About Composites Fiber Modeling

To make full use of Composites Fiber Modeling capabilities, additional data can be provided to obtain a more accurate simulation result.

The additional data include:

• Definition of a seed curve to constrain the fiber simulation along a curve.
• Definition of one or more orders of drape regions defined by contours on the surface of the ply.
• Selection of advanced propagation types to reflect the behavior of materials according to the manufacturing processes.
• Preview of the flat pattern to provide excellent feedback on the fiber simulation.

With the enhanced Producibility dialog box, Composites Fiber Modeling allows you to define specific parameters and to store them in Producibility params.x under each ply:

• Simulation: Advanced parameters affect the actual fiber simulation.
• Analysis: Advanced parameters affect the display of the fiber simulation results. They are used for additional display options provided by the solvers, which are added to the Deformation list.

The seed point is defined in the same way as for the standard fiber simulation.

Composites Fiber Modeling supports seed points located on the boundary of the ply. This reflects the normal practice where the application of the ply often begins along the edge of the region to cover.

In general, the material is unsheared at the seed point and shearing increases away from that point. The amount of shear in a ply made of a particular material increases with distance from the ply, and the degree of Gaussian curvature of the surface (this is a reflection of the double-curved nature of the surface, when applicable). To minimize shear, the seed point is located:

• To minimize the distance to any part of the ply boundary - so the middle would reduce shear, at the cost of being impractical.
• To minimize the Gaussian curvature near the start point. Begin draping on a region of near-zero Gaussian curvature if this is possible.

Example: Consider half of a pressure vessel that consists of a spherical cap with a cylindrical body. If the seed point is on the cylindrical body, which has zero Gaussian curvature and is developable, limited shearing only occurs over the spherical section.

By contrast, if the seed point is on the spherical section, shear builds up rapidly and the surface cannot even be covered.

A seed curve forces the warp, weft, or bias directions of the fabric along a curve.

The curve must pass through the seed point, and extend to the ply boundaries. In the following example, the bias direction of the fabric has been forced along a curve through the middle of the spar. This results in a flat pattern with parallel sides, which is a tape.

The choice of whether the warp, weft, or bias directions are forced along the curve depends on the relative alignment of the nominal warp and weft directions at the seed point, and the seed curve at this point.

The woven construction of the fabric is defined by physical parameters that reflect the structure of the material.

In the image below:

• L1: Weft Length
• L2: Warp Length
• A1: Warp/Weft Angle
• B: Bias, B2: Bias2
• A2: Warp/Bias Angle
• A3: Warp/Bias2 Angle
• W: Warp

In Composites Fiber Modeling, the Warp/Weft Angle is defined under Simulation: Advanced parameters, and information is found under Material parameters, whereas the Warp and Weft length ratio is defined under the Mesh parameters. For the purposes of defining the unit cell, it is the ratio of warp and weft lengths that is important. So, if warp and weft lengths are the same, the bias directions are at +/- 45 degrees.

The axis forced along the seed curve is the closest to the seed curve at the seed point. For example, if the Warp/Weft Angle is 90 degrees and the warp and weft lengths are identical, and if the seed curve lies 30 degrees from the nominal warp direction, the bias direction is closest to the seed curve and this is the direction forced along the seed curve.

This snapping to the nearest direction means that the actual and nominal warp directions at the seed point can be slightly different - 15 degrees in the case above. This must be accounted for when using the inspection tool.

The seed point must be located on the seed curve. In practice, a tolerance of 1% of the square root of the ply area is allowed to permit small separations between seed point and seed curve resulting from the thickness update capability, for example.

You can select the way the warp angle is calculated with respect to the guide curve.

Snap to Axes

The initial warp direction at the seed point is calculated with respect to the guide curve. The closest of the material warp, weft, or bias directions are created on the guide curve.

This results in a robust solution but means that the actual warp direction is not necessarily the same as the theoretical direction.

The image below shows the bias points of an initial producibility mesh being snapped to the guide curve, resulting in a warp direction differing from the theoretical direction at the seed point.

Constant Angle

The initial warp direction at the seed point is calculated with respect to the guide curve, and the warp angle is kept constant at this value along the whole guide curve.

If the theoretical warp/weft or bias directions are within 0.1 degrees of the guide curve at the seed point, that axis is snapped onto the guide curve to promote robustness of the solution.

The image below shows the initial producibility mesh at an angle of approximately 30 degrees to the guide curve, with the actual warp direction matching the theoretical direction at the seed point.

About Curve

A producibility mesh of one cell width is draped along the guide curve. As this strip is narrow, little shearing of the material is introduced.

This option models unsheared material being forced along the guide curve, followed by subsequent lateral laydown to the edge of the ply.

Note: This constraint may lead to the need to insert a split into the material in the same way as for an Order of Drape simulation with a concave edge.

