiber near the net boundary as shown below.


Plies42.jpg6. Click OK, and click the Link Geometry button for the second curve. As this is a simple case where the V-dart curves are linear, the second curve only needs to consist of one point. This point should be placed along the other yellow fiber near the net boundary.


Plies43.jpg7. Click OK. Now the dart geometry needs to be created in NX by clicking on the Generate Dart Feature at the bottom of the “create V-Shape Dart” form.


8. Once the dart has been generated, click OK twice to return to the “modify Plies” form and run producibility again to see the effect of the dart on the ply’s draping. The simulation results in the mesh below:



And additional patch ply can be created using the V-shaped dart geometry as its boundary (and an origin point on the upper flange of the part, in the gap created by the V-dart). Doing so will result in a butt-splice, but the dart curves can obviously be offset if an overlap splice is required. 

Material Width

When running the producibility simulation, a message often appeared saying that “The material width is exceeded”. This means that the bolt or roll width of the actual material used in the ply is not great enough to cover the entire ply area. The ply will therefore need to be spliced into two or more separate plies.

1. Open the model, select Ply in the left-hand Applications tree, double-click P003 to modify it, and navigate to the Net Geometry tab. Note the section labeled, “Material Width Lines”.



2. As long as the ply’s producibility simulation is up-to-date, pressing the Preview button will highlight where the edge of the material would end, as shown below:Plies47.jpg3. The “Material Width Offset” field can be used to vary the position of the material width lines. Leave this value at 0 and press Generate to create a curve exactly where the material width line originally highlighted.

4. Since our goal is a ply that doesn’t violate the material’s width, click the Link Geometry button next to Boundary and select the material width line:


Plies48.jpg5. Finally, run the net producibility simulation and the material width warning will no longer appear. The simulation should look like this:



Net/Extended Analysis

In addition to these tools for accurately defining plies in Fibersim, the software also provides analytical feedback. Geometric properties for any given ply, such as Area, Perimeter, and MOI Tensor are calculated based on the ply’s producibility results. From these values, Weight and Cost are determined, given the specified material’s Areal Weight and Cost Per Weight parameters (as defined in the MaterialsDB.xml file). These results can be seen on the Analysis tab of the “create/modify Plies” form as shown below:



Flat Pattern Generation

One of the most important downstream deliverables in Fibersim’s ply-based design workflow is the flat pattern. Although accurate flat patterns may be the ultimate goal in a ply-based design, as alluded to before, generating flat patterns can be a very useful intermediate step in determining appropriate dart or splicing geometry. 

In order to generate a flat pattern, the producibility simulation must be up-to-date and free of any errors.

Please open the example model and start Fibersim. Select Ply from the Applications tree and highlight P002 in the Ply list on the right. 


Plies51.jpg2. If the net producibility is not up-to-date (if a simulation mesh does not appear in the NX graphics window), run the simulation now. Once complete, click on the Net Flat Pattern button on the tool bar as shown below:


Plies52.jpg3. Please examine the flat pattern created in the NX graphics window:



In most cases, of course, the extended flat pattern is actually required for manufacture. If so, the ply’s extended producibility simulation needs to be run. Then, the extended flat pattern can be generated by clicking the appropriate button on the toolbar.


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