The order of drape capability allows you to model the exact sequence used to apply fabric to surfaces on the shop floor.

The order of drape regions define smaller surfaces that are covered in one sequence, and which provide a stable initial condition for subsequent application of material.

In the example below, three order of drape regions are defined:

• The first order of drape region marked in red (1) must contain the seed point (X), otherwise it is ignored.
• At the end of the first sequence, the area defined by the second order of drape region marked in green (2) is covered by fabric using the first order of drape as a starting point.
• The third order of drape region (3) is similarly covered next.
• Finally, all the remaining area of the ply is covered.
  
  

Do not define too many order of drape regions on a single ply. Usually, between one and two regions are sufficient.

Try to define order of drape regions on relatively flat regions of the surface: The position of the seed point is not significant, and this lack of sensitivity follows through the simulation, resulting in a robust final result, that is easily manufacturable.

In general, it is best if the summed boundaries of all the order of drape regions are straight or convex. If they are concave with respect to a warp or weft fiber, the fiber in the fabric might be over-constrained as shown below.

Here, the first order of drape region will drape perfectly well with its shape. But consider the red fiber as the drape extends beyond the first region. Here, the fiber suddenly becomes over-constrained as the drape extends across the concave boundary, so that the fibers become kinematically inadmissible.

To handle this situation, Composites Fiber Modeling unsticks fabric on one branch of the semi-circle (keeping the branch where the red line is longest) to remove the excessive constraint. This behavior is followed if the Inadmissible Mode is specified as Delete, that is, the constraint is deleted as required.

This situation of kinematic inadmissibility can be resolved by placing a dart in the middle of the semi-circular region using the limit contour capability as shown below. This dart removes the excessive constraint and allows draping to proceed.

You can force Composites Fiber Modeling to keep the fabric stuck to the order of drape regions during subsequent extensions if Inadmissible Mode is specified as Cut. In this case, kinematically inadmissible fibers are avoided by automatically generating a rough cut in the middle of a bounded length in an over-constrained fiber, such as that shown in red in the figure above. This cut is created only if the strain required in the fiber to allow it to conform to the surface exceeds the inadmissible tolerance value set, or the displacement mismatch at the cut location exceeds half a step length in the appropriate direction. This approach has the advantage of keeping the flat pattern in an order of drape region constant during subsequent draping, but forces you to define a dart to specify the flat pattern shape around the dart accurately. This methodology is suitable for highest accuracy on components like curved frames where it is impossible to avoid concave order of drape boundaries.

Seed curve and order of drape can be combined if required, but the seed curve (and seed point) should lie in the first order of drape region.

For CFM FEFlatten propagation types, the Edge smoothing distance parameter controls to which degree the material sticks to the surface:

• In this method, a given 3D surface geometry is meshed with triangular finite elements that are as close in size and shape as possible within the constraints of the geometry and chosen step length. The element associated with the Seed Point is then defined on a 2D flatten plane and adjacent elements are laid down in a radially expanding way until the edge of the ply is reached. Double curvature in the surface means that both the new element and the already-flattened elements are geometrically incompatible and have to deform when connected.
  
  
• When a new element is added to the flatten shape, only already-flattened elements within this distance to the free edge are allowed to deform, while other elements are fixed. Below, the cross represents the seed point. The solid line represents the free edge. The Edge smoothing distance is the distance between the solid and the dotted line.
  
  The default distance value (20mm) represents the size of a finger or a tool used to drape material over a surface.

A smooth region is an area where the simulation can ignore awkward bumps/hollows for an initial global simulation. The subsequent local draping goes from the smooth region boundary toward the middle of the bumps/hollows.

Many composite structures contain "bumps" - a ubiquitous example is that of a sandwich panel incorporating cores. When laying plies over the bumps, the material necessarily shears to conform to the highly curved surface covering the bumps. This shearing deformation is indicated by the red fibers on the J-panel below.

However, it is often required to define plies that lie around the bumps, but do not cover them. As most fiber simulations cannot accept holes in the surface used for simulation, the underlying surface that includes the bumps is used for the simulation, so high shear is still predicted whereas the surface excluding bumps may be rather simple and not need excessive shear.

Draping result with smooth regions

The smooth region is defined by an area and a curve.

• The area, with or without an order of drape, specifies a developable region.
• The smooth region curve defines a bump inside to be filled with a smooth surface using point, tangent, or curvature.

In the image below:

• 1 represents the bump.
• 2 represents the manual trimming area.
• 3 represents the smooth region boundary.
• 4 represents the required dart.
• 5 represents the finished inner boundary.

The example below has an interior contour. Without a smooth region, draping occurs over the entire surface causing strain to be distributed in places where it is undesirable. With a smooth region curve:

• The smooth region is filled with a surface and combined with the ply shell.
• This new smooth surface is simulated to create a low strain surface giving good edges.
• Next, the area inside the smooth region curve is added, and any strain is propagated into this region. Generally, causing the need for darts.
• The blue arrows show the interior contour. The pink arrow shows the smooth region curve.
  
  
• This is the difference of flat patterns.
  
  

The Mesh parameters define the architecture of the fabric, and the step lengths used for the fiber simulation.

Regarding the architecture of the fabric, the ratio between warp and weft is constant for a particular material. Use this material property consistently. This is especially important when using a seed curve.

The fiber simulation in Composites Fiber Modeling is relatively insensitive to step length. This makes the simulation accurate for highly curved surfaces, and reduces sensitivity to step length.

Consider the following channel section. For a wide variety of step lengths (between 1 and 50 mm), the Composites Fiber Modeling result predicted the same flat pattern. By contrast, the standard fiber simulation gave very different results depending on the chosen step length, and large step lengths lead to very poor flat pattern results.

As a result of the insensitivity to step length, there is no need to have very small step lengths and this can, in fact, be counter-productive.

Fine detail on the edge of the ply is picked up irrespective of the step length.

As you can see in the image below, the warp and weft lines (blue) at the spacing requested by the user have crossed the line defining the ply boundary (orange) at the location marked with circles (green).

The line showing the flat pattern (yellow) still contains the fine detail of the edge, which reflects the local strain in the ply.

There is no need to specify a small spacing to pick up details. The

The propagation type reflects the way in which the fabric covers the surface. As a consequence, different propagation types lead to different shapes and measures for the flattened ply.

Consider fabric placed on a surface beginning at the seed point with the principal warp and weft fibers defining seed curves being shown in red below. If the red lines are constrained to the surface, the placement of the fabric bounded by the green line is then uniquely defined.

Extending the material further to the ply boundary is done one free edge at a time. When extending a free edge, the fabric behaves as a trellis and so the exact direction of growth is not fully defined. Therefore, assumptions must be made of the direction of growth based on the material behavior and manufacturing method. Composites Fiber Modeling provides several different propagation modes to cover the most important manufacturing options, that fall into three categories.

• Optimized Energy and Optimized Maxshear modes use a biaxial material model and a circular extension strategy to model the hand layup of woven fabrics.
• Second, the Tape Steering and Tape Angle modes use a strip extension strategy to simulate the application of biaxial or uniaxial tape respectively in a way that 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 by automated means, subject to the detailed characteristics of the tape laying machine are not known by Composites Fiber Modeling.
• Finally, the FEFlatten propagation mode predicts the flattening of plies using finite element analysis techniques and is applicable to fabrics without a biaxial or uniaxial microstructure.

The 3D draped pattern for Composites Fiber Modeling is displayed in the same way as the standard fiber simulation.

The result can be used for inspection and 2D-3D transfer, and can be stored as curves in the model.

When using the enhanced producibility panel, users can access specialized display modes relevant to the fiber simulation. These give additional insight into the deformation of the ply over and above the degree of pure shear visualized in the standard Shearing Angle display mode. Composites Fiber Modeling currently supports display of the Steering Radius (not FEFlatten), and Axial Strain (only CFM FEFlatten).

Thickness Update provides a powerful capability to offset the simulation mesh from the ply definition surface. The result is a very accurate flat pattern of plies offset from the mold surface (far better than offsetting by a constant thickness). However, for complex layups, there are situations where the resultant updated surface becomes uneven.

As an example, consider the case of a two plies that are separated by a small gap (This may be intended by the designer to promote resin flow).

Here, the surface simulation mesh (in yellow at the bottom) is necessarily defined to respect ply boundaries on the surface, but when updated, results in a surface (in red on top).

Since Composites Fiber Modeling produces very accurate flat patterns as it follows the surface absolutely, it is disproportionately affected by surface quality. In fact, the surface roughness described above is seamlessly handled by Composites Fiber Modeling, but the effect of generalized crossing of hundreds of ply boundaries can be unpredictable. To solve this problem while taking advantage of the power of Thickness Update, smooth the simulation surface by choosing a maximum slope of the offset facets measured from their baseline position as shown below.

This effectively removes simulation surface artifacts and results in stable simulations with better results.

Composites Fiber Modeling computes the flat pattern instantaneously as an intrinsic feature of its simulation.

This ensures:

• Accuracy, as it is an intrinsic part of the simulation process,
• Speed, as no separate process is involved.
• Clarity, as it is possible to visualize the 2D flat pattern at the same time as the 3D draped pattern.

To display the flat pattern at the same time as the draped pattern, select the Display Flat Pattern check box. The flat pattern is displayed in a plane parallel to the surface at the seed point, and with the correct orientation.

The feedback allowed by this feature gives the user immediate confidence in the quality of the fiber simulation.